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MICHAEL SHORT: OK.

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00:00:24,000 --> 00:00:26,375
I think things have been
getting pretty derivy lately,

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00:00:26,375 --> 00:00:28,750
so I wanted to shift gears to
something a little bit more

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00:00:28,750 --> 00:00:30,070
practical.

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00:00:30,070 --> 00:00:33,310
So I started alluding to this
hypothetical radiation source

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00:00:33,310 --> 00:00:35,697
I might have right
here, and things

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00:00:35,697 --> 00:00:37,780
like if you have a source
of known activity, which

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00:00:37,780 --> 00:00:40,000
we calculated
yesterday, and you have

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00:00:40,000 --> 00:00:44,210
a detector of
unknown efficiency,

17
00:00:44,210 --> 00:00:45,940
how do you know what
the efficiency is?

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00:00:45,940 --> 00:00:48,220
How do you know what, let's say,
your dose distance relationship

19
00:00:48,220 --> 00:00:48,903
is?

20
00:00:48,903 --> 00:00:50,570
And how do you calculate
all this stuff?

21
00:00:50,570 --> 00:00:54,130
So let's take the general
situation that we're

22
00:00:54,130 --> 00:00:56,090
starting to work out.

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00:00:56,090 --> 00:00:59,210
Let's say we have a
Geiger counter right here.

24
00:00:59,210 --> 00:01:01,150
That's our GM tube.

25
00:01:01,150 --> 00:01:07,870
And we have a
point source that's

26
00:01:07,870 --> 00:01:10,905
emitting things
in all directions.

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00:01:10,905 --> 00:01:12,530
Let's go with the
stuff from yesterday.

28
00:01:12,530 --> 00:01:14,000
Let's say it's a
cobalt 60 source.

29
00:01:17,960 --> 00:01:23,210
It's now 0.52 microcurie.

30
00:01:23,210 --> 00:01:26,810
The question is, how many counts
do you expect in this detector

31
00:01:26,810 --> 00:01:29,030
when it's a certain
distance away?

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00:01:29,030 --> 00:01:31,460
So I've actually laser-cut out
a little Geiger counter jig

33
00:01:31,460 --> 00:01:32,480
from a previous class.

34
00:01:32,480 --> 00:01:34,370
And you guys can
all do this too.

35
00:01:34,370 --> 00:01:36,890
Who here has been
to the IDC before?

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00:01:36,890 --> 00:01:37,430
A couple.

37
00:01:37,430 --> 00:01:38,805
The international
design center--

38
00:01:38,805 --> 00:01:41,430
so they've got a laser cutter
that you can sign up to use,

39
00:01:41,430 --> 00:01:43,020
which is where I did this.

40
00:01:43,020 --> 00:01:45,770
And it's set to just
take a Geiger counter

41
00:01:45,770 --> 00:01:48,620
and put your sources
at some fixed distance

42
00:01:48,620 --> 00:01:51,410
away so you can discover
the dose distance

43
00:01:51,410 --> 00:01:53,060
relationship with things.

44
00:01:53,060 --> 00:01:55,250
Speaking of, does anybody
know what the relationship

45
00:01:55,250 --> 00:01:58,545
is between dose and distance or
measured activity and distance?

46
00:02:01,970 --> 00:02:02,830
Yeah, Luke.

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00:02:02,830 --> 00:02:04,080
AUDIENCE: [INAUDIBLE] r cubed.

48
00:02:04,080 --> 00:02:05,310
MICHAEL SHORT: Close.

49
00:02:05,310 --> 00:02:08,190
It's, let's say, the
measured activity

50
00:02:08,190 --> 00:02:15,000
would be proportional
to 1 over r squared.

51
00:02:15,000 --> 00:02:18,170
Who knows where this comes from?

52
00:02:18,170 --> 00:02:20,500
I'll move the source a bit
away to lessen the beeping.

53
00:02:23,320 --> 00:02:23,820
Yeah.

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00:02:23,820 --> 00:02:25,860
AUDIENCE: Well, the flux
of particles coming out

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00:02:25,860 --> 00:02:31,508
is just [INAUDIBLE] over the
surface area of [INAUDIBLE]

56
00:02:31,508 --> 00:02:34,890
and the [INAUDIBLE]
is 4 pi r squared.

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00:02:34,890 --> 00:02:36,240
MICHAEL SHORT: Yeah, exactly.

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00:02:36,240 --> 00:02:39,720
If you were to draw
a hypothetical sphere

59
00:02:39,720 --> 00:02:43,170
around the source right
here, then you've got,

60
00:02:43,170 --> 00:02:45,750
let's say, a detector
that's roughly rectangular

61
00:02:45,750 --> 00:02:47,220
with a fixed area.

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00:02:47,220 --> 00:02:50,850
Let's say it's got a half
length L and a half width

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00:02:50,850 --> 00:02:53,340
W. Then the area--

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00:02:53,340 --> 00:02:56,730
I'm sorry, let's just
say length L, width W--

65
00:02:56,730 --> 00:03:00,900
would be just L times
W. And actually,

66
00:03:00,900 --> 00:03:05,130
what Chris mentioned as
the solid angle subtended

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00:03:05,130 --> 00:03:07,230
by this detector right here--

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00:03:07,230 --> 00:03:11,340
in other words, at a
certain distance r away,

69
00:03:11,340 --> 00:03:13,273
how much of this sphere--

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00:03:13,273 --> 00:03:15,690
how much does the area of this
sphere-- does this detector

71
00:03:15,690 --> 00:03:16,797
take up?

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00:03:16,797 --> 00:03:18,630
In other words, how
many of these gamma rays

73
00:03:18,630 --> 00:03:21,360
are going to go in a different
direction than the detector,

74
00:03:21,360 --> 00:03:24,450
versus how many we'll
actually enter the detector?

75
00:03:24,450 --> 00:03:26,640
And a simple formula
for the solid angle

76
00:03:26,640 --> 00:03:29,400
is just the surface
area of whatever

77
00:03:29,400 --> 00:03:34,490
you've got over r squared.

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00:03:34,490 --> 00:03:37,610
It's a pretty good approximation
to the solid angle of something

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00:03:37,610 --> 00:03:40,278
for very long distances,
and it's probably the one

80
00:03:40,278 --> 00:03:41,570
that you'll see in the reading.

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00:03:41,570 --> 00:03:45,290
But I wanted to show you the
actual formula, in this case,

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00:03:45,290 --> 00:03:46,970
for a rectangle--

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00:03:46,970 --> 00:03:48,750
solid angle comparison.

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00:03:48,750 --> 00:03:49,970
Good, that's up there.

85
00:03:53,580 --> 00:03:56,400
So let's say on the
x-axis, right here,

86
00:03:56,400 --> 00:03:59,940
this would be distance
from the source

87
00:03:59,940 --> 00:04:01,405
to the detector in meters.

88
00:04:01,405 --> 00:04:03,780
And I've said that we've got
some sort of a detector that

89
00:04:03,780 --> 00:04:06,540
is 2.5 by 10 meters in size.

90
00:04:06,540 --> 00:04:08,550
That's an enormous detector.

91
00:04:08,550 --> 00:04:10,620
Let's actually switch it
to the units right here.

92
00:04:10,620 --> 00:04:15,480
So this is roughly
10 centimeters long.

93
00:04:15,480 --> 00:04:19,910
So let's change
our length to 0.1.

94
00:04:19,910 --> 00:04:23,150
And what do you think the
width of this Geiger counter

95
00:04:23,150 --> 00:04:24,792
is in meters?

96
00:04:24,792 --> 00:04:26,085
AUDIENCE: A centimeter

97
00:04:26,085 --> 00:04:28,410
MICHAEL SHORT: A centimeter.

98
00:04:28,410 --> 00:04:31,325
0.01.

99
00:04:31,325 --> 00:04:33,700
We're going to have to change
our axes so we can actually

100
00:04:33,700 --> 00:04:34,330
see the graph.

101
00:04:34,330 --> 00:04:37,620
So instead of looking all the
way out to 15 meters away,

102
00:04:37,620 --> 00:04:39,870
let's look one meter
away, maybe less.

103
00:04:39,870 --> 00:04:42,736
This whole thing is
probably 50 centimeters.

104
00:04:46,210 --> 00:04:48,140
And we'll take a look there.

105
00:04:48,140 --> 00:04:51,260
And what we notice is
that except for extremely

106
00:04:51,260 --> 00:04:54,980
short distances, this
approximate formula

107
00:04:54,980 --> 00:04:57,420
for the solid angle--
or in other words,

108
00:04:57,420 --> 00:05:01,820
if I were to draw a sphere
around the source that's

109
00:05:01,820 --> 00:05:03,823
the radius of the distance
between the source

110
00:05:03,823 --> 00:05:05,240
and the detector,
how much of that

111
00:05:05,240 --> 00:05:08,060
sphere's area does
the detector take up?

112
00:05:08,060 --> 00:05:11,330
This approximate
formula-- the blue curve--

113
00:05:11,330 --> 00:05:13,760
is a pretty good
approximation of the red curve

114
00:05:13,760 --> 00:05:16,970
until you get really, really
close to 5 centimeters

115
00:05:16,970 --> 00:05:20,180
away, or about this
distance right here.

116
00:05:20,180 --> 00:05:24,860
Does anyone know why this
formula would break down?

117
00:05:24,860 --> 00:05:28,980
What happens as r goes to 0?

118
00:05:31,510 --> 00:05:34,330
What happens to our solid
angle or our approximation

119
00:05:34,330 --> 00:05:36,800
for our solid angle?

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00:05:36,800 --> 00:05:38,106
AUDIENCE: Goes to Infinity

121
00:05:38,106 --> 00:05:41,000
MICHAEL SHORT: It goes
to infinity, right?

122
00:05:41,000 --> 00:05:43,820
Can a detector actually
take up infinity area

123
00:05:43,820 --> 00:05:45,950
on, well, anything?

124
00:05:45,950 --> 00:05:47,900
Never mind that unit sphere.

125
00:05:47,900 --> 00:05:49,140
Not quite.

126
00:05:49,140 --> 00:05:52,250
If you were to take this
detector and bring the radius

127
00:05:52,250 --> 00:05:55,730
down to 0 so that the
source and the detector,

128
00:05:55,730 --> 00:05:58,130
if not counting for the
thickness of the plastic,

129
00:05:58,130 --> 00:06:02,295
were right upside each other, if
that solid angle went to, well,

130
00:06:02,295 --> 00:06:04,295
infinity , then the count
should go to infinity,

131
00:06:04,295 --> 00:06:07,590
and it does not compute.

132
00:06:07,590 --> 00:06:09,770
Does anyone know how many--

133
00:06:09,770 --> 00:06:13,760
first of all, who here has
heard of solid angle before?

134
00:06:13,760 --> 00:06:16,878
So a little more
than half of you.

135
00:06:16,878 --> 00:06:17,795
That's getting clicky.

136
00:06:17,795 --> 00:06:19,880
I'm going to turn that off.

137
00:06:19,880 --> 00:06:22,320
Solid angle is kind of the
analog to regular old angle,

138
00:06:22,320 --> 00:06:24,090
except in 3D.

139
00:06:24,090 --> 00:06:26,880
So instead of looking
at things in radians,

140
00:06:26,880 --> 00:06:31,630
this has the unit of
what's called steradians--

141
00:06:31,630 --> 00:06:44,220
steradians-- with a full sphere
taking up 4pi steradians.

142
00:06:44,220 --> 00:06:46,290
Interestingly
enough, 4pi is also

143
00:06:46,290 --> 00:06:49,757
the surface area of a unit
sphere with radius of 1.

144
00:06:49,757 --> 00:06:51,090
So that's where this comes from.

145
00:06:51,090 --> 00:06:54,660
If something were to completely
cover a unit sphere--

146
00:06:54,660 --> 00:06:58,042
like, if you were to, let's say,
encase a light source in tin

147
00:06:58,042 --> 00:07:01,470
foil completely, and say,
how much of that solid angle

148
00:07:01,470 --> 00:07:02,640
does the tin foil encase?

149
00:07:02,640 --> 00:07:05,580
It would be 4pi
steradians, regardless

150
00:07:05,580 --> 00:07:10,310
of the size of the sphere or how
much tin foil you had to use.

151
00:07:10,310 --> 00:07:13,840
So this pretty simple formula
isn't the best approximation

152
00:07:13,840 --> 00:07:14,340
for it.

153
00:07:14,340 --> 00:07:16,432
And I'm not going to go
through the derivation,

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00:07:16,432 --> 00:07:17,890
because like I
said, today is going

155
00:07:17,890 --> 00:07:20,120
to be a more practical nature.

156
00:07:20,120 --> 00:07:22,730
There is a more complex
and rigorous formula

157
00:07:22,730 --> 00:07:25,670
for the solid
angle of something,

158
00:07:25,670 --> 00:07:28,550
let's say, in this case,
a rectangle of length L

159
00:07:28,550 --> 00:07:33,080
and with W, from a certain
distance r, or, in this case,

160
00:07:33,080 --> 00:07:35,530
on our graph, x away
from the sphere.

161
00:07:35,530 --> 00:07:38,115
And you can actually see
that red curve right there.

162
00:07:38,115 --> 00:07:39,740
Once you get to a
few centimeters away,

163
00:07:39,740 --> 00:07:42,650
it's pretty close.

164
00:07:42,650 --> 00:07:46,340
Anyone want to guess what the
maximum value of the red curve

165
00:07:46,340 --> 00:07:48,600
is?

166
00:07:48,600 --> 00:07:50,940
If I take this source
and slam it right up next

167
00:07:50,940 --> 00:07:57,000
to the detector,
how much of sphere

168
00:07:57,000 --> 00:07:59,902
is the detector subtending?

169
00:07:59,902 --> 00:08:00,800
AUDIENCE: 2pi

170
00:08:00,800 --> 00:08:02,960
MICHAEL SHORT: 2pi--
half the sphere.

171
00:08:02,960 --> 00:08:05,240
Because let's say this
whole side of the source

172
00:08:05,240 --> 00:08:08,390
is completely obscured by the
detector and this whole side

173
00:08:08,390 --> 00:08:10,310
is free to move.

174
00:08:10,310 --> 00:08:13,990
And if you look really closely,
yep, at 0, the correct formula

175
00:08:13,990 --> 00:08:16,130
does give you 2pi steradians.

176
00:08:16,130 --> 00:08:18,970
Which is to say that half the
gamma rays leaving the source

177
00:08:18,970 --> 00:08:21,010
would enter the detector.

178
00:08:21,010 --> 00:08:23,860
I didn't say anything
about get counted yet.

179
00:08:23,860 --> 00:08:26,130
That's where the detector
efficiency comes in.

180
00:08:26,130 --> 00:08:28,570
And that's something we're
going to be measuring today,

181
00:08:28,570 --> 00:08:32,140
which is why I have my
big bag of burnt bananas.

182
00:08:32,140 --> 00:08:35,710
These are the ashes of roughly
50 pounds of bananas charred

183
00:08:35,710 --> 00:08:39,490
to a crisp at about 250
Fahrenheit for 12 hours in most

184
00:08:39,490 --> 00:08:42,549
of the dorms and a couple
of the frat houses.

185
00:08:42,549 --> 00:08:45,040
So last year, I had the
students, everyone, take home

186
00:08:45,040 --> 00:08:47,018
about 50 pounds of
bananas or 50 bananas--

187
00:08:47,018 --> 00:08:47,810
I forget which one.

188
00:08:47,810 --> 00:08:49,450
It was a lot.

189
00:08:49,450 --> 00:08:50,890
And we did some
distributed labor.

190
00:08:50,890 --> 00:08:53,890
So everybody peeled the
bananas, put them in the oven,

191
00:08:53,890 --> 00:08:56,530
baked them, separated
off the tin foil,

192
00:08:56,530 --> 00:08:58,480
baked off as much water
and sugar as possible

193
00:08:58,480 --> 00:09:01,882
to concentrate the
potassium 40 in the banana.

194
00:09:01,882 --> 00:09:03,340
So there's a reason
I've been using

195
00:09:03,340 --> 00:09:06,700
potassium 40 as a lot of
examples in this class,

196
00:09:06,700 --> 00:09:08,560
because you're full of it.

197
00:09:08,560 --> 00:09:10,690
That's pretty much the
short answer of it.

198
00:09:10,690 --> 00:09:11,740
If you eat bananas--

199
00:09:11,740 --> 00:09:13,510
which, I think most
of you guys do--

200
00:09:13,510 --> 00:09:16,060
you're intaking a fair bit of
radioactive potassium, which

201
00:09:16,060 --> 00:09:18,880
is a positron
emitter, and also it

202
00:09:18,880 --> 00:09:21,800
does electron capture
and all that fun stuff.

203
00:09:21,800 --> 00:09:23,650
So today, what we're
going to be doing

204
00:09:23,650 --> 00:09:28,050
is calculating the
activity of one banana.

205
00:09:28,050 --> 00:09:30,360
But that's kind of a very
difficult thing to do.

206
00:09:30,360 --> 00:09:33,150
So anyone know how radioactive
one banana actually

207
00:09:33,150 --> 00:09:35,490
is in any units at all?

208
00:09:38,930 --> 00:09:42,250
Whatever it is, it's very,
very, very, very little.

209
00:09:42,250 --> 00:09:45,070
One banana contains a
minuscule but measurable amount

210
00:09:45,070 --> 00:09:46,630
of radioactivity.

211
00:09:46,630 --> 00:09:48,970
And one of the ways to
boost your confidence

212
00:09:48,970 --> 00:09:50,470
on any sort of
radiation measurement

213
00:09:50,470 --> 00:09:54,360
is to boost your signal strength
or to boost your counting time.

214
00:09:54,360 --> 00:09:57,190
And because I don't want to
count for the next seven years,

215
00:09:57,190 --> 00:10:00,700
we've concentrated the ashes
of 50 pounds of bananas in here

216
00:10:00,700 --> 00:10:02,980
to boost your signal
strength, which

217
00:10:02,980 --> 00:10:05,290
is going to boost
your count rate, which

218
00:10:05,290 --> 00:10:08,830
is the intro I want to give
to statistics certainty

219
00:10:08,830 --> 00:10:11,560
and counting.

220
00:10:11,560 --> 00:10:15,070
So let's take one of
the homework problems

221
00:10:15,070 --> 00:10:17,163
as a motivating example.

222
00:10:17,163 --> 00:10:19,330
You guys, did anyone notice
the extra credit problem

223
00:10:19,330 --> 00:10:21,170
on the homework?

224
00:10:21,170 --> 00:10:24,462
Let's start talking about
how we'd go about that.

225
00:10:24,462 --> 00:10:26,170
That should motivate
the rest of the day.

226
00:10:29,930 --> 00:10:33,660
So I'll pull up that
problem set, number 4--

227
00:10:33,660 --> 00:10:35,730
which, by the way, is due
Thursday, not Tuesday,

228
00:10:35,730 --> 00:10:38,950
because we have no
class on Tuesday.

229
00:10:38,950 --> 00:10:41,170
That was a surprise
to me, but whatever.

230
00:10:44,230 --> 00:10:45,700
I'll still be here.

231
00:10:45,700 --> 00:10:46,995
We don't get holidays--

232
00:10:46,995 --> 00:10:47,650
just you guys.

233
00:10:51,420 --> 00:10:54,770
So bonus question-- go do this.

234
00:10:54,770 --> 00:10:58,412
So we all know that smoking is
a major source of radioactivity.

235
00:10:58,412 --> 00:10:59,870
And if you think
about it, it's not

236
00:10:59,870 --> 00:11:02,540
just the smoke that contains
those radiation particles,

237
00:11:02,540 --> 00:11:04,160
it's got to be the
cigarettes, cigars,

238
00:11:04,160 --> 00:11:06,180
and other smokables themselves.

239
00:11:06,180 --> 00:11:09,140
And so I was thinking, there's
no better concentrated source

240
00:11:09,140 --> 00:11:11,095
of smoking radioactivity
than a smoke shop.

241
00:11:11,095 --> 00:11:13,220
There's one out at [INAUDIBLE]
at the end of the T.

242
00:11:13,220 --> 00:11:15,440
There's probably some
closer to campus.

243
00:11:15,440 --> 00:11:17,840
But I know there's a whole
bunch that are T accessible.

244
00:11:17,840 --> 00:11:20,150
And so I was thinking it'd
be neat for us to find out,

245
00:11:20,150 --> 00:11:23,710
how radioactive is it
to work in a smoke shop?

246
00:11:23,710 --> 00:11:26,030
Because there's all these
radon decay-- oh, yeah?

247
00:11:26,030 --> 00:11:26,780
You actually know.

248
00:11:26,780 --> 00:11:29,030
AUDIENCE: You know you have to
be 21 to go into a smoke shop?

249
00:11:29,030 --> 00:11:30,322
MICHAEL SHORT: Are you serious?

250
00:11:30,322 --> 00:11:32,005
But you have to be 18 to smoke.

251
00:11:32,005 --> 00:11:32,630
AUDIENCE: Yeah.

252
00:11:32,630 --> 00:11:34,025
It's a Cambridge, Boston law.

253
00:11:34,025 --> 00:11:35,150
MICHAEL SHORT: Interesting.

254
00:11:35,150 --> 00:11:37,310
We may have to leave
the city for this one.

255
00:11:37,310 --> 00:11:38,780
[LAUGHTER]

256
00:11:38,780 --> 00:11:40,960
What about Somerville?

257
00:11:40,960 --> 00:11:41,602
I think--

258
00:11:41,602 --> 00:11:43,310
AUDIENCE: It's still--
you're not allowed

259
00:11:43,310 --> 00:11:44,310
to go into there either.

260
00:11:44,310 --> 00:11:45,910
It's all of Massachusetts now.

