1 00:00:00,950 --> 00:00:03,320 The following content is provided under a Creative 2 00:00:03,320 --> 00:00:04,710 Commons license. 3 00:00:04,710 --> 00:00:06,920 Your support will help MIT OpenCourseWare 4 00:00:06,920 --> 00:00:11,010 continue to offer high quality educational resources for free. 5 00:00:11,010 --> 00:00:13,580 To make a donation or to view additional materials 6 00:00:13,580 --> 00:00:17,540 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:17,540 --> 00:00:18,420 at ocw.mit.edu. 8 00:00:22,928 --> 00:00:24,470 MICHAEL SHORT: Anyway, today is going 9 00:00:24,470 --> 00:00:27,380 to be a lot lighter than the past few days, which have been 10 00:00:27,380 --> 00:00:28,760 heavy on theory and new stuff. 11 00:00:28,760 --> 00:00:30,530 And I want to focus today on what can you 12 00:00:30,530 --> 00:00:33,580 do with the photon and ion interactions with matter. 13 00:00:33,580 --> 00:00:35,330 So we're going to go through a whole bunch 14 00:00:35,330 --> 00:00:37,940 of different analytical and materials 15 00:00:37,940 --> 00:00:40,580 characterization techniques that use the stuff that we've 16 00:00:40,580 --> 00:00:42,745 been learning and see what you can actually do. 17 00:00:42,745 --> 00:00:44,120 And I'll be drawing from examples 18 00:00:44,120 --> 00:00:46,340 from the open literature, from textbooks, 19 00:00:46,340 --> 00:00:47,430 and from my own work. 20 00:00:47,430 --> 00:00:50,470 So stuff I was doing here on my PhD thesis 21 00:00:50,470 --> 00:00:54,620 is actually a direct result of what do we do here in 22.01. 22 00:00:54,620 --> 00:00:57,433 So a quick review just to get it all on the board of what 23 00:00:57,433 --> 00:00:58,350 we've been looking at. 24 00:01:02,540 --> 00:01:05,459 So I don't hit anyone on the way in. 25 00:01:05,459 --> 00:01:15,350 We talked about different photon interactions, which include 26 00:01:15,350 --> 00:01:16,580 the photoelectric effect. 27 00:01:19,560 --> 00:01:22,110 Let's say this will be the energy 28 00:01:22,110 --> 00:01:26,910 of the scattered whatever, and this will be its cross section. 29 00:01:26,910 --> 00:01:31,430 We talked about Compton scattering. 30 00:01:31,430 --> 00:01:33,297 We talked about pair production. 31 00:01:36,220 --> 00:01:41,500 For the photoelectric effect, the energy of the photoelectron 32 00:01:41,500 --> 00:01:44,620 comes off like the energy of the gamma 33 00:01:44,620 --> 00:01:48,220 ray minus some very small difference, the binding 34 00:01:48,220 --> 00:01:50,050 energy of the electron. 35 00:01:50,050 --> 00:01:52,730 Let's just call it Eb. 36 00:01:52,730 --> 00:01:56,523 And this effect starts when you hit what's 37 00:01:56,523 --> 00:01:57,565 called the work function. 38 00:02:00,713 --> 00:02:02,380 I'm just going to put this all up there, 39 00:02:02,380 --> 00:02:04,390 so when we explain the analytical techniques, 40 00:02:04,390 --> 00:02:06,430 we can point to different bits of this 41 00:02:06,430 --> 00:02:09,189 and explain why we use these different things. 42 00:02:12,340 --> 00:02:16,480 The cross-section, I made sure to keep this handy, 43 00:02:16,480 --> 00:02:18,830 so I don't want to lose it. 44 00:02:18,830 --> 00:02:20,930 Strongly proportional with z. 45 00:02:20,930 --> 00:02:25,350 So the cross-section comes out of another line. 46 00:02:25,350 --> 00:02:26,540 What was it proportional to? 47 00:02:26,540 --> 00:02:29,330 Oh yeah, this is nuts. 48 00:02:29,330 --> 00:02:35,160 It's like z to the fifth over energy to the 7/2, 49 00:02:35,160 --> 00:02:36,860 which says that for higher z materials, 50 00:02:36,860 --> 00:02:39,800 the photoelectron yield is much, much stronger, 51 00:02:39,800 --> 00:02:43,340 and it's way more likely that way lower energy. 