261
00:11:45,910 --> 00:11:46,720
MICHAEL SHORT: Wow.

262
00:11:46,720 --> 00:11:47,790
AUDIENCE: So [INAUDIBLE]

263
00:11:47,790 --> 00:11:48,770
[INTERPOSING VOICES]

264
00:11:48,770 --> 00:11:50,395
AUDIENCE: [INAUDIBLE]
you can buy them.

265
00:11:50,395 --> 00:11:51,480
It's still late-stage.

266
00:11:51,480 --> 00:11:53,170
It's like town-to-town.

267
00:11:53,170 --> 00:11:54,850
Most of the Boston area is 21.

268
00:11:54,850 --> 00:11:56,878
But once you leave Boston--

269
00:11:56,878 --> 00:11:57,920
MICHAEL SHORT: It varies.

270
00:11:57,920 --> 00:11:58,410
AUDIENCE: Yeah.

271
00:11:58,410 --> 00:11:58,490
MICHAEL SHORT: Yeah.

272
00:11:58,490 --> 00:12:00,490
I don't think it is where
I'm-- from Swampscott,

273
00:12:00,490 --> 00:12:02,597
I don't think it's 21.

274
00:12:02,597 --> 00:12:04,430
But that's kind of up
on the commuter rails.

275
00:12:04,430 --> 00:12:05,900
You don't want to
go to Swampscott.

276
00:12:05,900 --> 00:12:08,000
At any rate, I would
think that, OK,

277
00:12:08,000 --> 00:12:10,790
it's probably a fairly
radioactive place to work.

278
00:12:10,790 --> 00:12:13,100
But the question is, how
long would you actually have

279
00:12:13,100 --> 00:12:15,560
to bring a detector
in and count in order

280
00:12:15,560 --> 00:12:19,130
to be sure that there's any
sort of measurable difference?

281
00:12:19,130 --> 00:12:22,880
And so, without deriving all
of this stuff about binomial,

282
00:12:22,880 --> 00:12:25,100
Poisson, and normal
statistics, I'll say,

283
00:12:25,100 --> 00:12:26,810
that's in the reading for today.

284
00:12:26,810 --> 00:12:28,790
I want to show you
some practical uses

285
00:12:28,790 --> 00:12:30,800
and applications of this stuff.

286
00:12:30,800 --> 00:12:33,410
Let's say you were
to measure some count

287
00:12:33,410 --> 00:12:35,580
rate in some experiment.

288
00:12:35,580 --> 00:12:39,660
And we'll put this in
units of counts per minute,

289
00:12:39,660 --> 00:12:43,040
which would be the number of
counts divided by the counting

290
00:12:43,040 --> 00:12:43,670
time.

291
00:12:43,670 --> 00:12:46,750
That's about as
simple as it gets.

292
00:12:46,750 --> 00:12:49,950
From Poisson
statistics, you can say

293
00:12:49,950 --> 00:12:54,940
that the standard deviation
of that count rate

294
00:12:54,940 --> 00:13:00,280
is actually just the square
root of the count rate divided

295
00:13:00,280 --> 00:13:01,430
by time.

296
00:13:01,430 --> 00:13:04,390
And that's kind of the
simple thing right here.

297
00:13:04,390 --> 00:13:06,430
But usually, in these
sorts of experiments,

298
00:13:06,430 --> 00:13:08,740
if you want to know how
much more radioactive is

299
00:13:08,740 --> 00:13:12,530
one place than another, you
have to take a background count.

300
00:13:12,530 --> 00:13:15,490
So if I wanted to know how much
activity that source was giving

301
00:13:15,490 --> 00:13:18,100
off, there is lots of
background radiation

302
00:13:18,100 --> 00:13:20,290
that we'll be going
over in about a month.

303
00:13:20,290 --> 00:13:23,320
I would have to sit
here for quite a while

304
00:13:23,320 --> 00:13:26,110
and wait for the slow clicks
of whatever background

305
00:13:26,110 --> 00:13:28,370
radiation is in the room--

306
00:13:28,370 --> 00:13:32,365
there we go-- to get enough
of a count right going on.

307
00:13:32,365 --> 00:13:35,368
As you can imagine, the
slower the count rate,

308
00:13:35,368 --> 00:13:37,660
the less certain you can be
that the number that you're

309
00:13:37,660 --> 00:13:39,770
measuring is actually accurate.

310
00:13:39,770 --> 00:13:42,370
So the idea here is that
this standard deviation

311
00:13:42,370 --> 00:13:46,270
is a measure of confidence that
your value is actually right.

312
00:13:46,270 --> 00:13:50,920
So the two things that
you could do to decrease

313
00:13:50,920 --> 00:13:52,810
this standard deviation--

314
00:13:52,810 --> 00:13:57,440
you could increase
your counting time.

315
00:13:57,440 --> 00:13:58,510
Why is there a C on top?

316
00:13:58,510 --> 00:14:00,020
That doesn't look right.

317
00:14:00,020 --> 00:14:01,260
It actually is OK.

318
00:14:01,260 --> 00:14:02,480
Yeah.

319
00:14:02,480 --> 00:14:04,190
Yeah, there we go.

320
00:14:04,190 --> 00:14:06,310
So by counting for
longer you can decrease

321
00:14:06,310 --> 00:14:08,620
your standard deviation.

322
00:14:08,620 --> 00:14:10,330
This is going to take forever.

323
00:14:10,330 --> 00:14:12,400
It actually takes
about 67 minutes,

324
00:14:12,400 --> 00:14:14,290
because we've already
done this calculation,

325
00:14:14,290 --> 00:14:18,273
to get a 95% confidence
on 5% uncertainty

326
00:14:18,273 --> 00:14:19,690
for this sort of
background count.

327
00:14:19,690 --> 00:14:22,540
I mean, how many counts
we have so far, like, 12?

328
00:14:22,540 --> 00:14:24,000
14?

329
00:14:24,000 --> 00:14:26,590
Yeah, not very many.

330
00:14:26,590 --> 00:14:29,230
Then you've got to be able
to subtract that count

331
00:14:29,230 --> 00:14:33,010
rate from whatever your
source actually is.

332
00:14:33,010 --> 00:14:35,080
And the way that you
actually measure this

333
00:14:35,080 --> 00:14:37,000
is pretty straightforward.

334
00:14:37,000 --> 00:14:38,830
The way that you do
error subtraction

335
00:14:38,830 --> 00:14:41,653
is not as straightforward.

336
00:14:41,653 --> 00:14:43,570
So let's say we're going
to separate these two

337
00:14:43,570 --> 00:14:48,870
experiments into a
background experiment, which

338
00:14:48,870 --> 00:14:50,610
we're actually going
to do in an hour.

339
00:14:50,610 --> 00:14:52,318
When we want to count
these banana ashes,

340
00:14:52,318 --> 00:14:54,600
we're going to have
to count radiation

341
00:14:54,600 --> 00:14:57,060
coming from the detector
itself, which will account

342
00:14:57,060 --> 00:15:00,780
for cosmic rays, contamination
in the detector, whatever else

343
00:15:00,780 --> 00:15:03,120
might have been spilled in
there from previous samples.

344
00:15:03,120 --> 00:15:08,860
And we're also going to take
some sort of gross count

345
00:15:08,860 --> 00:15:15,440
rate, which will be our
background plus the net count

346
00:15:15,440 --> 00:15:17,060
rate of our actual source.

347
00:15:17,060 --> 00:15:19,360
And that's what we're going for.

348
00:15:19,360 --> 00:15:22,245
So the net count
rate is pretty easy.

349
00:15:22,245 --> 00:15:25,070
It's just the gross count
rate minus the background--

350
00:15:25,070 --> 00:15:27,720
let's keep the
symbols the same--

351
00:15:27,720 --> 00:15:28,860
count rate.

352
00:15:28,860 --> 00:15:34,220
Does anyone know how to
quantify the uncertainty

353
00:15:34,220 --> 00:15:37,510
of this net count rate?

354
00:15:37,510 --> 00:15:38,670
Do you just add the two?

355
00:15:48,285 --> 00:15:50,410
Well, in this case, we have
to account for the fact

356
00:15:50,410 --> 00:15:53,800
that radiation
emission from anything

357
00:15:53,800 --> 00:15:56,020
is a truly random process.

358
00:15:56,020 --> 00:15:57,530
So it's actually random.

359
00:15:57,530 --> 00:16:00,370
There is no correlation between
when one particle leaves

360
00:16:00,370 --> 00:16:02,470
and the next particles
going to leave.

361
00:16:02,470 --> 00:16:04,840
And because it's a
truly random process,

362
00:16:04,840 --> 00:16:08,290
these errors in the background
rate and the gross rate

363
00:16:08,290 --> 00:16:11,317
could add together or could
subtract from each other.

364
00:16:11,317 --> 00:16:13,150
In other words, one
might be a little higher

365
00:16:13,150 --> 00:16:15,692
than it should be, one might be
a little lower than it should

366
00:16:15,692 --> 00:16:16,300
be.

367
00:16:16,300 --> 00:16:23,740
If you just add together
the two standard deviations,

368
00:16:23,740 --> 00:16:25,870
you actually always
get an overestimate

369
00:16:25,870 --> 00:16:30,180
of the true error,
because you're not

370
00:16:30,180 --> 00:16:32,820
accounting for the fact that
these two experiments may have

371
00:16:32,820 --> 00:16:34,867
partially canceling errors.

372
00:16:34,867 --> 00:16:37,200
So in this case, that would
be your worst case scenario,

373
00:16:37,200 --> 00:16:39,390
which is not your
most likely scenario.

374
00:16:39,390 --> 00:16:42,700
What you actually want is to
do what's called uncertainty

375
00:16:42,700 --> 00:16:45,960
in quadrature,
where you actually

376
00:16:45,960 --> 00:16:50,700
add up the sum of the square
roots of those errors.

377
00:16:50,700 --> 00:16:52,890
It kind of looks like the
magnitude of a vector,

378
00:16:52,890 --> 00:16:53,640
doesn't it?

379
00:16:53,640 --> 00:16:56,193
It kind of looks exactly like
the magnitude of a vector.

380
00:16:56,193 --> 00:16:58,110
So in this way, you're
accounting for the fact

381
00:16:58,110 --> 00:17:00,660
that more error
in each experiment

382
00:17:00,660 --> 00:17:03,930
does increase the error
on whatever net experiment

383
00:17:03,930 --> 00:17:06,390
you're doing, but not linearly.

384
00:17:06,390 --> 00:17:09,390
Because sometimes you have
partially canceling errors.

385
00:17:09,390 --> 00:17:12,450
And with enough statistics,
if you count for long enough

386
00:17:12,450 --> 00:17:15,450
or you count enough counts, then
these things, on average, are

387
00:17:15,450 --> 00:17:20,025
going to add in quadrature,
which will come out to--

388
00:17:20,025 --> 00:17:24,160
and I want to make sure we don't
have any typos, so I'll just

389
00:17:24,160 --> 00:17:26,020
keep the notes with me--

390
00:17:26,020 --> 00:17:30,280
so you'd need the background
count over the background time

391
00:17:30,280 --> 00:17:33,295
squared, plus those.

392
00:17:35,920 --> 00:17:37,613
There we go.

393
00:17:37,613 --> 00:17:39,280
And so, now, I'd like
to pose a question

394
00:17:39,280 --> 00:17:44,860
to you, the same one that's
here in the problem set--

395
00:17:44,860 --> 00:17:47,020
how long do you have to
count in the smoke shop

396
00:17:47,020 --> 00:17:49,200
to be 95% percent sure?

397
00:17:49,200 --> 00:17:52,227
So let's say your count
rate's 5% uncertain.

398
00:17:52,227 --> 00:17:54,310
And we're going to spend
the rest of today's class

399
00:17:54,310 --> 00:17:58,340
taking apart that statement and
getting at what it should be.

400
00:17:58,340 --> 00:18:00,320
So again, what we
want to say is,

401
00:18:00,320 --> 00:18:07,440
how do you know that we're
95% confident of our count

402
00:18:07,440 --> 00:18:12,540
rate plus or minus 5% error?

403
00:18:15,550 --> 00:18:17,637
That's the main
question for today.

404
00:18:17,637 --> 00:18:18,970
Does anyone know how we'd start?

405
00:18:23,237 --> 00:18:24,570
Anyone get to the reading today?

406
00:18:27,234 --> 00:18:28,650
I see some smiles.

407
00:18:28,650 --> 00:18:29,788
OK.

408
00:18:29,788 --> 00:18:31,080
We'll start from scratch, then.

409
00:18:34,475 --> 00:18:46,110
All right, So who here has
heard of a normal distribution

410
00:18:46,110 --> 00:18:47,160
before?

411
00:18:47,160 --> 00:18:48,210
A lot of you guys.

412
00:18:48,210 --> 00:18:49,080
Great.

413
00:18:49,080 --> 00:18:53,490
The idea here is that with
enough counting statistics,

414
00:18:53,490 --> 00:18:57,270
this very rare event binomial
distribution approaches

415
00:18:57,270 --> 00:19:00,900
a normal distribution, where
you can say if you measure

416
00:19:00,900 --> 00:19:03,540
a certain count rate-- let's say
this would be your mean count

417
00:19:03,540 --> 00:19:04,040
rate--

418
00:19:11,540 --> 00:19:14,450
to limits of plus
or minus 1 sigma

419
00:19:14,450 --> 00:19:22,680
or one standard deviation,
1 sigma gives you about 68%

420
00:19:22,680 --> 00:19:30,160
confidence in your result.
Yeah, I spelled it right.

421
00:19:30,160 --> 00:19:33,370
The reason for that is that if
you go plus or minus 1 sigma

422
00:19:33,370 --> 00:19:37,810
away from your true
average right here,

423
00:19:37,810 --> 00:19:42,880
you've filled in 68% of the area
under this normal distribution.

424
00:19:42,880 --> 00:19:52,040
Similarly, if you go plus
2 sigma or minus 2 sigma,

425
00:19:52,040 --> 00:19:57,400
it's around 95% confident.

426
00:19:57,400 --> 00:20:00,223
3 sigma is getting
towards 99 point--

427
00:20:00,223 --> 00:20:01,390
what was the number, again--

428
00:20:01,390 --> 00:20:03,840
I think it's 6.

429
00:20:03,840 --> 00:20:08,130
Maybe it's more like 98.5%.

430
00:20:08,130 --> 00:20:10,590
And then so on, and
so on, and so on.

431
00:20:10,590 --> 00:20:13,495
There's actually societies
called 6 sigma societies.

432
00:20:13,495 --> 00:20:15,120
And the way that they
get their name is

433
00:20:15,120 --> 00:20:17,220
we're so confident of
things we can predict them

434
00:20:17,220 --> 00:20:20,730
to 6 sigma, which is some
99 point a large number

435
00:20:20,730 --> 00:20:24,970
of nines percentage of the area
under a normal distribution.

436
00:20:24,970 --> 00:20:27,750
So if I ask you,
how long do you have

437
00:20:27,750 --> 00:20:31,590
to count to be 95%
confident in your result,

438
00:20:31,590 --> 00:20:34,500
you have to give an answer
that will relate two

439
00:20:34,500 --> 00:20:36,890
times this standard deviation.

440
00:20:36,890 --> 00:20:39,650
And now we know the formula for
standard deviation of this net

441
00:20:39,650 --> 00:20:41,600
counting experiment.

442
00:20:41,600 --> 00:20:44,570
So we can formulate
our equation thusly--

443
00:20:44,570 --> 00:20:49,040
let's say in order to be 95%
confident, in other words, 2

444
00:20:49,040 --> 00:20:55,920
sigma, that our counting rate is
within 5% of the actual value,

445
00:20:55,920 --> 00:21:00,130
in other words, plus
or minus 5% error,

446
00:21:00,130 --> 00:21:02,710
we put our error
percentage here,

447
00:21:02,710 --> 00:21:04,930
and our true net
count rate there.

448
00:21:04,930 --> 00:21:10,210
So this part right here
tells us the 95% confidence.

449
00:21:10,210 --> 00:21:15,520
This part right here
is our 5% error.

450
00:21:15,520 --> 00:21:18,190
And that part right
there is our count rate.

451
00:21:21,680 --> 00:21:25,370
So then we can substitute in
our expression for sigma--

452
00:21:25,370 --> 00:21:27,950
our uncertainty in quadrature--
and find out things like,

453
00:21:27,950 --> 00:21:30,710
well, it depends on what the
information we're given is.

454
00:21:30,710 --> 00:21:32,870
Let's say before you
go to the smoke shop,

455
00:21:32,870 --> 00:21:36,590
you take your Geiger counter,
and for an extremely long time

456
00:21:36,590 --> 00:21:39,170
you count the background
counts somewhere.

457
00:21:39,170 --> 00:21:46,160
So let's say in this problem
the known quantities--

458
00:21:46,160 --> 00:21:48,500
we know our
background count rate,

459
00:21:48,500 --> 00:21:51,480
because you can do that
at your leisure at home.

460
00:21:51,480 --> 00:21:56,550
And when I did this, it came out
to about 25 counts per minute.

461
00:21:56,550 --> 00:22:01,530
And known is the
background counting time.

462
00:22:01,530 --> 00:22:05,190
And when I did this, to
get within 95% confidence

463
00:22:05,190 --> 00:22:08,475
of 5% error, I had to
do this for 67 minutes.

464
00:22:12,440 --> 00:22:14,150
And now, all that's
left is we want

465
00:22:14,150 --> 00:22:22,740
to relate our net count rate
and our gross counting time,

466
00:22:22,740 --> 00:22:25,620
or our gross count rate and
our gross counting time,

467
00:22:25,620 --> 00:22:28,240
because it's the same thing.

468
00:22:28,240 --> 00:22:29,860
So this is actually
how you decide

469
00:22:29,860 --> 00:22:32,320
how long you have to
sit in the smoke shop

470
00:22:32,320 --> 00:22:36,730
to count in order to
satisfy what we asked for--

471
00:22:36,730 --> 00:22:41,840
95% confidence that your
count rate is 5% error.

472
00:22:41,840 --> 00:22:44,950
So let's start
substituting this out.

473
00:22:44,950 --> 00:22:46,960
That's not mine, so we
can get rid of that.

474
00:23:00,190 --> 00:23:02,880
So we'll take that expression
and substitute in everything

475
00:23:02,880 --> 00:23:04,050
we can.

476
00:23:04,050 --> 00:23:11,030
So 0.05 C n equals 2 sigma.

477
00:23:11,030 --> 00:23:14,760
And there's our
sigma expression,

478
00:23:14,760 --> 00:23:18,140
which I'll rewrite right here.

479
00:23:18,140 --> 00:23:24,860
So we have see C b
over t b squared plus C

480
00:23:24,860 --> 00:23:28,790
g over t g squared.

481
00:23:31,870 --> 00:23:32,380
What's next?

482
00:23:34,980 --> 00:23:37,942
How do we relate t g and C g?

483
00:23:45,195 --> 00:23:47,070
Well, let's start with
the easy stuff, right?

484
00:23:47,070 --> 00:23:49,007
What can we cancel, or
square, or whatever?

485
00:23:49,007 --> 00:23:50,090
Just somebody yell it out.

486
00:23:53,938 --> 00:23:57,418
AUDIENCE: Do we have
numbers for these counts?

487
00:23:57,418 --> 00:23:58,210
MICHAEL SHORT: Yep.

488
00:23:58,210 --> 00:24:03,508
So we have numbers for C b
and t b, but not C g and t g.

489
00:24:03,508 --> 00:24:05,050
We have not yet
answered the question

490
00:24:05,050 --> 00:24:07,510
when you go into the smoke
shop and talk to the owner,

491
00:24:07,510 --> 00:24:08,770
and he says, fine,
you're going to sit here

492
00:24:08,770 --> 00:24:10,030
with the radiation detector.

493
00:24:10,030 --> 00:24:13,060
How long do you have to be
here, looking all weird?

494
00:24:13,060 --> 00:24:15,520
You want to have an answer.

495
00:24:15,520 --> 00:24:19,200
And so if you get some
initial estimate of C g,

496
00:24:19,200 --> 00:24:22,170
you can tell him this is my
approximate t g, at which point

497
00:24:22,170 --> 00:24:24,212
he or she will say
yes or no, depending

498
00:24:24,212 --> 00:24:25,170
on how they're feeling.

499
00:24:27,800 --> 00:24:30,130
So why don't we just
start, divide by 2, right?

500
00:24:34,320 --> 00:24:36,084
Divide by 2.

501
00:24:36,084 --> 00:24:39,720
0.025.

502
00:24:39,720 --> 00:24:41,830
We can square both sides.

503
00:24:41,830 --> 00:24:43,860
And there's a C n there.

504
00:24:43,860 --> 00:24:51,556
Square both sides, and
we end up with 0.000625 C

505
00:24:51,556 --> 00:25:01,030
n squared equals C b
over t b squared plus C

506
00:25:01,030 --> 00:25:04,640
g over t g squared.

507
00:25:07,030 --> 00:25:08,530
There's lots of
ways to go about it.

508
00:25:08,530 --> 00:25:10,740
I want to make sure I
do the efficient one.

509
00:25:10,740 --> 00:25:14,980
Oh, I'm sorry those
aren't squared.

510
00:25:14,980 --> 00:25:22,100
Because our standard deviations
had the square root in them.

511
00:25:22,100 --> 00:25:22,867
There we go.

512
00:25:22,867 --> 00:25:23,700
That's more like it.

513
00:25:26,370 --> 00:25:28,912
What's next?

514
00:25:28,912 --> 00:25:30,120
We've got too many variables.

515
00:25:30,120 --> 00:25:30,620
Yeah?