52 00:02:43,340 --> 00:02:45,140 So you can imagine if you wanted to use 53 00:02:45,140 --> 00:02:47,240 this in an analytical technique, and you 54 00:02:47,240 --> 00:02:49,610 want to study which photoelectrons come from which 55 00:02:49,610 --> 00:02:53,570 elements, you might think to use a low energy photon to excite 56 00:02:53,570 --> 00:02:57,410 them, not a high energy photon, because like we had done 57 00:02:57,410 --> 00:02:59,180 a couple of times before, if we draw 58 00:02:59,180 --> 00:03:05,295 our energy versus major cross-section range, 59 00:03:05,295 --> 00:03:07,670 we had a graph that looks something like this, where this 60 00:03:07,670 --> 00:03:08,878 was the photoelectric effect. 61 00:03:12,410 --> 00:03:15,230 This was Compton scattering. 62 00:03:15,230 --> 00:03:19,240 This is pair production. 63 00:03:19,240 --> 00:03:21,007 And so by knowing what energy-- 64 00:03:21,007 --> 00:03:21,590 oh, I'm sorry. 65 00:03:21,590 --> 00:03:23,850 That's supposed to be z. 66 00:03:23,850 --> 00:03:27,840 And this would give you the dominant process 67 00:03:27,840 --> 00:03:29,990 that each the combination of energy and z. 68 00:03:29,990 --> 00:03:32,540 So if you know what energy photons you've got 69 00:03:32,540 --> 00:03:36,120 and what you're looking for, well, there you go. 70 00:03:36,120 --> 00:03:36,620 Let's see. 71 00:03:36,620 --> 00:03:42,367 What was the energy of the Compton electron? 72 00:03:42,367 --> 00:03:43,700 Remember the wavelength formula. 73 00:03:47,850 --> 00:03:54,470 It was like alpha 1 minus cosine theta over-- 74 00:03:54,470 --> 00:03:55,145 let's see. 75 00:03:59,090 --> 00:04:03,170 Another 1 minus cosine theta. 76 00:04:03,170 --> 00:04:05,320 In came the gamma ray energy. 77 00:04:05,320 --> 00:04:07,378 What was the part that came beforehand? 78 00:04:07,378 --> 00:04:09,170 That's why I have this here because I don't 79 00:04:09,170 --> 00:04:10,378 want to write anything wrong. 80 00:04:12,855 --> 00:04:14,605 It's good to have it all up there at once. 81 00:04:19,450 --> 00:04:22,290 1. 82 00:04:22,290 --> 00:04:24,090 Yeah. 83 00:04:24,090 --> 00:04:25,200 That's all I was missing. 84 00:04:25,200 --> 00:04:26,370 Cool. 85 00:04:26,370 --> 00:04:29,160 And the cross-section for Compton scattering 86 00:04:29,160 --> 00:04:33,300 scaled something like z over energy, something pretty 87 00:04:33,300 --> 00:04:38,090 simple, not nearly as strong as pair production 88 00:04:38,090 --> 00:04:39,740 or photoelectric effect, so you can 89 00:04:39,740 --> 00:04:42,050 think Compton scattering happens much more 90 00:04:42,050 --> 00:04:44,360 dominantly at low z or the other two 91 00:04:44,360 --> 00:04:45,945 don't really happen that much at low 92 00:04:45,945 --> 00:04:48,540 z, whichever way you want to think of it. 93 00:04:48,540 --> 00:04:50,900 And for pair production, you get a whole mess of stuff. 94 00:04:50,900 --> 00:04:53,210 You get positrons coming out. 95 00:04:53,210 --> 00:04:57,860 You get a bunch of 511 keV gamma rays 96 00:04:57,860 --> 00:05:00,350 and all sorts of other things you can detect. 97 00:05:00,350 --> 00:05:01,820 And the cross-section, this one's 98 00:05:01,820 --> 00:05:04,460 got the funny scaling term. 99 00:05:09,530 --> 00:05:11,440 This one, yeah. 100 00:05:11,440 --> 00:05:12,800 It's like z squared log. 101 00:05:16,830 --> 00:05:23,470 Energy over mec squared, so some z squared kind of dependence. 102 00:05:23,470 --> 00:05:26,580 So let's keep those up for now. 103 00:05:26,580 --> 00:05:30,250 Let's get the electron ones in. 104 00:05:30,250 --> 00:05:32,440 AUDIENCE: [INAUDIBLE] mez squared? 105 00:05:32,440 --> 00:05:33,860 MICHAEL SHORT: Was it z squared? 106 00:05:33,860 --> 00:05:35,600 Let me check. 107 00:05:35,600 --> 00:05:36,830 No, that's a c. 108 00:05:36,830 --> 00:05:37,750 AUDIENCE: [INAUDIBLE] 109 00:05:37,750 --> 00:05:38,583 MICHAEL SHORT: Yeah. 110 00:05:38,583 --> 00:05:41,670 Yeah, just make sure that's clearly a c squared. 