516
00:25:30,620 --> 00:25:34,233
AUDIENCE: I think there's still
a square value [INAUDIBLE]

517
00:25:34,233 --> 00:25:35,900
MICHAEL SHORT: Isn't
there still a what?

518
00:25:35,900 --> 00:25:37,400
AUDIENCE: Isn't
there a square value

519
00:25:37,400 --> 00:25:39,698
still under the [INAUDIBLE]?

520
00:25:39,698 --> 00:25:41,240
MICHAEL SHORT:
Because, in this case,

521
00:25:41,240 --> 00:25:44,680
the standard deviation is
the square root of the count

522
00:25:44,680 --> 00:25:46,900
rate over the time.

523
00:25:46,900 --> 00:25:48,310
So the standard
deviation squared

524
00:25:48,310 --> 00:25:51,860
is just count rate
overtime time.

525
00:25:51,860 --> 00:25:54,930
Was there an earlier
expression we have to correct?

526
00:25:54,930 --> 00:25:55,572
Yep.

527
00:25:55,572 --> 00:25:56,870
[LAUGHTER]

528
00:25:56,870 --> 00:25:59,790
That's where it came from.

529
00:25:59,790 --> 00:26:02,210
That's right.

530
00:26:02,210 --> 00:26:03,140
That's not.

531
00:26:03,140 --> 00:26:05,390
Because that's right.

532
00:26:05,390 --> 00:26:06,690
There we go.

533
00:26:06,690 --> 00:26:07,190
Good.

534
00:26:07,190 --> 00:26:08,820
Good, tracing out that.

535
00:26:08,820 --> 00:26:09,320
OK.

536
00:26:09,320 --> 00:26:15,032
Now that everything is
corrected here, what's next?

537
00:26:15,032 --> 00:26:16,240
We've got too many variables.

538
00:26:16,240 --> 00:26:17,770
Yeah?

539
00:26:17,770 --> 00:26:20,520
AUDIENCE: [INAUDIBLE]
the standard deviation

540
00:26:20,520 --> 00:26:24,090
have units of [INAUDIBLE]?

541
00:26:24,090 --> 00:26:26,610
MICHAEL SHORT: Not
quite, because there's

542
00:26:26,610 --> 00:26:27,670
a count rate in here.

543
00:26:27,670 --> 00:26:29,250
So the units of
standard deviation,

544
00:26:29,250 --> 00:26:32,940
if this is square root
of count rate over time,

545
00:26:32,940 --> 00:26:37,110
which is the same as number of
counts times time over time,

546
00:26:37,110 --> 00:26:37,766
right?

547
00:26:37,766 --> 00:26:38,600
AUDIENCE: OK.

548
00:26:38,600 --> 00:26:39,790
MICHAEL SHORT: Yeah.

549
00:26:39,790 --> 00:26:45,787
Because again, a count
rate is a number over--

550
00:26:45,787 --> 00:26:46,370
where'd it go.

551
00:26:46,370 --> 00:26:47,980
AUDIENCE: Number
over time squared.

552
00:26:47,980 --> 00:26:48,230
MICHAEL SHORT: Yeah.

553
00:26:48,230 --> 00:26:49,610
Number over time squared.

554
00:26:49,610 --> 00:26:52,665
That doesn't sound right though.

555
00:26:52,665 --> 00:26:53,165
Let's see.

556
00:26:59,877 --> 00:27:00,460
Hold on a sec.

557
00:27:04,082 --> 00:27:05,540
Although the standard
deviation has

558
00:27:05,540 --> 00:27:08,750
got to have the same units
as the count rate itself,

559
00:27:08,750 --> 00:27:10,740
because they're additive, right?

560
00:27:10,740 --> 00:27:13,190
Because they usually express
some count rate plus or minus

561
00:27:13,190 --> 00:27:15,217
either sigma or 2
sigma, so they've

562
00:27:15,217 --> 00:27:16,550
got to have the same count rate.

563
00:27:16,550 --> 00:27:19,560
So standard deviations are
expressed in counts per minute

564
00:27:19,560 --> 00:27:21,700
if your counts are expressed
in counts per minute.

565
00:27:21,700 --> 00:27:24,400
OK, cool.

566
00:27:24,400 --> 00:27:26,650
So we've got too many
variables, but it's

567
00:27:26,650 --> 00:27:29,290
easy to get rid of one of
them, either C n or C g.

568
00:27:29,290 --> 00:27:30,040
Do you a question?

569
00:27:30,040 --> 00:27:32,325
AUDIENCE: No, I was just
going to say [INAUDIBLE]..

570
00:27:32,325 --> 00:27:33,200
MICHAEL SHORT: Great.

571
00:27:33,200 --> 00:27:34,825
So you were going to
say the same thing

572
00:27:34,825 --> 00:27:36,100
that I was going to do.

573
00:27:36,100 --> 00:27:37,480
Cool.

574
00:27:37,480 --> 00:27:43,890
So we'll take out our C n, and
we'll stick in a C g minus C b.

575
00:27:49,320 --> 00:27:52,980
And we're trying to isolate t
g as a function of C g or vice

576
00:27:52,980 --> 00:27:53,970
versa.

577
00:27:53,970 --> 00:27:55,560
There's a lot of C
g's and not a lot

578
00:27:55,560 --> 00:27:59,480
of t g's, so let's just
keep the t g on its own.

579
00:27:59,480 --> 00:28:07,050
So we'll have 0.000625
C g minus C b squared.

580
00:28:07,050 --> 00:28:11,650
Then I'm going to subtract C
b over t b from both sides.

581
00:28:17,330 --> 00:28:25,820
Minus C b over t b
equals C g over t g.

582
00:28:25,820 --> 00:28:28,810
And do I have to go through the
rest the math with you guys?

583
00:28:28,810 --> 00:28:31,640
I think, at this point, we've
got it pretty much solved.

584
00:28:31,640 --> 00:28:36,200
We divide everything
by C g, flip it over,

585
00:28:36,200 --> 00:28:37,700
and you end up
with-- actually, I've

586
00:28:37,700 --> 00:28:40,130
already written out
the expression, which

587
00:28:40,130 --> 00:28:41,360
I want to show you guys here.

588
00:28:44,350 --> 00:28:47,620
Back to smoke shop
counting time.

589
00:28:47,620 --> 00:28:50,110
So I want to show you
some of the implications

590
00:28:50,110 --> 00:28:51,760
of this expression.

591
00:28:51,760 --> 00:28:56,140
That number right there is
just a more exact part-- a bit

592
00:28:56,140 --> 00:28:57,370
of 2 Sigma.

593
00:28:57,370 --> 00:29:02,890
Instead of 0.05, we had
something much, much closer.

594
00:29:02,890 --> 00:29:06,350
So what I want us to look
at is this graph right here.

595
00:29:06,350 --> 00:29:09,490
We've got a nice relation
now between the count

596
00:29:09,490 --> 00:29:11,200
rate and counts per minute--

597
00:29:11,200 --> 00:29:14,440
and it was the gross count
rate and the required counting

598
00:29:14,440 --> 00:29:17,580
time to get to that
5% uncertainty.

599
00:29:17,580 --> 00:29:19,330
Well, there's a couple
of interesting bits

600
00:29:19,330 --> 00:29:20,762
about this equation.

601
00:29:20,762 --> 00:29:22,470
What are some of the
features you notice?

602
00:29:26,970 --> 00:29:27,470
Yeah.

603
00:29:27,470 --> 00:29:32,478
AUDIENCE: The count rate is
extremely low for [INAUDIBLE]..

604
00:29:32,478 --> 00:29:33,270
MICHAEL SHORT: Yes.

605
00:29:33,270 --> 00:29:35,160
If the count rate
is extremely low,

606
00:29:35,160 --> 00:29:37,260
it's going to take an
infinite amount of time.

607
00:29:37,260 --> 00:29:39,700
You're absolutely
right on some level.

608
00:29:39,700 --> 00:29:41,880
So if we have that
expression right

609
00:29:41,880 --> 00:29:44,730
there-- so let me just
actually get it all the way out

610
00:29:44,730 --> 00:29:45,773
so we can see.

611
00:29:45,773 --> 00:29:47,190
Because I want to
show you some of

612
00:29:47,190 --> 00:29:49,260
the math-related
implications for this.

613
00:29:49,260 --> 00:29:53,160
So if we had our counting time--

614
00:29:53,160 --> 00:29:54,890
what do we have--

615
00:29:54,890 --> 00:30:12,610
C g over 0.025 C g minus C b
squared, minus C b over t b,

616
00:30:12,610 --> 00:30:15,880
at what point is this
equation undefined?

617
00:30:23,768 --> 00:30:24,400
Yeah, Sean.

618
00:30:24,400 --> 00:30:26,233
AUDIENCE: [INAUDIBLE]
question [INAUDIBLE],,

619
00:30:26,233 --> 00:30:29,690
using the second one
after the [INAUDIBLE]..

620
00:30:29,690 --> 00:30:31,670
MICHAEL SHORT: That's right.

621
00:30:31,670 --> 00:30:33,170
So like Sean said,
for the condition

622
00:30:33,170 --> 00:30:38,870
where 0.025 C g
minus C b-- let's

623
00:30:38,870 --> 00:30:45,140
just call it C net squared
minus equals C b over t b,

624
00:30:45,140 --> 00:30:48,260
this equation is
actually undefined.

625
00:30:48,260 --> 00:30:51,500
Which means that if
your C b and t b-- let's

626
00:30:51,500 --> 00:30:58,790
say if the uncertainty from
your background counting rate

627
00:30:58,790 --> 00:31:00,620
experiment is such
that you can never

628
00:31:00,620 --> 00:31:03,410
get the total
uncertainty down to let's

629
00:31:03,410 --> 00:31:07,130
say 5% error with
95% confidence,

630
00:31:07,130 --> 00:31:10,250
you can't actually
run that experiment.

631
00:31:10,250 --> 00:31:13,400
Because these uncertainties
are added in quadrature,

632
00:31:13,400 --> 00:31:16,010
if you're trying to reduce
sigma down to a value

633
00:31:16,010 --> 00:31:20,120
below that already,
how can you do that?

634
00:31:20,120 --> 00:31:23,903
You can't have a negative
standard deviation, right?

635
00:31:23,903 --> 00:31:25,570
So what this actually
means is that when

636
00:31:25,570 --> 00:31:28,570
you're designing this
experiment, even if you count

637
00:31:28,570 --> 00:31:32,330
for 67 minutes at 25
counts per minute,

638
00:31:32,330 --> 00:31:34,510
like we can now out in
the air, that might not

639
00:31:34,510 --> 00:31:37,480
be enough to discern the
activity of the smoke

640
00:31:37,480 --> 00:31:38,980
shop, or the source,
or whatever you

641
00:31:38,980 --> 00:31:43,850
happen to be looking at to 95%
confidence within 5% error.

642
00:31:43,850 --> 00:31:45,850
And so let's actually
look at that on the graph.

643
00:31:45,850 --> 00:31:53,520
If we keep on scrolling up just
by adding stuff to the y-axis,

644
00:31:53,520 --> 00:31:56,280
eventually we see that
it gets all straight.

645
00:31:56,280 --> 00:32:01,740
And right here, at about
49 counts a minute,

646
00:32:01,740 --> 00:32:04,380
suspiciously close to
the background counts,

647
00:32:04,380 --> 00:32:07,800
you'll never actually be able
to get within this confidence

648
00:32:07,800 --> 00:32:09,275
and error interval.

649
00:32:09,275 --> 00:32:10,650
So there's always
some trade-offs

650
00:32:10,650 --> 00:32:13,200
you can make in your experiment.

651
00:32:13,200 --> 00:32:17,510
Let's see-- there it is.

652
00:32:17,510 --> 00:32:19,270
So sometimes, do
you necessarily have

653
00:32:19,270 --> 00:32:23,940
to be 95% confident
of your result?

654
00:32:23,940 --> 00:32:25,770
Depends on what you're doing.

655
00:32:25,770 --> 00:32:30,430
Or do you necessarily have
to get within 5% error?

656
00:32:30,430 --> 00:32:33,940
That's probably the one you
can start to sacrifice first.

657
00:32:33,940 --> 00:32:36,220
So usually, you want to
be confident of whatever

658
00:32:36,220 --> 00:32:38,500
result you're saying and
be confident that you're

659
00:32:38,500 --> 00:32:41,680
giving acceptable bounds.

660
00:32:41,680 --> 00:32:44,980
So you can remain at 95%
confidence, which means--

661
00:32:47,850 --> 00:32:51,310
where did part go--

662
00:32:51,310 --> 00:32:56,920
which means keep your 2 Sigma,
but you can then increase

663
00:32:56,920 --> 00:32:59,360
your allowable percent error.

664
00:32:59,360 --> 00:33:01,180
So if you can't get
within 5% error--

665
00:33:01,180 --> 00:33:03,820
and I believe the homework
doesn't actually say that

666
00:33:03,820 --> 00:33:06,510
for a reason--

667
00:33:06,510 --> 00:33:08,820
yeah, we don't tell
what error to choose.

668
00:33:08,820 --> 00:33:11,070
But we do say try to
get a 95% confidence.

669
00:33:11,070 --> 00:33:13,350
So then the question is,
for a reasonable counting

670
00:33:13,350 --> 00:33:18,990
time, to what error can you
get within 95% confidence?

671
00:33:18,990 --> 00:33:21,930
The more error you allow,
the shorter time you

672
00:33:21,930 --> 00:33:23,135
have to count for.

673
00:33:23,135 --> 00:33:25,260
And I want to show you
graphically how some of that

674
00:33:25,260 --> 00:33:28,830
stuff interplay with each other.

675
00:33:28,830 --> 00:33:31,370
Let's say you were to
increase your counting time,

676
00:33:31,370 --> 00:33:33,690
which we can do
here with a slider.

677
00:33:33,690 --> 00:33:36,140
So for the same
background counting rate,

678
00:33:36,140 --> 00:33:37,700
if you increase
the counting time,

679
00:33:37,700 --> 00:33:41,645
what happens to the uncertainty
on your background experiment?

680
00:33:41,645 --> 00:33:44,365
Does it go up, down, or nothing?

681
00:33:44,365 --> 00:33:45,670
AUDIENCE: It goes down.

682
00:33:45,670 --> 00:33:46,310
MICHAEL SHORT: It's
going to go down.

683
00:33:46,310 --> 00:33:46,810
Yeah.

684
00:33:46,810 --> 00:33:49,240
Count for longer-- the
uncertainty goes down.

685
00:33:49,240 --> 00:33:53,050
I'm going to have to change
the bounds here to something

686
00:33:53,050 --> 00:33:55,510
more reasonable.

687
00:33:55,510 --> 00:33:59,360
So we were at 67 minutes.

688
00:33:59,360 --> 00:34:03,620
And now, notice, as you
increase your counting time,

689
00:34:03,620 --> 00:34:06,920
even though you haven't
changed the counting rate,

690
00:34:06,920 --> 00:34:08,840
it then takes less
time to distinguish

691
00:34:08,840 --> 00:34:10,100
whatever your source is.

692
00:34:10,100 --> 00:34:14,447
So let's count for less
time in the background,

693
00:34:14,447 --> 00:34:16,489
you have to count for more
time in the experiment

694
00:34:16,489 --> 00:34:19,520
until it just kind of explodes.

695
00:34:19,520 --> 00:34:21,110
Count for more time
in the background,

696
00:34:21,110 --> 00:34:23,152
you have to count for less
time in the experiment

697
00:34:23,152 --> 00:34:25,850
in order to get to the
uncertainty and confidence you

698
00:34:25,850 --> 00:34:27,110
want to get to.

699
00:34:27,110 --> 00:34:29,090
So if you doubled your
background count time

700
00:34:29,090 --> 00:34:35,810
from 67 minutes to
134, then you can

701
00:34:35,810 --> 00:34:39,742
measure count rates as low as
42 counts per minute gross.

702
00:34:39,742 --> 00:34:42,409
So when you start going into the
smoke shop, you can, let's say,

703
00:34:42,409 --> 00:34:46,280
count for a few minutes and
get some very crude estimate

704
00:34:46,280 --> 00:34:48,085
of the counting
rate and then decide

705
00:34:48,085 --> 00:34:50,210
how long you have to let
your background accumulate

706
00:34:50,210 --> 00:34:53,000
so you can distinguish the
activity in the smoke shop

707
00:34:53,000 --> 00:34:56,801
to within some confidence
and some error.

708
00:34:56,801 --> 00:34:57,564
Yes.

709
00:34:57,564 --> 00:35:00,147
AUDIENCE: So does the background
in the case of the smoke shop

710
00:35:00,147 --> 00:35:03,015
just the area right
outside of it?

711
00:35:03,015 --> 00:35:05,410
Instead of the inside?

712
00:35:05,410 --> 00:35:07,540
MICHAEL SHORT: It's
definitely location dependent.

713
00:35:07,540 --> 00:35:10,452
So we will get into
background counts and sources

714
00:35:10,452 --> 00:35:12,160
of background radiation
in about a month.

715
00:35:12,160 --> 00:35:13,810
But to give you a
quick flash-forward,

716
00:35:13,810 --> 00:35:16,675
it depends on your elevation to
say how much of the atmosphere

717
00:35:16,675 --> 00:35:18,640
is protecting you
from cosmic rays.

718
00:35:18,640 --> 00:35:20,230
It definitely
depends on location.

719
00:35:20,230 --> 00:35:22,522
So in New Hampshire, the
background count's quite a bit

720
00:35:22,522 --> 00:35:24,880
higher, because there's a
lot of granite deposits,

721
00:35:24,880 --> 00:35:30,460
and granite can be upwards of
52 parts per million radium.

722
00:35:30,460 --> 00:35:32,140
Conway granite in
particular, named

723
00:35:32,140 --> 00:35:35,390
after Conway, New Hampshire,
is pretty rich in radium ore.

724
00:35:35,390 --> 00:35:36,640
Oh, is that where you're from?

725
00:35:36,640 --> 00:35:37,182
AUDIENCE: No.

726
00:35:37,182 --> 00:35:38,230
My last name is Conway.

727
00:35:38,230 --> 00:35:39,290
MICHAEL SHORT: Oh, there you go.

728
00:35:39,290 --> 00:35:39,800
OK.

729
00:35:39,800 --> 00:35:40,300
[LAUGHTER]

730
00:35:40,300 --> 00:35:41,385
Yeah.

731
00:35:41,385 --> 00:35:42,010
It's also neat.

732
00:35:42,010 --> 00:35:45,850
You can use background counts
as a radiation altimeter.

733
00:35:45,850 --> 00:35:48,700
One of my graduate students
actually built a Geiger counter

734
00:35:48,700 --> 00:35:50,890
interface to an Arduino,
where you could actually

735
00:35:50,890 --> 00:35:52,900
tell what the height
you were flying at

736
00:35:52,900 --> 00:35:56,140
is by the amount of
background radiation increase.

737
00:35:56,140 --> 00:35:59,178
So certainly it's going to
depend where you are, right?

738
00:35:59,178 --> 00:36:01,220
But you want to make sure
that you're in an area,

739
00:36:01,220 --> 00:36:03,400
to answer Sean's question,
representative of where

740
00:36:03,400 --> 00:36:04,780
the smoke shop is.

741
00:36:04,780 --> 00:36:06,322
So you can't go
into the reactor,

742
00:36:06,322 --> 00:36:07,780
and drop this in
the core, and say,

743
00:36:07,780 --> 00:36:09,130
I'm doing a background count.

744
00:36:09,130 --> 00:36:11,560
That's not a valid experiment.

745
00:36:11,560 --> 00:36:14,980
So yeah, you'd want to be,
I don't know, same block.

746
00:36:14,980 --> 00:36:16,930
That would be a pretty good.

747
00:36:16,930 --> 00:36:18,970
And then go in there
and see, can you

748
00:36:18,970 --> 00:36:21,100
measure any sort
of increase, get

749
00:36:21,100 --> 00:36:24,100
a crude estimate of your C g--

750
00:36:24,100 --> 00:36:26,380
your gross count rate.

751
00:36:26,380 --> 00:36:29,380
Use this formula right here to
estimate how much time you'd

752
00:36:29,380 --> 00:36:30,760
have to wait.

753
00:36:30,760 --> 00:36:35,140
So for example, let's shrink
our y-axis down a little

754
00:36:35,140 --> 00:36:38,515
and be more optimistic
than we probably should.

755
00:36:38,515 --> 00:36:40,390
Let's say you go in
there and you get a count

756
00:36:40,390 --> 00:36:41,830
rate of 100 counts per minute.

757
00:36:41,830 --> 00:36:44,610
That would do that
would surprise me.

758
00:36:44,610 --> 00:36:47,880
You'd only have to count
for an extra 28 minutes

759
00:36:47,880 --> 00:36:55,210
to nail that net count rate
with 95% confidence to 5% error.

760
00:36:55,210 --> 00:36:56,960
Let's say now, what
happens if we increase

761
00:36:56,960 --> 00:36:58,830
the allowable percent error?

762
00:36:58,830 --> 00:37:03,110
So let's say 10% error
would be acceptable.

763
00:37:03,110 --> 00:37:07,748
We just take that
number and double it.

764
00:37:07,748 --> 00:37:09,290
Then, all of a
sudden, you don't have

765
00:37:09,290 --> 00:37:12,930
to count for nearly as long.

766
00:37:12,930 --> 00:37:17,270
So again at 5% error,
which means a 0.25 here,

767
00:37:17,270 --> 00:37:18,950
at 100 counts per
minute, you'd have

768
00:37:18,950 --> 00:37:22,460
to count for about 30 minutes.

769
00:37:22,460 --> 00:37:26,880
If you're willing
to accept 10% error,

770
00:37:26,880 --> 00:37:32,113
it goes down to seven
minutes and 18 seconds.

771
00:37:32,113 --> 00:37:33,780
So do you guys see
the general interplay

772
00:37:33,780 --> 00:37:37,290
between confidence, percent
error, counting time,

773
00:37:37,290 --> 00:37:39,990
and counting rate?

774
00:37:39,990 --> 00:37:43,070
Who here is built an NSE
Geiger counter before?

775
00:37:43,070 --> 00:37:43,570
Awesome.

776
00:37:43,570 --> 00:37:46,000
So this is definitely a
try-it-at-home kids kind

777
00:37:46,000 --> 00:37:46,990
of thing.

778
00:37:46,990 --> 00:37:49,480
If you want to find out
is something radioactive,

779
00:37:49,480 --> 00:37:52,150
this is what you can actually
use to answer the question,

780
00:37:52,150 --> 00:37:56,590
is it discernibly radioactive
to within some limit of error

781
00:37:56,590 --> 00:37:59,462
or limit of confidence?

782
00:37:59,462 --> 00:38:01,920
That's what we're going to be
doing here with a much, much,

783
00:38:01,920 --> 00:38:04,250
much more sensitive detector.

784
00:38:04,250 --> 00:38:08,750
So the only thing missing
from our complete picture

785
00:38:08,750 --> 00:38:11,480
of going from the activity of
a source, which we've shown you

786
00:38:11,480 --> 00:38:14,510
how to count, to dealing with
the solid angle, which is just

787
00:38:14,510 --> 00:38:17,660
a simple formula, to
dealing with statistics

788
00:38:17,660 --> 00:38:21,380
and uncertainty, is now the
efficiency of this detector.

789
00:38:21,380 --> 00:38:23,960
Out of the number of
radiation quanta or whatever

790
00:38:23,960 --> 00:38:27,290
that enter the detector,
how many interact,

791
00:38:27,290 --> 00:38:29,877
and how many leave
out the other side?

792
00:38:29,877 --> 00:38:32,210
That's we're going to be
spending most of the next month

793
00:38:32,210 --> 00:38:36,470
on when we do ion, photon,
electron, and neutron

794
00:38:36,470 --> 00:38:38,510
interactions with matter.

795
00:38:38,510 --> 00:38:41,060
So we'll find out-- what's the
probability per unit length

796
00:38:41,060 --> 00:38:43,730
that each one undergoes
an interaction, what kind

797
00:38:43,730 --> 00:38:47,540
of interactions do they
undergo, and then we'll

798
00:38:47,540 --> 00:38:48,830
complete this actual picture.

799
00:38:48,830 --> 00:38:52,550
So you can take a source of,
let's say, unknown activity,

800
00:38:52,550 --> 00:38:55,880
put it a known distance
away from a known detector

801
00:38:55,880 --> 00:38:58,220
with a known
efficiency, and back out

802
00:38:58,220 --> 00:39:01,670
what the activity of that
source is with accuracy.

803
00:39:01,670 --> 00:39:04,160
That's what you're going to
start doing on this homework

804
00:39:04,160 --> 00:39:06,630
as well for the banana lab.

805
00:39:06,630 --> 00:39:08,750
The only thing you don't
know is the activity

806
00:39:08,750 --> 00:39:10,260
of this bag of bananas.

807
00:39:10,260 --> 00:39:12,260
But we're going to give
you all the information,

808
00:39:12,260 --> 00:39:14,180
like the efficiency
of the detector

809
00:39:14,180 --> 00:39:15,747
and the geometry
of the detector,

810
00:39:15,747 --> 00:39:17,330
and you're going to
be able to measure

811
00:39:17,330 --> 00:39:20,150
the number of
potassium 40 counts

812
00:39:20,150 --> 00:39:22,320
that the detector picks up.

813
00:39:22,320 --> 00:39:25,870
So by taking-- let's see
where we have some space left.

814
00:39:28,410 --> 00:39:30,170
We had a little bit here.

815
00:39:30,170 --> 00:39:35,030
So by taking that
number of counts

816
00:39:35,030 --> 00:39:37,010
and dividing by, let's
say, the efficiency

817
00:39:37,010 --> 00:39:39,230
of the detector, where
that efficiency is

818
00:39:39,230 --> 00:39:45,170
going to range from 0 to 1,
probably much closer to 0,

819
00:39:45,170 --> 00:39:50,720
and also dividing by, let's
say, your solid angle over 4

820
00:39:50,720 --> 00:39:53,960
pi to account for how many
of the emitted potassium

821
00:39:53,960 --> 00:39:57,740
40 gamma rays actually
get into the detector

822
00:39:57,740 --> 00:40:05,658
and dividing by 2 gamma
rays per disintegration--

823
00:40:05,658 --> 00:40:07,290
I think that's what
we had last time.

824
00:40:07,290 --> 00:40:09,620
Or was that cobalt 60?

825
00:40:09,620 --> 00:40:10,760
Yeah.

826
00:40:10,760 --> 00:40:12,900
We've been using cobalt
60 as an example.

827
00:40:12,900 --> 00:40:16,200
So remember, we had two gamma
rays emitted per cobalt 60

828
00:40:16,200 --> 00:40:18,200
disintegration on average.

829
00:40:18,200 --> 00:40:23,830
Then you can get to the
actual activity of the source.

830
00:40:23,830 --> 00:40:27,100
Once you know the activity
of this bag of bananas,

831
00:40:27,100 --> 00:40:29,470
you can then divide by either
the mass of one banana,

832
00:40:29,470 --> 00:40:31,140
or the number of
bananas, or whatever

833
00:40:31,140 --> 00:40:32,140
to get the final answer.

834
00:40:35,185 --> 00:40:37,560
That's what we're going to
spend the rest of today doing.

835
00:40:37,560 --> 00:40:40,333
So since it's getting
on five out of five of,

836
00:40:40,333 --> 00:40:42,750
do you guys have any questions
about what we covered today

837
00:40:42,750 --> 00:40:43,958
or what we're about to go do?

838
00:40:51,710 --> 00:40:53,360
AUDIENCE: You said
that for solid angle

839
00:40:53,360 --> 00:40:54,840
you wouldn't do this.

840
00:40:54,840 --> 00:40:55,650
MICHAEL SHORT: Yep.

841
00:40:55,650 --> 00:40:57,720
AUDIENCE: So for
solid angle, it's

842
00:40:57,720 --> 00:41:00,140
[INAUDIBLE] to the surface
area over y squared.

843
00:41:00,140 --> 00:41:03,510
And in this situation,
does solid angle over 4 pi

844
00:41:03,510 --> 00:41:08,968
mean that you can only have a
maximum of half of the sphere?

845
00:41:08,968 --> 00:41:10,260
MICHAEL SHORT: Not necessarily.

846
00:41:10,260 --> 00:41:12,010
Let's say you were to
encase your detector

847
00:41:12,010 --> 00:41:14,910
in an infinite medium
of radiation material.

848
00:41:14,910 --> 00:41:16,900
Then you could subtend 4 pi.

849
00:41:16,900 --> 00:41:19,800
So the idea here is that if
you captured every single gamma

850
00:41:19,800 --> 00:41:22,660
ray, your solid
angle would be 4 pi.

851
00:41:22,660 --> 00:41:27,030
So if your solid angle
is 4 pi, then that

852
00:41:27,030 --> 00:41:32,490
would equal-ish the area
over r squared of your thing.

853
00:41:32,490 --> 00:41:35,520
But this is actually not
that good of an approximation

854
00:41:35,520 --> 00:41:39,270
when you put a source very,
very up close to a detector.

855
00:41:39,270 --> 00:41:41,760
So there are actual formulas
for solid angle, where

856
00:41:41,760 --> 00:41:44,850
the real formula
for a solid angle,

857
00:41:44,850 --> 00:41:47,550
you actually end up
having to do a surface

858
00:41:47,550 --> 00:41:53,460
integral of the sine,
which accounts for the fact

859
00:41:53,460 --> 00:41:55,770
that the object that you
have might be, let's say,

860
00:41:55,770 --> 00:41:58,310
tilted towards or away
from the detector,

861
00:41:58,310 --> 00:42:05,843
times some differential d phi
d theta of this unit sphere.

862
00:42:05,843 --> 00:42:07,260
So you'll have to
integrate to say

863
00:42:07,260 --> 00:42:13,740
how many of these little d
phi d thetas are actually

864
00:42:13,740 --> 00:42:15,840
subtended by your detector.

865
00:42:15,840 --> 00:42:17,940
And the value of that
actual surface integral

866
00:42:17,940 --> 00:42:20,430
gives you the real solid angle.

867
00:42:20,430 --> 00:42:22,405
That's the super
simple one if you just

868
00:42:22,405 --> 00:42:24,030
know the area of
something and you know

869
00:42:24,030 --> 00:42:26,580
that you're kind of far away.

870
00:42:26,580 --> 00:42:29,820
But again, whenever possible,
use the exact formula.

871
00:42:32,380 --> 00:42:34,010
So any other questions?

872
00:42:34,010 --> 00:42:34,795
Yeah, Sean.

873
00:42:34,795 --> 00:42:36,420
AUDIENCE: You said
that that expression

874
00:42:36,420 --> 00:42:41,870
is a true statement
[INAUDIBLE] per second, right?

875
00:42:41,870 --> 00:42:43,970
MICHAEL SHORT: The two
gammas per cobalt 60?

876
00:42:43,970 --> 00:42:44,470
This one?

877
00:42:44,470 --> 00:42:45,220
AUDIENCE: Yeah.

878
00:42:45,220 --> 00:42:46,928
MICHAEL SHORT: That
accounts for the fact

879
00:42:46,928 --> 00:42:51,287
that if you remember the
decay diagram for cobalt 60,

880
00:42:51,287 --> 00:42:52,120
how does that decay?

881
00:42:52,120 --> 00:42:55,570
By beta emission.

882
00:42:55,570 --> 00:42:57,850
It goes to one
energy level, and it

883
00:42:57,850 --> 00:43:04,990
tends to go down by two
gamma decays to nickel 60.

884
00:43:04,990 --> 00:43:06,610
So each time it
gives off a gamma ray

885
00:43:06,610 --> 00:43:09,230
to one level and a gamma
ray to another level.

886
00:43:09,230 --> 00:43:12,010
So in this case, one
becquerel of cobalt 60

887
00:43:12,010 --> 00:43:15,860
would give off two
gamma rays per second.

888
00:43:15,860 --> 00:43:19,000
So if you're measuring a number
of counts, and each count,

889
00:43:19,000 --> 00:43:21,340
one gamma ray was
responsible, you

890
00:43:21,340 --> 00:43:23,260
have to then divide
by the number of gamma

891
00:43:23,260 --> 00:43:25,900
rays per disintegration
on average in order

892
00:43:25,900 --> 00:43:28,856
to get the actual
activity of that source.

893
00:43:28,856 --> 00:43:32,050
Because remember, activity is
measured in disintegrations,

894
00:43:32,050 --> 00:43:36,070
not in number of
gamma rays emitted.

895
00:43:36,070 --> 00:43:37,940
That's the difference here.

896
00:43:37,940 --> 00:43:40,880
Dose-- you'd actually care about
how many gamma rays you absorb.

897
00:43:40,880 --> 00:43:43,690
But activity is how many atoms
are disintegrating per second.

898
00:43:46,860 --> 00:43:47,360
Yeah.

899
00:43:47,360 --> 00:43:51,700
AUDIENCE: What units of
cobalt 60 [INAUDIBLE]??

900
00:43:51,700 --> 00:43:53,385
MICHAEL SHORT: The
units of cobalt 60?

901
00:43:53,385 --> 00:43:54,890
AUDIENCE: It's just two gamma--

902
00:43:54,890 --> 00:43:58,040
MICHAEL SHORT: Oh, this would
be, like, atoms of cobalt 60.

903
00:43:58,040 --> 00:44:00,350
And those gamma rays
would be gammas per atom.

904
00:44:06,930 --> 00:44:11,690
So in this case, it's like two
gamma rays per atom of cobalt

905
00:44:11,690 --> 00:44:16,760
60 disintegrating, or better
yet, per disintegration.

906
00:44:20,000 --> 00:44:21,750
So you've got to know
what material you're

907
00:44:21,750 --> 00:44:24,120
looking at in order
to know how many gamma

908
00:44:24,120 --> 00:44:26,870
or how many betas or
more that you're going

909
00:44:26,870 --> 00:44:28,486
to get per disintegration.

910
00:44:33,593 --> 00:44:35,760
Who here has heard of this
uncertainty in quadrature

911
00:44:35,760 --> 00:44:38,006
before?

912
00:44:38,006 --> 00:44:39,190
There's a couple folks.

913
00:44:39,190 --> 00:44:40,240
OK.

914
00:44:40,240 --> 00:44:40,740
Yeah.

915
00:44:40,740 --> 00:44:43,673
The idea here is that, again,
if you just add the errors up,

916
00:44:43,673 --> 00:44:45,340
you're probably
overestimating the error

917
00:44:45,340 --> 00:44:46,465
and selling yourself short.

918
00:44:48,810 --> 00:44:49,860
Cool.

919
00:44:49,860 --> 00:44:53,700
In that case, if there's no
questions, let's go do this.

920
00:44:53,700 --> 00:44:55,380
So follow me to
the counting lab.

921
00:45:01,902 --> 00:45:02,610
MICHAEL AMES: OK.

922
00:45:02,610 --> 00:45:05,110
So this is my counting lab.

923
00:45:05,110 --> 00:45:08,100
These are three high-purity
germanium detectors.

924
00:45:08,100 --> 00:45:10,225
Have you explained high-purity
germanium detectors?

925
00:45:10,225 --> 00:45:11,010
MICHAEL SHORT: No, we haven't.

926
00:45:11,010 --> 00:45:11,890
MICHAEL AMES: OK.

927
00:45:11,890 --> 00:45:13,900
Have you explained
any detectors?

928
00:45:13,900 --> 00:45:14,750
MICHAEL SHORT: Just
the Geiger counter

929
00:45:14,750 --> 00:45:16,190
we were playing
around with today.

930
00:45:16,190 --> 00:45:16,898
MICHAEL AMES: OK.

931
00:45:16,898 --> 00:45:18,490
Well, here.

932
00:45:18,490 --> 00:45:25,460
Down in here there's a little
high-purity germanium crystal

933
00:45:25,460 --> 00:45:29,000
with a couple thousand
volts across it.

934
00:45:29,000 --> 00:45:31,860
When a gamma ray
goes into it, it

935
00:45:31,860 --> 00:45:34,370
makes some electron hole pairs.

936
00:45:34,370 --> 00:45:36,230
Nod when I say
electron hole pairs.

937
00:45:36,230 --> 00:45:38,460
OK, good.

938
00:45:38,460 --> 00:45:42,050
And basically, you get more
electron hole pairs the more

939
00:45:42,050 --> 00:45:44,270
energy of the gamma you have.

940
00:45:44,270 --> 00:45:47,540
So you collect the
current from that,

941
00:45:47,540 --> 00:45:51,470
and you get a little
pulse of current,

942
00:45:51,470 --> 00:45:53,090
and the height of
the pulse tells you

943
00:45:53,090 --> 00:45:54,800
how many hole pairs
you had, and then

944
00:45:54,800 --> 00:45:58,850
back it up to what the
energy or your gamma was.

945
00:45:58,850 --> 00:46:01,460
That works fine if you collect
all of the gamma energy.

946
00:46:01,460 --> 00:46:03,740
You don't always quite do that.

947
00:46:03,740 --> 00:46:05,920
Anyway, so that's how--

948
00:46:05,920 --> 00:46:08,060
You all can scooch up.

949
00:46:08,060 --> 00:46:09,830
There's not a whole
lot to see in there.

950
00:46:09,830 --> 00:46:10,926
MICHAEL SHORT:
It's worth a look.

951
00:46:10,926 --> 00:46:11,926
If you've never seen it.

952
00:46:11,926 --> 00:46:13,280
MICHAEL AMES: It's worth a look.

953
00:46:13,280 --> 00:46:14,810
You can't really
see the crystal.

954
00:46:14,810 --> 00:46:18,710
There's just an aluminum
cylinder in there.

955
00:46:18,710 --> 00:46:21,760
The black part is just
a carbon fiber window,

956
00:46:21,760 --> 00:46:26,660
because you don't want to
cut off the low energy gamma.

957
00:46:26,660 --> 00:46:29,448
So it's got a really thin
carbon fiber window on it.

958
00:46:29,448 --> 00:46:31,490
MICHAEL SHORT: What's with
the hundreds of pounds

959
00:46:31,490 --> 00:46:32,960
of copper around the side?

960
00:46:32,960 --> 00:46:34,040
MICHAEL AMES: What's with
the hundreds of pounds

961
00:46:34,040 --> 00:46:34,970
of copper on the side?

962
00:46:34,970 --> 00:46:37,310
There's not hundreds of
pounds of copper on the side.

963
00:46:37,310 --> 00:46:38,630
These guys are lead.

964
00:46:38,630 --> 00:46:40,070
MICHAEL SHORT: Ah-hah!

965
00:46:40,070 --> 00:46:42,000
MICHAEL AMES: Which
does two things--

966
00:46:42,000 --> 00:46:48,920
it shields the detectors from
the activity out here, from you

967
00:46:48,920 --> 00:46:51,890
guys, from the activities
coming out of here--

968
00:46:51,890 --> 00:46:54,530
because sometimes I'm counting
very low activity samples--

969
00:46:54,530 --> 00:46:56,240
and it also, if I'm
counting something

970
00:46:56,240 --> 00:47:00,260
that has a lot of activity, it
shields us from that activity.

971
00:47:00,260 --> 00:47:03,230
So it kind of goes both ways.

972
00:47:03,230 --> 00:47:08,540
The reason there's copper is
if you get a high energy gamma

973
00:47:08,540 --> 00:47:12,710
ray into some lead,
it makes x-rays.

974
00:47:12,710 --> 00:47:15,215
And it makes a
very nice 75 keV--

975
00:47:15,215 --> 00:47:16,830
do you guys know keV?

976
00:47:16,830 --> 00:47:17,330
Good.

977
00:47:17,330 --> 00:47:18,410
MICHAEL SHORT:
We've done x-rays.

978
00:47:18,410 --> 00:47:19,327
MICHAEL AMES: Awesome.

979
00:47:19,327 --> 00:47:22,400
So it's a really,
really nice 75 keV

980
00:47:22,400 --> 00:47:25,820
x-ray that interferes with
trying to count things

981
00:47:25,820 --> 00:47:28,710
around 75 keV, because you're
getting all these x-rays coming

982
00:47:28,710 --> 00:47:29,210
out of lead.

983
00:47:29,210 --> 00:47:31,160
So you line it
with copper, which

984
00:47:31,160 --> 00:47:36,740
makes a lower energy x-ray and
filters out the lead x-rays.

985
00:47:36,740 --> 00:47:41,810
So anyway, so this is I've
got two germanium detectors.

986
00:47:41,810 --> 00:47:44,840
That ones also germanium,
but it's a well detector.

987
00:47:44,840 --> 00:47:47,840
So it's got a little
one-centimeter hole

988
00:47:47,840 --> 00:47:52,850
in so you can stick a sample
right in the germanium.

989
00:47:52,850 --> 00:47:56,540
They're hooked up through
a little electronic box

990
00:47:56,540 --> 00:47:58,880
and go into the computer
over there that does

991
00:47:58,880 --> 00:48:00,870
all the peak height analysis.

992
00:48:00,870 --> 00:48:02,570
Oh, yeah, liquid
nitrogen [INAUDIBLE]..

993
00:48:02,570 --> 00:48:05,000
Thanks for pointing.

994
00:48:05,000 --> 00:48:08,930
Yeah, you cool the electronics
and everything down

995
00:48:08,930 --> 00:48:11,252
so it cuts out
the thermal noise.

996
00:48:11,252 --> 00:48:13,460
Because you're looking for
really tiny little signals

997
00:48:13,460 --> 00:48:16,050
here, so you cool
everything down.

998
00:48:16,050 --> 00:48:19,010
And that way, it's
not too noisy.

999
00:48:19,010 --> 00:48:21,830
These guys are OK warming up.

1000
00:48:21,830 --> 00:48:23,660
It doesn't destroy the detector.

1001
00:48:23,660 --> 00:48:26,750
The old detectors you had
to keep cold all the time.

1002
00:48:26,750 --> 00:48:31,940
And if they warmed up, then
they were just paperweights.

1003
00:48:31,940 --> 00:48:33,560
So this is just
the counting lab.

1004
00:48:33,560 --> 00:48:36,620
I've got an actual sample
counting in here right now.

1005
00:48:36,620 --> 00:48:38,780
We'll take a look at the
spectrum in a minute.

1006
00:48:38,780 --> 00:48:40,670
Your bananas are
going to go here.

1007
00:48:45,160 --> 00:48:49,860
And let's see if we
can smash it down.

1008
00:48:49,860 --> 00:48:50,360
Yeah.

1009
00:48:50,360 --> 00:48:52,400
Because it would be nice
if I can close the lid.

1010
00:48:55,860 --> 00:48:56,360
Oops.

1011
00:48:56,360 --> 00:48:57,527
MICHAEL SHORT: Well, almost.

1012
00:48:57,527 --> 00:48:58,402
MICHAEL AMES: Almost.

1013
00:48:58,402 --> 00:48:59,480
Well, smash this down.

1014
00:48:59,480 --> 00:49:01,360
Here, one you guys do this.

1015
00:49:01,360 --> 00:49:02,580
Here, you.

1016
00:49:02,580 --> 00:49:04,970
Smash that down until
it fits in there.

1017
00:49:04,970 --> 00:49:06,715
Although, don't break the bag.