111 00:05:44,350 --> 00:05:46,860 So now let's call it charged particle, 112 00:05:46,860 --> 00:05:50,520 or just more generally ion electron interactions. 113 00:05:54,080 --> 00:05:56,030 Since these are more fresh in our head, 114 00:05:56,030 --> 00:05:57,770 what are the three ways in which charged 115 00:05:57,770 --> 00:06:03,730 particles can interact with matter that we talked about? 116 00:06:03,730 --> 00:06:05,110 Just rattle off any one of them. 117 00:06:08,278 --> 00:06:09,320 AUDIENCE: Bremsstrahlung? 118 00:06:09,320 --> 00:06:11,362 MICHAEL SHORT: Yeah, Bremsstrahlung or radiative. 119 00:06:23,090 --> 00:06:24,968 What else? 120 00:06:24,968 --> 00:06:25,860 AUDIENCE: [INAUDIBLE] 121 00:06:25,860 --> 00:06:26,160 MICHAEL SHORT: Is what? 122 00:06:26,160 --> 00:06:27,035 AUDIENCE: Ionization. 123 00:06:27,035 --> 00:06:28,146 MICHAEL SHORT: Ionization. 124 00:06:33,010 --> 00:06:35,470 Which we'll call inelastic collisions. 125 00:06:38,260 --> 00:06:39,187 And? 126 00:06:39,187 --> 00:06:40,520 AUDIENCE: Rutherford scattering. 127 00:06:40,520 --> 00:06:42,644 MICHAEL SHORT: Yep, Rutherford scattering. 128 00:06:46,520 --> 00:06:50,220 Which are kind of elastic or hard sphere collisions. 129 00:06:50,220 --> 00:06:53,970 And if we had to make kind of a table of when 130 00:06:53,970 --> 00:06:56,340 do we care about which effect, let's 131 00:06:56,340 --> 00:07:01,680 say this was an ion or electron, scattering off 132 00:07:01,680 --> 00:07:08,590 of either electrons or nuclei, in either elastic or inelastic 133 00:07:08,590 --> 00:07:09,090 ways. 134 00:07:12,940 --> 00:07:14,640 First of all, when do we actually 135 00:07:14,640 --> 00:07:17,550 care about elastic scattering off 136 00:07:17,550 --> 00:07:20,820 of electrons, which would be hard sphere collisions off 137 00:07:20,820 --> 00:07:21,570 of electrons? 138 00:07:26,080 --> 00:07:30,900 To help get you going, in an elastic collision, 139 00:07:30,900 --> 00:07:35,310 the maximum energy transfer can be this formula gamma 140 00:07:35,310 --> 00:07:38,370 times the incoming energy, where gamma 141 00:07:38,370 --> 00:07:42,780 is 4 times the incoming mass times the mass of whatever 142 00:07:42,780 --> 00:07:46,725 you're hitting over n plus big m squared. 143 00:07:49,270 --> 00:07:55,930 Let's say if one of these masses was mass of an electron. 144 00:07:55,930 --> 00:07:58,635 What is gamma approximately equal for most cases? 145 00:08:06,500 --> 00:08:08,750 Well, let's say this was like electrons scattering off 146 00:08:08,750 --> 00:08:11,540 of protons or vice versa. 147 00:08:11,540 --> 00:08:13,130 How much energy could an electron 148 00:08:13,130 --> 00:08:17,540 transfer to a proton in an elastic collision? 149 00:08:17,540 --> 00:08:19,610 Basically zero. 150 00:08:19,610 --> 00:08:23,600 The only time which this actually matters 151 00:08:23,600 --> 00:08:28,220 is if it's an electron hitting another electron, in which case 152 00:08:28,220 --> 00:08:30,860 you can have pretty significant energy transfer. 153 00:08:30,860 --> 00:08:35,270 So I'd say for elastic collisions off of electrons, 154 00:08:35,270 --> 00:08:39,830 you only care about those for other electrons. 155 00:08:42,400 --> 00:08:45,290 And I'm going to put in low energy electrons. 156 00:08:45,290 --> 00:08:47,930 Why do we only care about them for low energy electrons? 157 00:08:47,930 --> 00:08:51,530 Or in other words, what are the other methods of stopping power 158 00:08:51,530 --> 00:08:52,850 or interaction-- yeah, Chris. 159 00:08:52,850 --> 00:08:55,562 AUDIENCE: [INAUDIBLE] 160 00:08:55,562 --> 00:08:56,520 MICHAEL SHORT: Exactly. 