1018
00:49:06,715 --> 00:49:07,670
Oh!

1019
00:49:07,670 --> 00:49:08,762
OK, we'll get another bag.

1020
00:49:08,762 --> 00:49:09,970
AUDIENCE: Oh, did I break it?

1021
00:49:09,970 --> 00:49:10,887
MICHAEL AMES: It's OK.

1022
00:49:10,887 --> 00:49:13,050
It's just banana ash.

1023
00:49:13,050 --> 00:49:14,780
We'll find another bag.

1024
00:49:14,780 --> 00:49:16,230
It's OK.

1025
00:49:16,230 --> 00:49:19,500
You know, I'm all
about making mistakes.

1026
00:49:19,500 --> 00:49:21,080
AUDIENCE: [INAUDIBLE]

1027
00:49:21,080 --> 00:49:24,780
MICHAEL AMES: Yeah, yeah, yeah,
just be a little more gentle.

1028
00:49:24,780 --> 00:49:29,250
We'll throw some duct tape
on it, and it'll be fine.

1029
00:49:29,250 --> 00:49:33,990
So you're looking for potassium
40 in your bananas, correct?

1030
00:49:33,990 --> 00:49:37,260
Where else do you think
we got potassium 40?

1031
00:49:37,260 --> 00:49:40,667
Or do you think there's any
other potassium 40 in the room?

1032
00:49:40,667 --> 00:49:41,507
AUDIENCE: In us.

1033
00:49:41,507 --> 00:49:42,590
MICHAEL AMES: Yeah, right.

1034
00:49:42,590 --> 00:49:47,640
So when you do the banana
count, we frequently

1035
00:49:47,640 --> 00:49:51,900
take a spectrum on this
with the lid closed,

1036
00:49:51,900 --> 00:49:53,520
and we always see potassium 40.

1037
00:49:53,520 --> 00:49:55,440
There's potassium 40 everywhere.

1038
00:49:55,440 --> 00:49:58,500
So after we get the
count of the bananas,

1039
00:49:58,500 --> 00:50:00,150
we'll take a background count.

1040
00:50:00,150 --> 00:50:02,040
You'll want to subtract
the two signals.

1041
00:50:02,040 --> 00:50:04,150
MICHAEL SHORT: We just
did 15 minutes ago.

1042
00:50:04,150 --> 00:50:05,960
MICHAEL AMES: You're
so ahead of me.

1043
00:50:05,960 --> 00:50:08,348
OK, I think that's all--

1044
00:50:08,348 --> 00:50:09,390
Is this going to fit now?

1045
00:50:09,390 --> 00:50:10,384
AUDIENCE: [INAUDIBLE]

1046
00:50:10,384 --> 00:50:11,092
MICHAEL AMES: OK.

1047
00:50:14,360 --> 00:50:17,000
Close enough.

1048
00:50:17,000 --> 00:50:18,350
I've got this thing--

1049
00:50:18,350 --> 00:50:20,030
I've got a whole bunch
of little spacers

1050
00:50:20,030 --> 00:50:22,760
if I'm counting
something that's hot.

1051
00:50:22,760 --> 00:50:24,610
And by hot, I mean
radioactive hot.

1052
00:50:28,200 --> 00:50:30,290
I'll space it out
a little further.

1053
00:50:30,290 --> 00:50:31,995
AUDIENCE: Need a
little more smashing?

1054
00:50:31,995 --> 00:50:33,245
MICHAEL AMES: No, that's fine.

1055
00:50:33,245 --> 00:50:35,050
We just got to close the lid.

1056
00:50:35,050 --> 00:50:40,190
And if I've got something
that's very radioactive,

1057
00:50:40,190 --> 00:50:43,280
I'll just space it out
away from the detector.

1058
00:50:43,280 --> 00:50:45,770
If you've got something
that's really hot,

1059
00:50:45,770 --> 00:50:47,780
it just kind of swamps
out the electronics.

1060
00:50:47,780 --> 00:50:50,480
MICHAEL SHORT: We did just go
for a solid angle too, today.

1061
00:50:50,480 --> 00:50:51,605
MICHAEL AMES: There you go.

1062
00:50:53,990 --> 00:50:56,210
Is there anything else
I want to say in here?

1063
00:50:56,210 --> 00:50:58,570
No, let's move this way.

1064
00:50:58,570 --> 00:51:04,060
This is the spectrum
I'm collecting on MIT 1.

1065
00:51:04,060 --> 00:51:07,780
Right now, I don't know--
how long has that been going?

1066
00:51:07,780 --> 00:51:11,080
Half a day-- less than that.

1067
00:51:11,080 --> 00:51:14,620
Anyway, so this is
a sample of quartz

1068
00:51:14,620 --> 00:51:17,770
that was irradiated
next to the reactor.

1069
00:51:17,770 --> 00:51:20,755
You guys are going to do
shorts in like a month--

1070
00:51:20,755 --> 00:51:21,880
did you bring your samples?

1071
00:51:21,880 --> 00:51:23,190
MICHAEL SHORT:
We're getting them.

1072
00:51:23,190 --> 00:51:24,160
MICHAEL AMES: OK, good.

1073
00:51:24,160 --> 00:51:25,638
Anyway, this is a
sample of quartz

1074
00:51:25,638 --> 00:51:27,930
that was irradiated in the
same spot you guys are going

1075
00:51:27,930 --> 00:51:30,850
to do your irradiation,
sort of in the graphite

1076
00:51:30,850 --> 00:51:33,580
region of the reactor.

1077
00:51:33,580 --> 00:51:37,090
The reason we're running
it is the people who

1078
00:51:37,090 --> 00:51:41,470
are looking at this quartz
want to run it for 80 hours,

1079
00:51:41,470 --> 00:51:43,840
and we'd like to know if
there are any impurities in it

1080
00:51:43,840 --> 00:51:46,030
that'll cause grief--

1081
00:51:46,030 --> 00:51:48,800
meaning a lot of activity
when it comes out.

1082
00:51:48,800 --> 00:51:50,320
So we run it for a short period.

1083
00:51:50,320 --> 00:51:52,780
I think this ran six hours.

1084
00:51:52,780 --> 00:51:55,450
And it's just a
little tiny piece.

1085
00:51:55,450 --> 00:51:58,620
And so I can look at the gamma
spectrum coming out of this.

1086
00:51:58,620 --> 00:52:02,980
So you can see, there's a
whole mess of peaks in here.

1087
00:52:02,980 --> 00:52:05,845
This one-- you see that?

1088
00:52:05,845 --> 00:52:07,720
You see that lovely,
little peak right there?

1089
00:52:07,720 --> 00:52:08,320
Can you all see that?

1090
00:52:08,320 --> 00:52:08,820
Nod.

1091
00:52:08,820 --> 00:52:09,670
Yeah, OK.

1092
00:52:09,670 --> 00:52:10,900
So that's the full spectrum.

1093
00:52:10,900 --> 00:52:11,567
That's the peak.

1094
00:52:11,567 --> 00:52:14,950
That's a tungsten 187 peak.

1095
00:52:14,950 --> 00:52:19,627
So I did put up one little
thing right behind you.

1096
00:52:19,627 --> 00:52:21,460
Have you all seen the
chart of the nuclides?

1097
00:52:21,460 --> 00:52:22,098
This thing?

1098
00:52:22,098 --> 00:52:23,140
MICHAEL SHORT: Every day.

1099
00:52:23,140 --> 00:52:23,890
MICHAEL AMES: Every day!

1100
00:52:23,890 --> 00:52:24,390
Good.

1101
00:52:24,390 --> 00:52:27,040
I've got one of
these on every wall

1102
00:52:27,040 --> 00:52:30,370
in every lab in office and
a little handbook Yeah.

1103
00:52:30,370 --> 00:52:36,428
So the tungsten 186
activates into tungsten 187.

1104
00:52:36,428 --> 00:52:38,470
So if you've looked at
the chart of the nuclides,

1105
00:52:38,470 --> 00:52:41,410
you can tell that there's
all the sort of parameters

1106
00:52:41,410 --> 00:52:45,790
you would need to calculate
how much activation

1107
00:52:45,790 --> 00:52:50,660
you'd get based on neutron
flux, and time, and cross.

1108
00:52:50,660 --> 00:52:56,170
The 28.43, that's the
abundance of that isotope.

1109
00:52:56,170 --> 00:52:59,590
You can see the sigma
gamma 38, that's

1110
00:52:59,590 --> 00:53:04,010
the cross section
for thermal neutrons.

1111
00:53:04,010 --> 00:53:08,500
And so that's how likely
you'll get from 186 to 187.

1112
00:53:08,500 --> 00:53:10,090
187, that's the half-life--

1113
00:53:10,090 --> 00:53:12,130
23.9 hours.

1114
00:53:12,130 --> 00:53:13,800
So with all of that--

1115
00:53:13,800 --> 00:53:16,690
oh, and underneath
the 23.9, you've

1116
00:53:16,690 --> 00:53:19,090
got what the gammas are--

1117
00:53:19,090 --> 00:53:21,867
685, 479.

1118
00:53:21,867 --> 00:53:23,200
it's got a whole mess of gammas.

1119
00:53:23,200 --> 00:53:25,570
So that's a bunch of the
gammas in here for that.

1120
00:53:25,570 --> 00:53:31,390
So you could, knowing
how big that peak is,

1121
00:53:31,390 --> 00:53:34,510
what the efficiency of the
detector is for collecting that

1122
00:53:34,510 --> 00:53:39,190
peak in that geometry, the
half-life, the cross set--

1123
00:53:39,190 --> 00:53:41,500
that whole mess of parameters--

1124
00:53:41,500 --> 00:53:45,560
back-calculate how much
tungsten is in the sample.

1125
00:53:45,560 --> 00:53:48,070
So that's kind of
how NAA works, which

1126
00:53:48,070 --> 00:53:49,420
I assume you've explained.

1127
00:53:49,420 --> 00:53:50,080
MICHAEL SHORT: We have.

1128
00:53:50,080 --> 00:53:50,410
MICHAEL AMES: OK.

1129
00:53:50,410 --> 00:53:51,490
MICHAEL SHORT:
Actually, the whole idea

1130
00:53:51,490 --> 00:53:53,282
behind doing those
short NAA activations is

1131
00:53:53,282 --> 00:53:54,657
these guys are
going to calculate

1132
00:53:54,657 --> 00:53:55,690
what's in their samples.

1133
00:53:55,690 --> 00:53:56,815
MICHAEL AMES: There you go.

1134
00:53:56,815 --> 00:53:58,450
MICHAEL SHORT: Once
we get the date.

1135
00:53:58,450 --> 00:54:00,490
MICHAEL AMES: But
that's not how I do NAA.

1136
00:54:00,490 --> 00:54:01,840
MICHAEL SHORT: [INAUDIBLE] We're
doing a simplified version.

1137
00:54:01,840 --> 00:54:02,350
MICHAEL AMES: Right, right.

1138
00:54:02,350 --> 00:54:02,860
No, no, no.

1139
00:54:02,860 --> 00:54:04,527
So there's two things
that you could do.

1140
00:54:04,527 --> 00:54:06,400
One of the things
you could do is

1141
00:54:06,400 --> 00:54:08,470
you take all those
nuclear parameters

1142
00:54:08,470 --> 00:54:11,110
and you calculate it just
from the peak height.

1143
00:54:11,110 --> 00:54:15,130
The other way that
everybody who does NAA--

1144
00:54:15,130 --> 00:54:17,020
almost everybody who does NAA--

1145
00:54:17,020 --> 00:54:18,790
is you run a standard material.

1146
00:54:18,790 --> 00:54:21,460
Any of you guys chemists
at any point in your life?

1147
00:54:21,460 --> 00:54:23,800
You all took some
chemistry at some point?

1148
00:54:23,800 --> 00:54:24,370
OK.

1149
00:54:24,370 --> 00:54:28,180
So you've run a standard,
which means a material

1150
00:54:28,180 --> 00:54:31,660
that how much tungsten is in
it or how much a whole mess

1151
00:54:31,660 --> 00:54:33,220
of other things are.

1152
00:54:33,220 --> 00:54:35,150
So I run a bunch of
different standards.

1153
00:54:35,150 --> 00:54:38,320
So along with this
piece of quartz,

1154
00:54:38,320 --> 00:54:43,660
I ran a standard, irradiated
it at the same time.

1155
00:54:43,660 --> 00:54:46,570
I'll count the quartz and
then I'll count the standard.

1156
00:54:46,570 --> 00:54:48,670
And by comparing
the peak heights

1157
00:54:48,670 --> 00:54:50,830
and doing all the decay
corrections and the weight

1158
00:54:50,830 --> 00:54:53,350
corrections, then I
calculate how much

1159
00:54:53,350 --> 00:54:55,310
tungsten is in my sample.

1160
00:54:55,310 --> 00:54:59,560
So I don't actually use the
cross sections, or the flux,

1161
00:54:59,560 --> 00:55:04,630
or any of that other stuff-- all
of those parameters disappear.

1162
00:55:04,630 --> 00:55:08,530
Notably, the detector efficiency
disappears out of the equation,

1163
00:55:08,530 --> 00:55:11,500
because that's the
parameter that you usually

1164
00:55:11,500 --> 00:55:14,290
have the funniest idea about.

1165
00:55:14,290 --> 00:55:16,240
And so you reduce
the uncertainty

1166
00:55:16,240 --> 00:55:20,620
in your concentration by doing
this sort of comparative method

1167
00:55:20,620 --> 00:55:22,710
with a standard.

1168
00:55:22,710 --> 00:55:24,430
That all make sense?

1169
00:55:24,430 --> 00:55:24,930
OK.

1170
00:55:24,930 --> 00:55:28,030
So when we run shorts,
I guess, in a month,

1171
00:55:28,030 --> 00:55:30,760
we'll take whatever
your samples are.

1172
00:55:30,760 --> 00:55:32,710
I've had feedback
about, oh, God,

1173
00:55:32,710 --> 00:55:35,320
you don't want to run
that many samples.

1174
00:55:35,320 --> 00:55:37,320
But we'll figure out how
many samples we'll run.

1175
00:55:37,320 --> 00:55:38,778
MICHAEL SHORT: It's
one per person.

1176
00:55:38,778 --> 00:55:39,430
[INAUDIBLE]

1177
00:55:39,430 --> 00:55:40,770
MICHAEL AMES: That's
a lot of shorts.

1178
00:55:40,770 --> 00:55:41,620
MICHAEL SHORT: In pairs, right?

1179
00:55:41,620 --> 00:55:42,500
MICHAEL AMES: Yeah.

1180
00:55:42,500 --> 00:55:47,050
So I'll show you how
the shorts get run.

1181
00:55:47,050 --> 00:55:51,760
So when we run your shorts,
we'll run your samples

1182
00:55:51,760 --> 00:55:54,220
and we'll run
standards, and then

1183
00:55:54,220 --> 00:55:55,840
you can do the
comparative method.

1184
00:55:55,840 --> 00:55:58,418
Or, if you feel like it,
you can do the other method,

1185
00:55:58,418 --> 00:55:59,585
depending on what exercise--

1186
00:55:59,585 --> 00:56:00,590
MICHAEL SHORT: The other method.

1187
00:56:00,590 --> 00:56:02,540
MICHAEL AMES: You're going
to do the other method.

1188
00:56:02,540 --> 00:56:04,170
You don't want to do
the standard method?

1189
00:56:04,170 --> 00:56:05,378
MICHAEL SHORT: Oh, no, no no.

1190
00:56:05,378 --> 00:56:07,850
We're drilling comprehension,
not [INAUDIBLE]..

1191
00:56:07,850 --> 00:56:09,822
MICHAEL AMES: Not practical?

1192
00:56:09,822 --> 00:56:10,322
Oh.

1193
00:56:10,322 --> 00:56:12,863
MICHAEL SHORT: What happens
if the computer break down?

1194
00:56:12,863 --> 00:56:14,780
MICHAEL AMES: Well, if
the computer goes down,

1195
00:56:14,780 --> 00:56:16,320
you can't get any data anyway.

1196
00:56:16,320 --> 00:56:17,720
MICHAEL SHORT: Oh, [INAUDIBLE].

1197
00:56:17,720 --> 00:56:20,370
MICHAEL AMES: I can do the
comparative one on an envelope.

1198
00:56:20,370 --> 00:56:24,260
Anyway-- well, we'll run
standards or not, depending

1199
00:56:24,260 --> 00:56:27,560
on how you guys are feeling.

1200
00:56:27,560 --> 00:56:28,380
So that's that.

1201
00:56:28,380 --> 00:56:28,880
Oh, right.

1202
00:56:28,880 --> 00:56:31,640
Let's count your bananas.

1203
00:56:31,640 --> 00:56:33,140
So this is detector 2.

1204
00:56:33,140 --> 00:56:37,480
We did an energy
calibration earlier today.

1205
00:56:37,480 --> 00:56:41,180
So actually, I've got a couple
of little button sources.

1206
00:56:41,180 --> 00:56:42,740
Have you seen the
button sources?

1207
00:56:42,740 --> 00:56:43,310
Yeah.

1208
00:56:43,310 --> 00:56:47,780
So that's just a couple of
cobalt 60 lines and a cesium

1209
00:56:47,780 --> 00:56:50,270
137 line down in here.

1210
00:56:50,270 --> 00:56:51,770
And I know where
those energies are,

1211
00:56:51,770 --> 00:56:54,440
so that just gets used to
calibrate the detectors.

1212
00:56:54,440 --> 00:56:56,270
MICHAEL SHORT: We were playing
around one of those cobalt 60

1213
00:56:56,270 --> 00:56:57,260
buttons today in class.

1214
00:56:57,260 --> 00:56:57,830
MICHAEL AMES: There you go.

1215
00:56:57,830 --> 00:56:59,420
MICHAEL SHORT: We mentioned the
two gammas per disintegration,

1216
00:56:59,420 --> 00:57:00,590
and there they are.

1217
00:57:00,590 --> 00:57:01,798
MICHAEL AMES: There they are.

1218
00:57:01,798 --> 00:57:04,280
They're kind of small there
because my buttons are probably

1219
00:57:04,280 --> 00:57:05,120
30 years old.

1220
00:57:05,120 --> 00:57:06,868
MICHAEL SHORT: Oh, I
got some fresh ones.

1221
00:57:06,868 --> 00:57:07,660
MICHAEL AMES: Yeah.

1222
00:57:07,660 --> 00:57:10,550
So anyway, we cleared that out.

1223
00:57:10,550 --> 00:57:13,130
And we just hit Start.

1224
00:57:13,130 --> 00:57:17,240
And we're not going to
see anything a while.

1225
00:57:17,240 --> 00:57:18,060
Where are we?

1226
00:57:18,060 --> 00:57:19,490
Oh here.

1227
00:57:19,490 --> 00:57:27,080
14-- anyway, your banana
peak will end up out in here.

1228
00:57:27,080 --> 00:57:29,940
So it'll take a while.

1229
00:57:29,940 --> 00:57:33,990
We're going to let this
count until Tuesday.

1230
00:57:33,990 --> 00:57:35,380
Because, why not?

1231
00:57:35,380 --> 00:57:37,380
And I don't feel like
coming in over the weekend

1232
00:57:37,380 --> 00:57:40,880
and turning it off.

1233
00:57:40,880 --> 00:57:41,580
So yeah.

1234
00:57:41,580 --> 00:57:45,390
So this is just picking up
all the gammas coming out

1235
00:57:45,390 --> 00:57:47,623
of the bananas, and
everything else that

1236
00:57:47,623 --> 00:57:49,290
happens to get through
the [INAUDIBLE],,

1237
00:57:49,290 --> 00:57:53,780
and all the contamination
on the inside of that.

1238
00:57:53,780 --> 00:57:54,840
And we just let it count.

1239
00:57:54,840 --> 00:57:58,020
And then you guys can
calculate how much

1240
00:57:58,020 --> 00:58:02,160
potassium 40 is in your ashes.

1241
00:58:02,160 --> 00:58:05,652
You'll need to do the
background subtraction.

1242
00:58:05,652 --> 00:58:06,360
I will give you--

1243
00:58:06,360 --> 00:58:07,410
MICHAEL SHORT: Do you
have background spectra?

1244
00:58:07,410 --> 00:58:08,520
MICHAEL AMES: Yeah.

1245
00:58:08,520 --> 00:58:12,150
We collect background
spectra once a month or so.

1246
00:58:12,150 --> 00:58:14,040
So I'll give you a
background spectra.

1247
00:58:14,040 --> 00:58:18,330
I will provide the efficiency
for this geometry, which

1248
00:58:18,330 --> 00:58:23,730
is pretty poorly
defined, because I've

1249
00:58:23,730 --> 00:58:25,080
got a program that'll do that.

1250
00:58:25,080 --> 00:58:27,450
And I can't give
you the program,

1251
00:58:27,450 --> 00:58:30,840
and it's a pain in the
neck to run anyway.

1252
00:58:30,840 --> 00:58:34,020
If we've got a really
well-defined geometry that's

1253
00:58:34,020 --> 00:58:38,310
not a big bag, usually I try to
count sort of point sources--

1254
00:58:38,310 --> 00:58:41,190
so I've got an efficiency
standard that I

1255
00:58:41,190 --> 00:58:44,550
can use that I know what
the disintegrations in that

1256
00:58:44,550 --> 00:58:46,290
are at a lot of
energies, and I use

1257
00:58:46,290 --> 00:58:48,870
that to do an efficiency
calibration, usually.

1258
00:58:48,870 --> 00:58:53,070
But I don't have an efficiency
standard that's that big.

1259
00:58:53,070 --> 00:58:56,280
It's just a point source.

1260
00:58:56,280 --> 00:58:58,680
And I think that's
the practical NAA.

1261
00:58:58,680 --> 00:59:01,945
From this end, did
that all makes sense?

1262
00:59:01,945 --> 00:59:03,570
I want you guys to
nod, not him to nod.

1263
00:59:03,570 --> 00:59:03,990
Yeah.

1264
00:59:03,990 --> 00:59:04,610
MICHAEL SHORT: Do you guys
have any questions for Mike

1265
00:59:04,610 --> 00:59:05,795
on what you've just heard?

1266
00:59:05,795 --> 00:59:08,170
Well-timed, because we were
just talking about this stuff

1267
00:59:08,170 --> 00:59:08,890
all week.

1268
00:59:08,890 --> 00:59:09,890
MICHAEL AMES: Good deal.

1269
00:59:09,890 --> 00:59:15,600
For neutron activation, that's
kind of a real common part

1270
00:59:15,600 --> 00:59:16,220
of the chart.

1271
00:59:16,220 --> 00:59:19,350
So there's the manganese,
iron, cobalt, nickel.

1272
00:59:19,350 --> 00:59:21,630
One of the things--

1273
00:59:21,630 --> 00:59:25,260
what you'd like, usually
when you're doing NAA,

1274
00:59:25,260 --> 00:59:29,157
is you want a nice
thermal neutron spectrum.

1275
00:59:29,157 --> 00:59:30,990
You know what thermal
neutron spectra means?

1276
00:59:30,990 --> 00:59:34,170
Real slow neutrons.

1277
00:59:34,170 --> 00:59:38,100
And they'll just give you
sort of an n gamma reaction.

1278
00:59:38,100 --> 00:59:41,970
So on that chart,
iron 58 to iron 59,

1279
00:59:41,970 --> 00:59:46,860
that's a nice n gamma reaction.

1280
00:59:46,860 --> 00:59:50,490
And that's the one I
use to analyze for iron.

1281
00:59:50,490 --> 00:59:52,050
If you're near the
reactor, you're

1282
00:59:52,050 --> 00:59:55,830
also getting some
fast neutrons, which

1283
00:59:55,830 --> 00:59:58,810
can give you an n p reaction.

1284
00:59:58,810 --> 01:00:01,320
So if you're looking
on the chart there,

1285
01:00:01,320 --> 01:00:04,680
cobalt 59, if you
get an n p reaction,

1286
01:00:04,680 --> 01:00:07,690
will also make the iron 59.

1287
01:00:07,690 --> 01:00:10,895
And that's a pain in the neck,
because if you've got iron,

1288
01:00:10,895 --> 01:00:13,020
you've always got a little
cobalt floating around--

1289
01:00:13,020 --> 01:00:14,760
you maybe need to
do a correction.

1290
01:00:14,760 --> 01:00:17,580
So in practical terms,
when you're running NAA,

1291
01:00:17,580 --> 01:00:22,530
you really want to avoid having
all these fast reactions.

1292
01:00:22,530 --> 01:00:24,000
There's usually an
energy threshold

1293
01:00:24,000 --> 01:00:27,594
for the fast reactions,
like 1 meV or so.

1294
01:00:27,594 --> 01:00:29,802
MICHAEL SHORT: Sound familiar
from the cube equation?

1295
01:00:29,802 --> 01:00:30,760
MICHAEL AMES: Yeah, OK.

1296
01:00:30,760 --> 01:00:31,560
Right.

1297
01:00:31,560 --> 01:00:34,895
The place where we do the
irradiations is very thermal.

1298
01:00:34,895 --> 01:00:37,380
It's got a very
low, fast spectrum.

1299
01:00:37,380 --> 01:00:40,290
So I don't usually have
to worry about that.

1300
01:00:40,290 --> 01:00:41,790
There's a couple
of times I actually

1301
01:00:41,790 --> 01:00:44,370
use the fast n p reaction.

1302
01:00:44,370 --> 01:00:47,790
If I want to measure nickel,
you can see nickel 58,

1303
01:00:47,790 --> 01:00:51,600
an n p reaction
will get cobalt 58.

1304
01:00:51,600 --> 01:00:56,100
And since there's
not a good reaction n

1305
01:00:56,100 --> 01:01:01,350
gamma from cobalt 57, cobalt
57 isn't around usually.

1306
01:01:01,350 --> 01:01:05,310
So that's how I measure
nickel, using n p reaction.

1307
01:01:05,310 --> 01:01:08,040
And I need to put the
rabbits into where I've

1308
01:01:08,040 --> 01:01:09,870
got a fast flux in the reactor.

1309
01:01:09,870 --> 01:01:13,080
Which, well, they've got a
couple of spots for that.

1310
01:01:13,080 --> 01:01:14,760
I try not to have
to measure nickel,

1311
01:01:14,760 --> 01:01:16,010
because it's pain in the neck.

1312
01:01:16,010 --> 01:01:19,760
But sometimes people
want to know nickel.

1313
01:01:19,760 --> 01:01:21,750
And we talked a
little about what

1314
01:01:21,750 --> 01:01:24,428
we've run in here
for types of samples.

1315
01:01:24,428 --> 01:01:26,220
MICHAEL SHORT: Well,
why don't you tell us?

1316
01:01:26,220 --> 01:01:27,480
MICHAEL AMES: OK, OK.

1317
01:01:27,480 --> 01:01:34,350
So back 15, 20, 25
years ago, we did

1318
01:01:34,350 --> 01:01:38,820
a ton of environmental
samples in this lab.

1319
01:01:38,820 --> 01:01:42,910
We had a whole three grad
students, myself included,

1320
01:01:42,910 --> 01:01:49,440
who did atmospheric particulate
matter, rain water, snow,

1321
01:01:49,440 --> 01:01:51,570
we even did some fog
collection, which

1322
01:01:51,570 --> 01:01:57,850
is kind of fun, ice cores, which
are old particulate deposition.

1323
01:01:57,850 --> 01:02:00,360
And it was all for trace
elements in those kind

1324
01:02:00,360 --> 01:02:02,580
of environmental samples--

1325
01:02:02,580 --> 01:02:05,460
also lake sediments.

1326
01:02:05,460 --> 01:02:09,510
Other analytical methods
have gotten a lot better,

1327
01:02:09,510 --> 01:02:11,460
and so they've kind
of caught up to NAA,

1328
01:02:11,460 --> 01:02:13,800
and you don't need a
reactor to run those.

1329
01:02:13,800 --> 01:02:15,930
So the environmental
side of this

1330
01:02:15,930 --> 01:02:17,580
has kind of quieted down a lot.

1331
01:02:17,580 --> 01:02:20,890
But it's still useful
for a bunch of things.

1332
01:02:20,890 --> 01:02:24,240
And so I do some work here now.

1333
01:02:24,240 --> 01:02:27,370
I also work in the NCORE group.

1334
01:02:27,370 --> 01:02:30,850
So that's a lot of my time,
rather than just this lab.

1335
01:02:33,670 --> 01:02:38,590
Practical things-- let's go take
a look at a couple other labs.

1336
01:02:38,590 --> 01:02:39,580
You're not on wheels?

1337
01:02:39,580 --> 01:02:41,190
You don't have a steady cam?

1338
01:02:41,190 --> 01:02:42,630
MICHAEL SHORT: I got a question.

1339
01:02:42,630 --> 01:02:42,880
MICHAEL AMES: OK.

1340
01:02:42,880 --> 01:02:43,570
You've got a question.

1341
01:02:43,570 --> 01:02:43,900
MICHAEL SHORT: What's the
weirdest thing you've ever

1342
01:02:43,900 --> 01:02:45,700
been asked to count?

1343
01:02:45,700 --> 01:02:48,880
MICHAEL AMES: The weirdest
thing I've been asked to count?

1344
01:02:48,880 --> 01:02:51,040
That's already activated, or?

1345
01:02:51,040 --> 01:02:52,290
MICHAEL SHORT: At all.

1346
01:02:52,290 --> 01:02:54,160
MICHAEL AMES: OK.

1347
01:02:54,160 --> 01:02:59,140
I don't know-- brain tissue.

1348
01:02:59,140 --> 01:03:05,200
Fish samples that we actually
did the fresh fish samples.

1349
01:03:05,200 --> 01:03:08,680
And you want to kind
of homogenize those.

1350
01:03:08,680 --> 01:03:11,290
And we had this kind
of titanium blender--

1351
01:03:11,290 --> 01:03:12,880
you remember the Bass-O-Matic?

1352
01:03:12,880 --> 01:03:16,420
We had this titanium blender
that we dropped the fish in,

1353
01:03:16,420 --> 01:03:18,940
and you completely
homogenized the fish,

1354
01:03:18,940 --> 01:03:21,040
and then you took a
little sample of it,

1355
01:03:21,040 --> 01:03:25,135
and freeze dried it, and
then analyzed it for mercury.

1356
01:03:25,135 --> 01:03:26,987
MICHAEL SHORT: [INAUDIBLE]

1357
01:03:26,987 --> 01:03:28,070
MICHAEL AMES: Yeah, right.

1358
01:03:28,070 --> 01:03:31,810
Because, I mean guys saw, the
rabbits are only this big,

1359
01:03:31,810 --> 01:03:34,480
and the samples I want
are only that big.

1360
01:03:34,480 --> 01:03:38,210
And so to get a
representative fish,

1361
01:03:38,210 --> 01:03:41,320
you want to kind of
make a fish smoothie

1362
01:03:41,320 --> 01:03:44,500
and then take a
sample out of that.

1363
01:03:44,500 --> 01:03:47,530
We did have a guy who
came to me and was

1364
01:03:47,530 --> 01:03:51,610
promising we were going to
do this giant study using

1365
01:03:51,610 --> 01:03:56,710
fingernails and toenails
for nutritional analysis.

1366
01:03:56,710 --> 01:04:01,570
He was working with a group
that looks at zinc deficiencies,

1367
01:04:01,570 --> 01:04:04,510
and fingernails and
toenails will give you

1368
01:04:04,510 --> 01:04:06,250
a good record of
how much zinc you've

1369
01:04:06,250 --> 01:04:09,100
had over the last
week, or month,

1370
01:04:09,100 --> 01:04:12,400
or whatever-- depend
where you cut the nails.

1371
01:04:12,400 --> 01:04:15,310
And so I was going to
get a couple of hundred

1372
01:04:15,310 --> 01:04:19,420
African children's toenails.

1373
01:04:19,420 --> 01:04:20,350
That didn't happen.

1374
01:04:20,350 --> 01:04:23,410
But I did analyze
my own toenails.

1375
01:04:23,410 --> 01:04:25,340
Well, if you went
to somebody who

1376
01:04:25,340 --> 01:04:28,960
was a little suspicious of
you, asking for toenails

1377
01:04:28,960 --> 01:04:32,530
is a lot easier than
asking for a blood sample.

1378
01:04:32,530 --> 01:04:37,510
Because people would give up
toenails-- it's not a big deal.

1379
01:04:37,510 --> 01:04:39,700
Have you ever seen the
movie or read the book

1380
01:04:39,700 --> 01:04:43,190
Civil Action, about the
superfund site in Woburn.

1381
01:04:43,190 --> 01:04:46,540
It was a big old superfund
site, and Woburn had arsenic

1382
01:04:46,540 --> 01:04:49,990
and chromium contamination.

1383
01:04:49,990 --> 01:04:52,090
There used to be a lab--

1384
01:04:52,090 --> 01:04:54,820
I forget which building it was
in-- that did a ton of research

1385
01:04:54,820 --> 01:04:55,690
there.

1386
01:04:55,690 --> 01:04:58,960
One of the things we
did in this lab was we

1387
01:04:58,960 --> 01:05:05,020
collected baby hair samples
from people's scrapbooks.

1388
01:05:05,020 --> 01:05:10,660
So we had baby hair
going back 50-60 years--

1389
01:05:10,660 --> 01:05:14,340
dated, because everybody
knew how old their kid was--

1390
01:05:14,340 --> 01:05:18,580
and we analyzed the hair samples
for arsenic and chromium,

1391
01:05:18,580 --> 01:05:21,910
and then we plotted
out where they were,

1392
01:05:21,910 --> 01:05:24,290
when the sample was taken,
and how close they were

1393
01:05:24,290 --> 01:05:27,220
to some contaminated wells.

1394
01:05:27,220 --> 01:05:30,370
And because we did a
fairly short of radiation,

1395
01:05:30,370 --> 01:05:32,380
after a while the
activities died down

1396
01:05:32,380 --> 01:05:34,910
and we gave the samples back.

1397
01:05:34,910 --> 01:05:39,040
And we found that it didn't
correlate with the well

1398
01:05:39,040 --> 01:05:42,010
water or the time when
the contamination was

1399
01:05:42,010 --> 01:05:45,340
the worst, which made
people happy in retrospect,

1400
01:05:45,340 --> 01:05:48,100
that the contamination
from that area

1401
01:05:48,100 --> 01:05:50,830
didn't get into the well water.

1402
01:05:50,830 --> 01:05:54,040
That was in the mid-90s or so.

1403
01:05:54,040 --> 01:05:56,870
Anyway, that was
one of my samples.

1404
01:05:56,870 --> 01:05:58,870
And the hair is a pain
in the neck to work with.

1405
01:05:58,870 --> 01:06:01,510
So I hope none of you
give me hair samples.

1406
01:06:01,510 --> 01:06:03,150
I won't run them.

1407
01:06:03,150 --> 01:06:08,430
So let's go down
the hall, this way.

1408
01:06:08,430 --> 01:06:10,246
You all got to follow.

1409
01:06:10,246 --> 01:06:14,960
And so this is
just a fine powder.

1410
01:06:14,960 --> 01:06:18,470
And it's fly ash from a
coal-fired power plant.

1411
01:06:18,470 --> 01:06:23,210
Fly ash means the ash that goes
up the smokestack, as opposed

1412
01:06:23,210 --> 01:06:25,820
to bottom ash which
is what falls down.

1413
01:06:25,820 --> 01:06:31,730
And so, they collect a whole
hundreds of kilograms of fly

1414
01:06:31,730 --> 01:06:35,990
ash, just homogenize it,
sieve it, send it out

1415
01:06:35,990 --> 01:06:39,510
to a lot of labs to analyze--

1416
01:06:39,510 --> 01:06:41,870
NIST is really good at this--

1417
01:06:41,870 --> 01:06:42,920
take all the data.

1418
01:06:42,920 --> 01:06:48,710
And so this ash is characterized
for about 20 elements or so.

1419
01:06:48,710 --> 01:06:51,400
So when I run my
samples, if I were

1420
01:06:51,400 --> 01:06:53,540
to run your samples
with standards,

1421
01:06:53,540 --> 01:06:58,100
I'd run a little bit of
this, 5, 6, 7 milligrams.

1422
01:06:58,100 --> 01:07:01,250
And I know what the
concentrations are in this.

1423
01:07:01,250 --> 01:07:04,220
And so that's how I do
the comparative method.

1424
01:07:04,220 --> 01:07:06,155
And so I got this.

1425
01:07:06,155 --> 01:07:07,280
And they all look the same.

1426
01:07:07,280 --> 01:07:11,540
And this is some soil from
Montana next to a mine,

1427
01:07:11,540 --> 01:07:15,350
so it's nicely contaminated
with some metals.

1428
01:07:15,350 --> 01:07:20,040
This is my IAEA mercury
and hair standard.

1429
01:07:20,040 --> 01:07:21,650
But again, it's just
a little powder.

1430
01:07:21,650 --> 01:07:24,440
And this is kind of what
everybody uses for standards.

1431
01:07:24,440 --> 01:07:27,487
And you just kind of have
a whole collection of them.

1432
01:07:27,487 --> 01:07:29,570
And depending on what
elements you're looking for,

1433
01:07:29,570 --> 01:07:34,130
you try to mix and
match them so you

1434
01:07:34,130 --> 01:07:35,600
cover what you
want without having

1435
01:07:35,600 --> 01:07:37,652
to run five or six of them.

1436
01:07:37,652 --> 01:07:42,400
This is my hot lab,
or one of my hot labs.

1437
01:07:42,400 --> 01:07:44,600
You guys, last week,
or whatever it was,

1438
01:07:44,600 --> 01:07:46,640
I came by-- so
this is the rabbit.

1439
01:07:46,640 --> 01:07:48,080
Those you who
weren't there, these

1440
01:07:48,080 --> 01:07:51,980
are called rabbits because
it's the little thing that runs

1441
01:07:51,980 --> 01:07:53,870
through the pneumatic tube.

1442
01:07:53,870 --> 01:07:56,220
You guys are doing
[INAUDIBLE] later today?

1443
01:07:56,220 --> 01:07:56,720
Yeah.

1444
01:07:56,720 --> 01:07:58,428
When you're sitting
at the control panel,

1445
01:07:58,428 --> 01:08:00,350
there's a button, I
think it's to the left,

1446
01:08:00,350 --> 01:08:02,450
and it says insert rabbit.

1447
01:08:02,450 --> 01:08:05,780
And that's what this
is referring to.

1448
01:08:05,780 --> 01:08:07,670
For longer radiations
there's a spot

1449
01:08:07,670 --> 01:08:11,090
in the basement in the reactor
where they can get these,

1450
01:08:11,090 --> 01:08:14,660
and they send them into
the irradiation location.

1451
01:08:14,660 --> 01:08:16,399
For short
irradiations, like what

1452
01:08:16,399 --> 01:08:18,149
you guys are going to
be doing in a month,

1453
01:08:18,149 --> 01:08:20,810
I send them in from here.

1454
01:08:20,810 --> 01:08:24,700
That's OK-- I just don't
want to bump into that thing.

1455
01:08:24,700 --> 01:08:26,970
So this is one end of
the pneumatic system.

1456
01:08:26,970 --> 01:08:30,670
And so I can put a couple
of samples in here.

1457
01:08:30,670 --> 01:08:34,430
I stick it in that little tube
there, call the control room

1458
01:08:34,430 --> 01:08:37,100
and say, OK, turn
a bunch of knobs,

1459
01:08:37,100 --> 01:08:38,580
and switches, and whatnot.

1460
01:08:38,580 --> 01:08:42,470
And it goes schwoonk,
and in about 15 seconds

1461
01:08:42,470 --> 01:08:44,359
it's next to the
reactor to the core

1462
01:08:44,359 --> 01:08:47,060
of the reactor in the graphite.

1463
01:08:47,060 --> 01:08:48,410
I usually run shorts.

1464
01:08:48,410 --> 01:08:51,800
I'll usually irradiate
for about 10 minutes.

1465
01:08:51,800 --> 01:08:55,109
We usually let the sample sit in
the reactor for a little while.

1466
01:08:55,109 --> 01:08:58,189
So the very short half-life
stuff decays away,

1467
01:08:58,189 --> 01:09:00,050
and then it comes back out here.

1468
01:09:00,050 --> 01:09:02,029
And the thing just kind
of shoots out there

1469
01:09:02,029 --> 01:09:04,990
and bounces into here.

1470
01:09:04,990 --> 01:09:08,569
And then pop open the
rabbit, and in that hood,

1471
01:09:08,569 --> 01:09:10,220
pull the samples out.

1472
01:09:10,220 --> 01:09:14,430
I usually try to
repackage the samples.

1473
01:09:14,430 --> 01:09:16,399
So this is partly why I
asked for stuff that's

1474
01:09:16,399 --> 01:09:19,939
one or two good solid pieces.

1475
01:09:19,939 --> 01:09:23,450
Because then I can take it out
of whatever it was irradiated,

1476
01:09:23,450 --> 01:09:27,170
put it in a clean bag
or vial, and that way we

1477
01:09:27,170 --> 01:09:31,655
don't have to do a blank
subtraction for the sample.

1478
01:09:31,655 --> 01:09:33,960
Does that make sense?

1479
01:09:33,960 --> 01:09:36,620
Because, otherwise, if
I take a little vial,

1480
01:09:36,620 --> 01:09:38,750
irradiate it, and
then count it, I'll

1481
01:09:38,750 --> 01:09:43,279
also have whatever elements
are in the vial on the thing.

1482
01:09:43,279 --> 01:09:47,270
For when I'm running standards--
and this is when if we're not

1483
01:09:47,270 --> 01:09:49,540
running standards you don't
have to worry about this--

1484
01:09:49,540 --> 01:09:53,420
that powdered standard stuff,
I never get that out of a bag.

1485
01:09:53,420 --> 01:09:55,160
Because you'd never
get all of it out,

1486
01:09:55,160 --> 01:09:58,880
and I'd have contamination
everywhere if I started cutting

1487
01:09:58,880 --> 01:09:59,990
open those bags.

1488
01:09:59,990 --> 01:10:03,350
So I do have to do a bag
correction for those.

1489
01:10:03,350 --> 01:10:05,480
So when I do when
an irradiation,

1490
01:10:05,480 --> 01:10:08,145
I always irradiate
a few empty bags,

1491
01:10:08,145 --> 01:10:09,770
and then you do a
correction for those.

1492
01:10:09,770 --> 01:10:13,040
Because the bags have got
aluminum, and antimony,

1493
01:10:13,040 --> 01:10:15,650
and a bunch of things in them.

1494
01:10:15,650 --> 01:10:17,510
And so then I take
a couple of samples,

1495
01:10:17,510 --> 01:10:19,460
I throw them in a lead pig--

1496
01:10:19,460 --> 01:10:21,770
so I've got a whole bunch
of these floating around--

1497
01:10:21,770 --> 01:10:25,380
and I run it down the hall,
and throw it on a detector,

1498
01:10:25,380 --> 01:10:26,210
and we count it.

1499
01:10:26,210 --> 01:10:30,260
When we're doing shorts,
I'll irradiate two samples

1500
01:10:30,260 --> 01:10:32,402
at a time, because I
have two detectors.

1501
01:10:32,402 --> 01:10:33,860
When I used to have
four detectors,

1502
01:10:33,860 --> 01:10:35,480
I ran for samples at a time.

1503
01:10:35,480 --> 01:10:40,370
So you irradiate it,
repackage it, count it.

1504
01:10:40,370 --> 01:10:43,880
While those pair of
samples are counting,

1505
01:10:43,880 --> 01:10:46,730
you come down here, you
irradiate the next two,

1506
01:10:46,730 --> 01:10:50,090
so that you're
just kind of always

1507
01:10:50,090 --> 01:10:53,150
irradiating and counting.

1508
01:10:53,150 --> 01:10:56,390
I usually do a 10-minute
irradiation for shorts.

1509
01:10:56,390 --> 01:11:00,740
I'll do a fairly quick
count-- five minutes-- right

1510
01:11:00,740 --> 01:11:02,690
after I get the
sample down there,

1511
01:11:02,690 --> 01:11:06,050
and that's looking for
stuff with half-lifes

1512
01:11:06,050 --> 01:11:07,640
under 10 minutes.

1513
01:11:07,640 --> 01:11:10,370
The shortest half-life I
look for is for aluminum.

1514
01:11:10,370 --> 01:11:12,317
It's 2 and 1/4 minutes.

1515
01:11:12,317 --> 01:11:14,400
But things usually have a
lot of aluminum in them,

1516
01:11:14,400 --> 01:11:18,410
so I see aluminum pretty well.

1517
01:11:18,410 --> 01:11:20,030
For shorts, I'll
count all the way up

1518
01:11:20,030 --> 01:11:25,340
to about sodium, which is
almost 15 hour half-life.

1519
01:11:25,340 --> 01:11:28,040
Longer stuff, I'll do a
longer irradiation to count.

1520
01:11:28,040 --> 01:11:30,200
There's a little overlap
on my shorts and longs.

1521
01:11:30,200 --> 01:11:33,300
That helps me do QA on things.

1522
01:11:33,300 --> 01:11:36,268
And if I run two standards,
I'll check the concentrations

1523
01:11:36,268 --> 01:11:37,560
from one standard to the other.

1524
01:11:37,560 --> 01:11:41,360
That's another little QA thing.

1525
01:11:41,360 --> 01:11:42,902
What else we got?

1526
01:11:42,902 --> 01:11:43,910
MICHAEL SHORT: What
question do you guys have?

1527
01:11:43,910 --> 01:11:44,305
MICHAEL AMES: Questions.

1528
01:11:44,305 --> 01:11:46,430
MICHAEL SHORT: Now that
you know how this done.

1529
01:11:46,430 --> 01:11:47,660
MICHAEL AMES: It's
pretty straightforward.

1530
01:11:47,660 --> 01:11:48,464
MICHAEL SHORT:
What sort of things

1531
01:11:48,464 --> 01:11:49,797
are you going to be bringing in?

1532
01:11:49,797 --> 01:11:51,357
MICHAEL AMES: Yeah,
what do we got?

1533
01:11:51,357 --> 01:11:55,062
AUDIENCE: Probably middle
Bronze age pottery shirts.

1534
01:11:55,062 --> 01:11:55,770
MICHAEL AMES: Oh.

1535
01:11:55,770 --> 01:11:56,590
Yeah, yeah.

1536
01:11:56,590 --> 01:11:57,380
OK.

1537
01:11:57,380 --> 01:12:02,930
There is a lot of
archeology that NAA

1538
01:12:02,930 --> 01:12:04,100
got used for that a lot.

1539
01:12:04,100 --> 01:12:06,620
I don't think we
ever did it here.

1540
01:12:06,620 --> 01:12:10,530
Fred Frey, who's a professor,
retired now, from EAPs--

1541
01:12:10,530 --> 01:12:12,350
Earth, Atmospheric,
and Planetary--

1542
01:12:12,350 --> 01:12:14,390
he did a lot of
geological samples.

1543
01:12:14,390 --> 01:12:18,920
And I forget where it was that
they did all the archeology.

1544
01:12:18,920 --> 01:12:20,540
One of the things
NAA is really good

1545
01:12:20,540 --> 01:12:23,930
for is rare earth
elements, which

1546
01:12:23,930 --> 01:12:27,350
are hard to measure
by other methods.

1547
01:12:27,350 --> 01:12:29,480
I can get very low
limits on that.

1548
01:12:29,480 --> 01:12:34,040
And by picking out various
rare earths and the ratios,

1549
01:12:34,040 --> 01:12:36,821
it can help identify where
things are from in the world.

1550
01:12:36,821 --> 01:12:37,654
MICHAEL SHORT: Yeah.

1551
01:12:37,654 --> 01:12:39,720
AUDIENCE: Can I use
a bird as a sample?

1552
01:12:39,720 --> 01:12:42,200
MICHAEL AMES: If you give me
a little, tiny piece of it.

1553
01:12:42,200 --> 01:12:42,742
AUDIENCE: OK.

1554
01:12:42,742 --> 01:12:44,075
MICHAEL AMES: I mean, you know--

1555
01:12:44,075 --> 01:12:44,960
I

1556
01:12:44,960 --> 01:12:46,585
AUDIENCE: Like, how
small [INAUDIBLE]??

1557
01:12:46,585 --> 01:12:48,377
MICHAEL AMES: Well,
see, that's the rabbit.

1558
01:12:48,377 --> 01:12:50,038
So it's definitely
got to fit in there.

1559
01:12:50,038 --> 01:12:50,580
AUDIENCE: OK.

1560
01:12:50,580 --> 01:12:52,710
MICHAEL AMES: The
thing I really like--

1561
01:12:52,710 --> 01:12:54,150
excuse me, where's my vials?

1562
01:12:57,330 --> 01:12:59,760
I used to have some
smaller ones up here.

1563
01:12:59,760 --> 01:13:04,350
But that should definitely
fit in one of those.

1564
01:13:04,350 --> 01:13:05,437
Like, see that guy.

1565
01:13:05,437 --> 01:13:06,473
AUDIENCE: OK

1566
01:13:06,473 --> 01:13:07,890
MICHAEL AMES: My
usual description

1567
01:13:07,890 --> 01:13:10,410
of what size sample
I like is if it's

1568
01:13:10,410 --> 01:13:13,830
a piece that you would pick
up with a pair of tweezers.

1569
01:13:13,830 --> 01:13:16,140
So not too small to pick up--

1570
01:13:16,140 --> 01:13:17,340
to be able to find.

1571
01:13:17,340 --> 01:13:19,875
So no powders.

1572
01:13:19,875 --> 01:13:21,750
And you could maybe get
it with your fingers.

1573
01:13:21,750 --> 01:13:26,130
But 20 milligrams, 50
milligrams, 100 milligrams

1574
01:13:26,130 --> 01:13:28,520
is just in the right ballpark.

1575
01:13:28,520 --> 01:13:30,020
AUDIENCE: OK.

1576
01:13:30,020 --> 01:13:33,520
MICHAEL SHORT: What else are
you guys thinking of bringing?

1577
01:13:33,520 --> 01:13:35,830
MICHAEL AMES: Doesn't matter.

1578
01:13:35,830 --> 01:13:39,070
We'll look at what
comes in, and--

1579
01:13:39,070 --> 01:13:40,930
yeah, I might veto
some things or not.

1580
01:13:40,930 --> 01:13:42,460
But we'll see.

1581
01:13:42,460 --> 01:13:43,442
We'll see what we got.

1582
01:13:43,442 --> 01:13:44,858
MICHAEL SHORT: OK.

1583
01:13:44,858 --> 01:13:47,155
AUDIENCE: What are
those little bricks for?

1584
01:13:47,155 --> 01:13:49,030
MICHAEL AMES: Well, we
got bricks everywhere.

1585
01:13:49,030 --> 01:13:52,660
So when I get the
sample out of there,

1586
01:13:52,660 --> 01:13:54,640
I do the repackaging in here.

1587
01:13:54,640 --> 01:13:57,700
And so this is just
shielding between the samples

1588
01:13:57,700 --> 01:14:00,400
I'm working on and myself.

1589
01:14:00,400 --> 01:14:01,960
I don't have my
dosimeter on now,

1590
01:14:01,960 --> 01:14:05,350
but I usually have got the
symmetry and a ring badge.

1591
01:14:05,350 --> 01:14:07,120
And then it kind
of comes over here,

1592
01:14:07,120 --> 01:14:09,610
and this is where
the heat sealer is.

1593
01:14:09,610 --> 01:14:12,950
So I can heat seal it here, and
then I'll have a pig over here.

1594
01:14:12,950 --> 01:14:14,950
MICHAEL SHORT: They're
just painted lead bricks?

1595
01:14:14,950 --> 01:14:16,390
MICHAEL AMES: Yeah, these
are just painted lead bricks.

1596
01:14:16,390 --> 01:14:19,130
And you know, these have
been here longer than I have.

1597
01:14:19,130 --> 01:14:22,210
And sometimes things
just are somewhere,

1598
01:14:22,210 --> 01:14:23,350
and you never move them.

1599
01:14:23,350 --> 01:14:26,140
These, I think, are
older than me too.

1600
01:14:26,140 --> 01:14:30,580
This lab has been doing NAA
since the '70s, I think.

1601
01:14:33,744 --> 01:14:34,715
Anybody else?

1602
01:14:34,715 --> 01:14:36,090
AUDIENCE: Is there
a single brick

1603
01:14:36,090 --> 01:14:38,940
that I could just hold
to see how heavy it is?

1604
01:14:38,940 --> 01:14:40,560
MICHAEL AMES: The
full size bricks--

1605
01:14:40,560 --> 01:14:43,770
like, that size, 2 inches,
by 4 inches, by 8 inches,

1606
01:14:43,770 --> 01:14:46,380
weighs about 25 pounds.

1607
01:14:46,380 --> 01:14:49,230
There's usually a bunch
of them floating around.

1608
01:14:49,230 --> 01:14:52,530
Here, you want this game?

1609
01:14:52,530 --> 01:14:55,627
That one's not quite full size.

1610
01:14:55,627 --> 01:14:56,210
AUDIENCE: Wow.

1611
01:14:56,210 --> 01:14:57,390
That's pretty heavy.

1612
01:14:57,390 --> 01:14:58,030
MICHAEL AMES: They're heavy.

1613
01:14:58,030 --> 01:14:58,712
They're lead.

1614
01:14:58,712 --> 01:14:59,920
Anybody else want to toss it?

1615
01:14:59,920 --> 01:15:00,855
No, OK.

1616
01:15:00,855 --> 01:15:04,740
[LAUGHTER]

1617
01:15:04,740 --> 01:15:06,840
When people ask me--

1618
01:15:06,840 --> 01:15:09,220
because I work in the
reactor, as well-- they say,

1619
01:15:09,220 --> 01:15:11,012
is there anything
dangerous in the reactor?

1620
01:15:11,012 --> 01:15:14,900
The dangerous thing is dropping
lead bricks on your feet.

1621
01:15:14,900 --> 01:15:16,770
So I've got steel toast.

1622
01:15:16,770 --> 01:15:19,855
If I miss the toe, I'd
probably break my--

1623
01:15:19,855 --> 01:15:21,620
I don't want to think about it.

1624
01:15:21,620 --> 01:15:24,150
And they move much bigger
things in the reactor.

1625
01:15:24,150 --> 01:15:25,740
Have you toured the reactor yet?

1626
01:15:25,740 --> 01:15:27,000
AUDIENCE: [INAUDIBLE]

1627
01:15:27,000 --> 01:15:29,000
MICHAEL AMES: So there's
that giant crane there,

1628
01:15:29,000 --> 01:15:31,713
and they move five-ton
pieces a shielding.

1629
01:15:31,713 --> 01:15:33,630
And that's the other
dangerous thing in there,

1630
01:15:33,630 --> 01:15:35,400
dropping really big things.

1631
01:15:35,400 --> 01:15:37,140
We've never dropped
anything that big.

1632
01:15:39,663 --> 01:15:42,080
I think somebody dropped a
steel plate on their foot once.

1633
01:15:42,080 --> 01:15:43,626
That was about the worst of it.

1634
01:15:43,626 --> 01:15:45,530
[LAUGHTER]

1635
01:15:45,530 --> 01:15:48,292
You know, like, four-foot,
half-inch steel-- boom.

1636
01:15:48,292 --> 01:15:50,250
MICHAEL SHORT: That's
what happened to my foot.

1637
01:15:50,250 --> 01:15:51,042
MICHAEL AMES: Yeah.

1638
01:15:51,042 --> 01:15:51,990
OK, good.

1639
01:15:51,990 --> 01:15:53,790
And people trip and
fall off ladders.

1640
01:15:53,790 --> 01:15:55,800
And it's the usual
industrial accidents.

1641
01:15:55,800 --> 01:15:59,347
AUDIENCE: [INAUDIBLE]
cut off your toe.

1642
01:15:59,347 --> 01:16:00,180
[INTERPOSING VOICES]

1643
01:16:00,180 --> 01:16:01,310
AUDIENCE: Well, my
toes are still here.

1644
01:16:01,310 --> 01:16:01,540
MICHAEL AMES: Good.

1645
01:16:01,540 --> 01:16:02,060
Yeah.

1646
01:16:02,060 --> 01:16:04,850
I mean, I've broken
a few, but not here.

1647
01:16:04,850 --> 01:16:05,850
MICHAEL SHORT: So, cool.

1648
01:16:05,850 --> 01:16:06,680
Thanks a ton, Mike.

1649
01:16:06,680 --> 01:16:07,490
MICHAEL AMES: Sure.

1650
01:16:07,490 --> 01:16:09,500
And I'll see you guys
in a month or something

1651
01:16:09,500 --> 01:16:12,860
and have fun
running the reactor.

1652
01:16:12,860 --> 01:16:15,230
FRANK WARMSLEY: Well,
good day, folks.

1653
01:16:15,230 --> 01:16:18,830
You guys are here to do an
experiment on the reactor.

1654
01:16:18,830 --> 01:16:20,220
It's in two parts.

1655
01:16:20,220 --> 01:16:22,305
The first part is
raising reactor power.

1656
01:16:25,630 --> 01:16:28,620
The first is raising
reactor power

1657
01:16:28,620 --> 01:16:32,950
using a low worth absorber
called a regulating rod.

1658
01:16:32,950 --> 01:16:35,490
And then the second part will
be lowering reactor power

1659
01:16:35,490 --> 01:16:37,410
using a high worth absorber.

1660
01:16:37,410 --> 01:16:40,620
And the high worth absorber,
things will moved much faster.

1661
01:16:40,620 --> 01:16:43,680
And we don't want
to run into a chance

1662
01:16:43,680 --> 01:16:45,760
if you accidentally
going too high,

1663
01:16:45,760 --> 01:16:49,050
so that's why we use a low
worth absorber on the way up

1664
01:16:49,050 --> 01:16:51,900
and a high worth
absorber on the way down.

1665
01:16:51,900 --> 01:16:53,970
And I just want to
show you the controls.

1666
01:16:53,970 --> 01:16:56,160
With me today is Tim.

1667
01:16:56,160 --> 01:16:58,260
To actually do
this experiment, we

1668
01:16:58,260 --> 01:17:00,900
need two licensed
people in here,

1669
01:17:00,900 --> 01:17:03,510
one at least has a
senior reactor operator.

1670
01:17:03,510 --> 01:17:05,370
Both Tim and I are
both senior licenses,

1671
01:17:05,370 --> 01:17:07,050
so we have that covered.

1672
01:17:07,050 --> 01:17:09,240
The only way you can actually
do these manipulations

1673
01:17:09,240 --> 01:17:11,870
are if you're in my
training program--

1674
01:17:11,870 --> 01:17:14,340
I'm the training supervisor
for the facility--

1675
01:17:14,340 --> 01:17:18,360
or you're in a program that
needs you to actually operate

1676
01:17:18,360 --> 01:17:19,770
the reactor.

1677
01:17:19,770 --> 01:17:24,540
And the program you guys
are in fits that definition.

1678
01:17:24,540 --> 01:17:27,840
So I just want to show you some
of the controls of the reactor.

1679
01:17:27,840 --> 01:17:30,630
First, we have our
shim blade controller.

1680
01:17:30,630 --> 01:17:36,600
This basically moves one of
six shim blades at a time.

1681
01:17:36,600 --> 01:17:38,790
The one that's selected
has a slide on it.

1682
01:17:38,790 --> 01:17:41,940
And we can change which one's
selected with the shim blade

1683
01:17:41,940 --> 01:17:44,730
selector switch.

1684
01:17:44,730 --> 01:17:46,710
This switch here is
a regulating rod.

1685
01:17:46,710 --> 01:17:50,750
This one will allow you to move
the regulating rod up and down.

1686
01:17:50,750 --> 01:17:52,440
Our blades our fixed
speed, meaning they

1687
01:17:52,440 --> 01:17:58,800
can only move at the exact
same rate at all times.

1688
01:17:58,800 --> 01:18:01,890
Moving the shim blade
in an upward direction

1689
01:18:01,890 --> 01:18:04,920
or the regulating rod
in upwards direction,

1690
01:18:04,920 --> 01:18:07,680
take an underhand
grip and pull up

1691
01:18:07,680 --> 01:18:11,100
or twist upwards until it stops.

1692
01:18:11,100 --> 01:18:13,890
Moving it just a little
bit doesn't move anything.

1693
01:18:13,890 --> 01:18:16,440
You have to move all
the way until it stops,

1694
01:18:16,440 --> 01:18:20,310
and then the absorber will
move in the outward direction.

1695
01:18:20,310 --> 01:18:23,220
If you want the blade
to stop just release it.

1696
01:18:23,220 --> 01:18:25,950
It's spring-loaded and will go
back to the neutral position

1697
01:18:25,950 --> 01:18:27,360
and stop moving.

1698
01:18:27,360 --> 01:18:30,030
If you want to drive something
in the inward position,

1699
01:18:30,030 --> 01:18:33,480
take an overhand grip
and twist downwards,

1700
01:18:33,480 --> 01:18:35,220
and that will drive
the absorber in.

1701
01:18:35,220 --> 01:18:36,990
Once again, let go.

1702
01:18:36,990 --> 01:18:40,800
It'll snap back up and stop
the motion of the blade

1703
01:18:40,800 --> 01:18:43,890
or the regulating rod.

1704
01:18:43,890 --> 01:18:46,770
The experiment we're doing is
basically change reactor power

1705
01:18:46,770 --> 01:18:48,810
by half a megawatt.

1706
01:18:48,810 --> 01:18:51,930
And we're currently
at 500 kilowatts

1707
01:18:51,930 --> 01:18:55,530
we're going bring the
reactor up to 1 megawatt

1708
01:18:55,530 --> 01:18:59,770
and then bring it back
down to 500 kilowatts.

1709
01:18:59,770 --> 01:19:03,030
So before we can do this, you
have to log into our log book

1710
01:19:03,030 --> 01:19:05,790
as a trainee on console.

1711
01:19:05,790 --> 01:19:08,310
We'll show you the proper
way to make the entries.

1712
01:19:08,310 --> 01:19:12,180
As you make those entries,
you'll go ahead and then

1713
01:19:12,180 --> 01:19:15,840
do the actual movement itself.

1714
01:19:15,840 --> 01:19:20,040
Sp the first one is going to
be using a regular rod to move

1715
01:19:20,040 --> 01:19:21,870
the reactor power up.

1716
01:19:21,870 --> 01:19:23,520
What's the reactor power?

1717
01:19:23,520 --> 01:19:25,860
We have about nine
different instruments

1718
01:19:25,860 --> 01:19:28,410
that tell us what the reactor
power is at all times.

1719
01:19:28,410 --> 01:19:30,660
But the ones we're going to
be paying attention to are

1720
01:19:30,660 --> 01:19:32,730
channel seven 7 and channel 9.

1721
01:19:32,730 --> 01:19:35,490
These two channels are what
we used to basically tell us

1722
01:19:35,490 --> 01:19:36,960
what the wrecked power is.

1723
01:19:36,960 --> 01:19:41,580
Channel 7 is what we control
our automatic control at.

1724
01:19:41,580 --> 01:19:43,620
If you watch the
regulating rod, you'll

1725
01:19:43,620 --> 01:19:46,890
see it move up and
down on its own.

1726
01:19:46,890 --> 01:19:49,710
That's because
it's changing power

1727
01:19:49,710 --> 01:19:52,540
based on what it sees
channel 7 is doing.

1728
01:19:52,540 --> 01:19:55,650
So if channel 7 sees that the
power level's going too low,

1729
01:19:55,650 --> 01:19:58,290
it'll cause the regulating
rod to drive outwards

1730
01:19:58,290 --> 01:20:00,000
to increase the
amount of neutrons

1731
01:20:00,000 --> 01:20:02,760
making the reactor power go up.

1732
01:20:02,760 --> 01:20:05,760
Channel 9 is a
linear power channel,

1733
01:20:05,760 --> 01:20:09,240
and it basically tells us
what the power level is based

1734
01:20:09,240 --> 01:20:11,640
on a chart that we create.

1735
01:20:11,640 --> 01:20:14,760
So it's not showing you
megawatts, or kilowatts,

1736
01:20:14,760 --> 01:20:17,670
or anything like that, it's
showing you a current coming

1737
01:20:17,670 --> 01:20:18,390
from a chamber.

1738
01:20:18,390 --> 01:20:20,220
And that current
is then converted

1739
01:20:20,220 --> 01:20:23,350
into megawatts and so forth.

1740
01:20:23,350 --> 01:20:28,620
So right now, we're at 500
kilowatts, 8.5 microamps,

1741
01:20:28,620 --> 01:20:30,190
on this channel.

1742
01:20:30,190 --> 01:20:33,945
And that's 8.5 microamps
equals 550 kilowatts.

1743
01:20:33,945 --> 01:20:35,820
You're going to be
bringing a record up to 1.

1744
01:20:35,820 --> 01:20:42,030
Megawatt and since it's linear,
it'll be double that-- so 17.1.

1745
01:20:42,030 --> 01:20:44,700
Now, you want to be careful
when you raise reactor power.

1746
01:20:44,700 --> 01:20:46,740
So when you start to
add power to the reactor

1747
01:20:46,740 --> 01:20:51,300
by raising a regulating rod, you
don't want to keep raising it

1748
01:20:51,300 --> 01:20:53,700
until you reach your
value, because you

1749
01:20:53,700 --> 01:20:56,410
have to actually stop the
power increase as well.

1750
01:20:56,410 --> 01:20:59,010
So we have two rules
that we have to follow--

1751
01:20:59,010 --> 01:21:03,210
one, at the power level
we're at, we have period--

1752
01:21:03,210 --> 01:21:04,710
the reactor period.

1753
01:21:04,710 --> 01:21:06,210
The reactor period
is amount of time

1754
01:21:06,210 --> 01:21:08,010
it takes reactor
power to increase.

1755
01:21:08,010 --> 01:21:09,600
At the power level
we're at, we're

1756
01:21:09,600 --> 01:21:13,500
not allowed to go shorter
than a 100-second period.

1757
01:21:13,500 --> 01:21:16,770
So here is one of
three periods meters--

1758
01:21:16,770 --> 01:21:21,000
one here, one here, which
is selectable between two

1759
01:21:21,000 --> 01:21:23,078
different meters.

1760
01:21:23,078 --> 01:21:24,870
So as you're pulling
up the regulating rod,

1761
01:21:24,870 --> 01:21:26,400
one of the things
you have to watch

1762
01:21:26,400 --> 01:21:29,520
is to make sure that the reactor
period doesn't go shorter

1763
01:21:29,520 --> 01:21:31,080
than a 100-second period.

1764
01:21:31,080 --> 01:21:33,390
If it does, you have
to stop pulling blades.

1765
01:21:33,390 --> 01:21:35,220
The other thing we
have to watch for

1766
01:21:35,220 --> 01:21:38,970
is to make sure that the
power level, channel 9,

1767
01:21:38,970 --> 01:21:41,070
doesn't exceed where
you're going to.

1768
01:21:41,070 --> 01:21:43,110
Not only not exceed,
but we also want

1769
01:21:43,110 --> 01:21:46,740
to make sure that you can
actually control the reactor.

1770
01:21:46,740 --> 01:21:48,870
It's called
feasibility of control.

1771
01:21:48,870 --> 01:21:51,420
And what that means is
when you get to about 80%

1772
01:21:51,420 --> 01:21:52,962
of the power level
you're going to--

1773
01:21:52,962 --> 01:21:54,420
since we're going
up to 1 megawatt,

1774
01:21:54,420 --> 01:21:56,340
that's about 800 kilowatts--

1775
01:21:56,340 --> 01:22:00,000
you want to be able to drive
the absorber in and hold

1776
01:22:00,000 --> 01:22:01,050
the absorber in.

1777
01:22:01,050 --> 01:22:03,600
You'll drive the
regulating rod inwards.

1778
01:22:03,600 --> 01:22:05,430
And watch that
channel nine value.

1779
01:22:05,430 --> 01:22:09,480
It'll slow until it actually
starts to go down again.

1780
01:22:09,480 --> 01:22:12,592
Once it reaches that value
and you see it going down,

1781
01:22:12,592 --> 01:22:14,550
you now know that you
could control the reactor

1782
01:22:14,550 --> 01:22:16,750
and keep it from going away--

1783
01:22:16,750 --> 01:22:19,080
rack power increasing
continuously.

1784
01:22:19,080 --> 01:22:21,120
So what we're going
to do is have you

1785
01:22:21,120 --> 01:22:24,240
when you reach 80% of the
power level you're going to,

1786
01:22:24,240 --> 01:22:27,570
which happens to
be 800 kilowatts,

1787
01:22:27,570 --> 01:22:30,540
you're going to start increasing
or lengthening the period

1788
01:22:30,540 --> 01:22:34,500
by driving the absorber
back in the regulating rod.

1789
01:22:34,500 --> 01:22:36,900
And you'll keep holding it
in until you see the number

1790
01:22:36,900 --> 01:22:40,275
not only stop increasing, but
actually go down a little bit.

1791
01:22:40,275 --> 01:22:42,150
As soon as you see it
going down a little bit

1792
01:22:42,150 --> 01:22:44,315
and go of the
regulating rod, You

1793
01:22:44,315 --> 01:22:45,940
haven't stopped the
power at this time,

1794
01:22:45,940 --> 01:22:50,390
you've just decreased
how fast it's going up.

1795
01:22:50,390 --> 01:22:52,160
And then the power
level will sill go up,

1796
01:22:52,160 --> 01:22:55,410
but a much slower rate
than it was before.

1797
01:22:55,410 --> 01:22:58,740
And once it reaches the power
level you want to stop at,

1798
01:22:58,740 --> 01:23:01,700
the 1 megawatt, keep driving
the regulating rod in

1799
01:23:01,700 --> 01:23:04,160
to hold it at that power level.

1800
01:23:04,160 --> 01:23:06,080
Once you're at that
power level, you're

1801
01:23:06,080 --> 01:23:08,900
going to make an entry
in a log book that

1802
01:23:08,900 --> 01:23:10,580
basically says you
made it to the power

1803
01:23:10,580 --> 01:23:12,680
level you're going to.

1804
01:23:12,680 --> 01:23:15,560
And then we'll go down in power.

1805
01:23:15,560 --> 01:23:17,660
So once again, you make
an entry in a log book

1806
01:23:17,660 --> 01:23:21,140
that says I'm going to lower
ranked power to 500 kilowatts,

1807
01:23:21,140 --> 01:23:23,350
and then this time
we'll use a shim blade.

1808
01:23:23,350 --> 01:23:26,030
The shim blade is worth a lot
more than a regulating rod--

1809
01:23:26,030 --> 01:23:30,470
about 10 times the
regulating rod, so things

1810
01:23:30,470 --> 01:23:32,053
will happen much faster.

1811
01:23:32,053 --> 01:23:33,470
So you'll be able
to drive this in

1812
01:23:33,470 --> 01:23:36,920
and reactor power will change
much faster than before.

1813
01:23:36,920 --> 01:23:39,410
Same thing-- as you get closer
to the power level you start

1814
01:23:39,410 --> 01:23:41,930
at, the 500 kilowatts,
you don't want

1815
01:23:41,930 --> 01:23:43,670
to undershoot and go too low.

1816
01:23:43,670 --> 01:23:48,500
So right around 600 kilowatts
or so, start driving or shim

1817
01:23:48,500 --> 01:23:50,847
blade out to slow down
how quickly the power

1818
01:23:50,847 --> 01:23:51,680
level is going down.

1819
01:23:54,240 --> 01:23:57,710
And once you get back to the
place where you started it,

1820
01:23:57,710 --> 01:24:00,650
we'll use a regulating
rod to fine tune it

1821
01:24:00,650 --> 01:24:03,210
to keep the reactor power
where would want it to be.

1822
01:24:03,210 --> 01:24:05,370
There'll be another
logbook entry,

1823
01:24:05,370 --> 01:24:07,627
and your time on the
console will be completed.

1824
01:24:10,830 --> 01:24:14,383
So with us today we actually
have two MIT students

1825
01:24:14,383 --> 01:24:16,050
who are actually in
my training program,

1826
01:24:16,050 --> 01:24:22,112
and they've actually done a lot
of these manipulations already.

1827
01:24:22,112 --> 01:24:23,070
AUDIENCE: Ladies first.

1828
01:24:23,070 --> 01:24:23,520
FRANK WARMSLEY: Sarah.

1829
01:24:23,520 --> 01:24:24,030
Let's go.

1830
01:24:24,030 --> 01:24:24,947
AUDIENCE: [INAUDIBLE].

1831
01:24:24,947 --> 01:24:29,030
It's been so long
since I've done one.

1832
01:24:29,030 --> 01:24:31,450
FRANK WARMSLEY: I'll take that.

1833
01:24:31,450 --> 01:24:35,020
So, normally, we sit and watch.

1834
01:24:35,020 --> 01:24:38,840
If, at any time, you don't feel
comfortable doing something,

1835
01:24:38,840 --> 01:24:39,940
let us know.

1836
01:24:39,940 --> 01:24:42,778
We'll ask you just to take
your hands off the console,

1837
01:24:42,778 --> 01:24:44,320
and we'll take care
of doing whatever

1838
01:24:44,320 --> 01:24:47,110
is necessary to keep
the reactor safe.

1839
01:24:47,110 --> 01:24:51,970
But be aware, we're
a factor of 10

1840
01:24:51,970 --> 01:24:55,078
lower than where we would
automatically scram at so.

1841
01:24:55,078 --> 01:24:56,620
So it would be very
difficult for you

1842
01:24:56,620 --> 01:24:58,037
to get to someplace
where it would

1843
01:24:58,037 --> 01:25:00,595
cause a problem without
us being able to stop it.

1844
01:25:06,750 --> 01:25:08,920
I don't know if you want
to move or anything,

1845
01:25:08,920 --> 01:25:16,060
but the supervisor normally
sits kind of right in your way

1846
01:25:16,060 --> 01:25:20,756
so that they can keep
eye on what's happening.

1847
01:25:20,756 --> 01:25:23,182
AUDIENCE: Are we doing doing
any announcing for these?

1848
01:25:25,900 --> 01:25:29,130
FRANK WARMSLEY: You can go
ahead and make the announcement

1849
01:25:29,130 --> 01:25:32,138
that we're starting
power manipulations,

1850
01:25:32,138 --> 01:25:34,680
and then the last person will
make an announcement that we're

1851
01:25:34,680 --> 01:25:36,898
done with power manipulations.

1852
01:25:44,157 --> 01:25:45,865
AUDIENCE: Commencing
power manipulations.

1853
01:25:45,865 --> 01:25:47,842
Commencing power manipulations.

1854
01:25:54,770 --> 01:25:57,350
FRANK WARMSLEY: Right now,
the reactor is on autocontrol.

1855
01:25:57,350 --> 01:25:58,820
And when we do
these manipulations,

1856
01:25:58,820 --> 01:26:02,290
the reactor operator is
going to take manual control.

1857
01:26:02,290 --> 01:26:04,400
That'll cause an
alarm to come in.

1858
01:26:04,400 --> 01:26:07,050
And this will only happen
for the first time.

1859
01:26:07,050 --> 01:26:09,590
So one of the things she's
going to do after she

1860
01:26:09,590 --> 01:26:10,833
makes her logbook entry--

1861
01:26:10,833 --> 01:26:12,250
AUDIENCE: Are we
filling this out?

1862
01:26:12,250 --> 01:26:14,167
FRANK WARMSLEY: No, we'll
do that at the end--

1863
01:26:14,167 --> 01:26:16,760
is she'll take manual
control of the reactor,

1864
01:26:16,760 --> 01:26:19,490
an alarm will come in on
console, and she'll answer it.

1865
01:26:19,490 --> 01:26:21,698
And that should be the only
time you hear this alarm,

1866
01:26:21,698 --> 01:26:23,780
because we'll leave
it on manual control

1867
01:26:23,780 --> 01:26:28,332
until the final participant
has done their manipulations.

1868
01:26:30,936 --> 01:26:31,769
AUDIENCE: All right.

1869
01:26:31,769 --> 01:26:34,905
I hope to get to 1 megawatt
at 17.11 [INAUDIBLE]..

1870
01:26:34,905 --> 01:26:35,697
FRANK WARMSLEY: OK.

1871
01:26:40,116 --> 01:26:43,553
AUDIENCE: [INAUDIBLE]

1872
01:26:46,499 --> 01:26:51,876
[ELECTRONIC SOUND]

1873
01:26:51,876 --> 01:26:53,459
FRANK WARMSLEY: Now,
she's pulling out

1874
01:26:53,459 --> 01:26:57,530
the red rod all the way.

1875
01:26:57,530 --> 01:27:00,270
You see the red rod
number going up.

1876
01:27:00,270 --> 01:27:02,910
The period is getting shorter.

1877
01:27:02,910 --> 01:27:04,560
It's no longer at infinity.

1878
01:27:04,560 --> 01:27:06,780
It's getting closer
to 100 second.

1879
01:27:06,780 --> 01:27:10,314
And channel 7 and channel
9 are increasing in value.

1880
01:27:20,160 --> 01:27:23,120
Another way you can see
it is we have a display

1881
01:27:23,120 --> 01:27:25,260
on the front the operator.

1882
01:27:25,260 --> 01:27:28,140
Those three
displays, two of them

1883
01:27:28,140 --> 01:27:31,530
are just for evaluation only.

1884
01:27:31,530 --> 01:27:34,950
We don't actually use those
to control the reactor.

1885
01:27:34,950 --> 01:27:37,650
They're based on a system
that hasn't been approved yet.

1886
01:27:37,650 --> 01:27:40,570
But we're testing them to
see how well they work.

1887
01:27:40,570 --> 01:27:43,890
So you can see that the
power level on the far left

1888
01:27:43,890 --> 01:27:45,498
is going up.

1889
01:27:45,498 --> 01:27:47,790
The middle one is showing
what the actual power level--

1890
01:27:47,790 --> 01:27:49,340
we started at 500 kilowatts.

1891
01:27:49,340 --> 01:27:52,320
It's already up to 630
kilowatts and increasing.

1892
01:27:52,320 --> 01:27:54,300
And the period that
was at infinity

1893
01:27:54,300 --> 01:27:55,916
is now around 160 seconds.

1894
01:28:11,852 --> 01:28:16,620
So she's watching, and she sees
the 800 kilowatt value here

1895
01:28:16,620 --> 01:28:20,350
on channel 7 or channel
9, and she's started

1896
01:28:20,350 --> 01:28:22,320
to driving the regulating ride.

1897
01:28:22,320 --> 01:28:27,990
So she's slowing down how quick
the power increases going.

1898
01:28:27,990 --> 01:28:31,170
And you see the
period lengthening.

1899
01:28:31,170 --> 01:28:34,530
It's no longer at
150-160 seconds.

1900
01:28:34,530 --> 01:28:37,440
It's going closer
to infinity again.

1901
01:28:37,440 --> 01:28:39,450
So she's proving that she
could stop the reactor

1902
01:28:39,450 --> 01:28:42,860
power if she continued driving
in this regulating rod.

1903
01:28:55,235 --> 01:28:56,918
AUDIENCE: [INAUDIBLE]
back on auto?

1904
01:28:56,918 --> 01:28:57,710
FRANK WARMSLEY: No.

1905
01:29:06,867 --> 01:29:09,095
She's closing in
on the 1 megawatt.

1906
01:29:21,030 --> 01:29:24,370
One of the things the note
is that when she started,

1907
01:29:24,370 --> 01:29:27,610
the [INAUDIBLE] was
around 0300, 0310,

1908
01:29:27,610 --> 01:29:29,710
and she's almost
right back to there.

1909
01:29:29,710 --> 01:29:33,430
When you raise reactor power,
you basically open up a valve

1910
01:29:33,430 --> 01:29:34,472
and let more neutrons in.

1911
01:29:34,472 --> 01:29:36,597
And when you get to the
place where you want to be,

1912
01:29:36,597 --> 01:29:38,350
you basically close
that valve again.

1913
01:29:38,350 --> 01:29:43,100
So you basically add reactivity
and then stop that reactivity

1914
01:29:43,100 --> 01:29:46,330
addition by bringing the
absorbers back to about

1915
01:29:46,330 --> 01:29:48,860
where they started from.

1916
01:29:48,860 --> 01:29:52,270
AUDIENCE: We're at 17.1

1917
01:29:52,270 --> 01:29:53,770
FRANK WARMSLEY:
We're at 1 megawatt?

1918
01:29:53,770 --> 01:29:54,935
AUDIENCE: Yeah.

1919
01:29:54,935 --> 01:29:57,185
FRANK WARMSLEY: Go ahead and
make your log book entry.

1920
01:30:01,940 --> 01:30:03,430
So once again, she
has experience.

1921
01:30:03,430 --> 01:30:05,880
She's been doing startups
and power manipulations

1922
01:30:05,880 --> 01:30:06,820
for a while.

1923
01:30:06,820 --> 01:30:08,590
When the rest of
you sit down here,

1924
01:30:08,590 --> 01:30:10,700
we'll guide you through those--

1925
01:30:10,700 --> 01:30:14,560
the log book entries that
she's making and so forth.

1926
01:30:14,560 --> 01:30:16,045
AUDIENCE: The [INAUDIBLE].

1927
01:30:34,360 --> 01:30:41,285
FRANK WARMSLEY: 30.6.

1928
01:30:41,285 --> 01:30:41,785
OK.

1929
01:30:46,240 --> 01:30:48,220
AUDIENCE: Should [INAUDIBLE]?

1930
01:30:48,220 --> 01:30:50,240
FRANK WARMSLEY: Yep.

1931
01:30:50,240 --> 01:30:59,170
So one of the things that could
change the reactor is xenon.

1932
01:30:59,170 --> 01:31:03,250
It's a poison that builds into
the reactor while we operate.

1933
01:31:03,250 --> 01:31:05,260
Poison in that it
absorbs neutrons

1934
01:31:05,260 --> 01:31:06,820
not leading to fission.

1935
01:31:06,820 --> 01:31:10,600
And it has two
ways of being made

1936
01:31:10,600 --> 01:31:13,480
and two ways of
having it removed.

1937
01:31:13,480 --> 01:31:18,040
One is direct from fission
and the other is decay.

1938
01:31:18,040 --> 01:31:19,420
That's the way it's produced.

1939
01:31:19,420 --> 01:31:25,290
The way it goes away is
basically absorbing a neutron

1940
01:31:25,290 --> 01:31:27,550
and decaying to another isotope.

1941
01:31:27,550 --> 01:31:31,032
AUDIENCE: [INAUDIBLE] half a
megawatt at 8.56 microamps.

1942
01:31:31,032 --> 01:31:31,824
FRANK WARMSLEY: OK.

1943
01:31:31,824 --> 01:31:33,260
AUDIENCE: Use the
same shim blade?

1944
01:31:33,260 --> 01:31:36,390
FRANK WARMSLEY:
Yep, use blade 6.

1945
01:31:36,390 --> 01:31:40,380
And what happens is when
we lower reactor power,

1946
01:31:40,380 --> 01:31:45,240
the way we remove most of
the xenon from burn up,

1947
01:31:45,240 --> 01:31:53,280
basically the neutrons being
absorbed by the fission

1948
01:31:53,280 --> 01:31:54,300
process.

1949
01:31:54,300 --> 01:32:00,300
The fact that we don't have the
reactor at a very high power

1950
01:32:00,300 --> 01:32:02,850
means that the amount
of xenon in the core

1951
01:32:02,850 --> 01:32:04,260
isn't being removed.

1952
01:32:04,260 --> 01:32:06,830
So we actually start--

1953
01:32:06,830 --> 01:32:09,510
the power would actually
want to go down on it's own.

1954
01:32:09,510 --> 01:32:12,180
So you would have to
do a lot of re-shims.

1955
01:32:12,180 --> 01:32:15,570
And for a while, that's a very
large amount of reactivity

1956
01:32:15,570 --> 01:32:17,901
that has to be compensated for.

1957
01:32:17,901 --> 01:32:19,710
For this experiment,
though, we actually

1958
01:32:19,710 --> 01:32:22,470
shut down the reactor
yesterday and we started up

1959
01:32:22,470 --> 01:32:23,350
early this morning.

1960
01:32:23,350 --> 01:32:27,480
So it's not a big
factor as it normally

1961
01:32:27,480 --> 01:32:31,055
would be after doing one of
these lowering reactor power.

1962
01:32:31,055 --> 01:32:33,180
AUDIENCE: Do you want to
have her do a re-shim now?

1963
01:32:33,180 --> 01:32:35,376
Or do you want me to?

1964
01:32:35,376 --> 01:32:36,168
FRANK WARMSLEY: No.

1965
01:32:36,168 --> 01:32:39,156
I think we'll be able to get
at least one more person.

1966
01:32:44,160 --> 01:32:47,190
So once again, she's
lowering reactor power.

1967
01:32:47,190 --> 01:32:50,970
You can see on the period meter,
she's at a negative period,

1968
01:32:50,970 --> 01:32:53,220
and the reactor
power is decreasing.

1969
01:32:53,220 --> 01:32:55,000
She's almost at 500 kilowatts.

1970
01:32:55,000 --> 01:32:58,890
She's driving the absorber
out again to slow down

1971
01:32:58,890 --> 01:33:00,936
how quickly the power
level is going down.

1972
01:33:03,830 --> 01:33:05,420
And when she's
done, the shim blade

1973
01:33:05,420 --> 01:33:09,350
will end up about at the
same point where it started,

1974
01:33:09,350 --> 01:33:13,014
the 13.42 inches out of
the bottom of the core.

1975
01:33:27,924 --> 01:33:30,409
AUDIENCE: It might not make
it all the way back up to

1976
01:33:30,409 --> 01:33:31,403
[INAUDIBLE].

1977
01:33:31,403 --> 01:33:33,888
FRANK WARMSLEY: It'll be close.

1978
01:33:33,888 --> 01:33:36,373
Compensate with the
reg rod if you need to.

1979
01:33:56,750 --> 01:34:07,720
30.8.

1980
01:34:07,720 --> 01:34:08,690
OK.

1981
01:34:08,690 --> 01:34:11,740
And that's the end
of the exercise.