161 00:08:56,520 --> 00:08:57,270 Yep. 162 00:08:57,270 --> 00:08:58,920 We already saw that Bremsstrahlung 163 00:08:58,920 --> 00:09:01,700 the radiated power scales with something like z 164 00:09:01,700 --> 00:09:04,120 squared over m squared. 165 00:09:04,120 --> 00:09:06,270 So with a really small mass and a really high z 166 00:09:06,270 --> 00:09:09,870 and also a higher energy, you end up 167 00:09:09,870 --> 00:09:12,240 radiating most of that power away as Bremsstrahlung. 168 00:09:12,240 --> 00:09:15,810 And there's not much of a chance of elastic collision. 169 00:09:15,810 --> 00:09:18,120 So we only care about low energy electrons 170 00:09:18,120 --> 00:09:21,660 when it comes to elastic collisions with electrons. 171 00:09:21,660 --> 00:09:25,410 For inelastic collisions with electrons, 172 00:09:25,410 --> 00:09:30,210 well, that's the hollow cylinder derivation 173 00:09:30,210 --> 00:09:32,280 that we had done from before where 174 00:09:32,280 --> 00:09:38,010 you have some particle with a mass m and a charge little ze, 175 00:09:38,010 --> 00:09:42,990 getting slightly deflected by feeling the pull-- 176 00:09:42,990 --> 00:09:46,020 depending on what charge it is, it could be towards or away-- 177 00:09:46,020 --> 00:09:50,340 of that electron away from some impact parameter B. 178 00:09:50,340 --> 00:09:54,090 So we care about this pretty much all the time. 179 00:09:54,090 --> 00:09:59,040 Electrons and ions or stripped bare nuclei 180 00:09:59,040 --> 00:10:01,290 actually matter in this case. 181 00:10:01,290 --> 00:10:04,287 For elastic collisions off of nuclei, 182 00:10:04,287 --> 00:10:05,870 this is what Rutherford scattering is. 183 00:10:05,870 --> 00:10:08,810 It's a simple hard simple hard sphere collisions, 184 00:10:08,810 --> 00:10:12,530 so this matters pretty much all the time. 185 00:10:12,530 --> 00:10:15,030 What about inelastic collisions with nuclei? 186 00:10:15,030 --> 00:10:18,545 What does an inelastic collision actually mean with a nucleus? 187 00:10:22,070 --> 00:10:26,360 So fusion could be one of them, but let's go more generally. 188 00:10:26,360 --> 00:10:30,290 We have some nuclear reaction, where it's the old thing 189 00:10:30,290 --> 00:10:34,880 that I keep drawing all the time of some little nucleus striking 190 00:10:34,880 --> 00:10:36,950 a large nucleus. 191 00:10:36,950 --> 00:10:38,630 In an inelastic collision, this is 192 00:10:38,630 --> 00:10:40,170 the case we haven't considered yet, 193 00:10:40,170 --> 00:10:43,130 but I want to show you what actually happens. 194 00:10:43,130 --> 00:10:46,280 In an inelastic collision, these two nuclei 195 00:10:46,280 --> 00:10:49,580 join together to form what's called 196 00:10:49,580 --> 00:10:53,450 a compound nucleus or CN, at which point 197 00:10:53,450 --> 00:10:57,420 it breaks apart in some other way. 198 00:10:57,420 --> 00:10:59,990 So there might be some different small particle 199 00:10:59,990 --> 00:11:03,770 and some different large particle coming off. 200 00:11:03,770 --> 00:11:05,300 But in an inelastic collision, it's 201 00:11:05,300 --> 00:11:08,360 almost like the incoming particle is absorbed 202 00:11:08,360 --> 00:11:10,820 and something else is readmitted. 203 00:11:10,820 --> 00:11:13,580 It could be that same particle at a different energy, 204 00:11:13,580 --> 00:11:17,500 and it could be a different energy altogether. 205 00:11:17,500 --> 00:11:19,360 So yeah, I'd say fusion is an example. 206 00:11:19,360 --> 00:11:21,417 It's kicked off by an inelastic collision, 207 00:11:21,417 --> 00:11:23,500 because you've got to have some sort of absorption 208 00:11:23,500 --> 00:11:26,560 event of the small nucleus by the big nucleus. 209 00:11:26,560 --> 00:11:29,480 And then, maybe if it fuses and just stays that way, 210 00:11:29,480 --> 00:11:31,960 it releases a ton of its binding energy, 211 00:11:31,960 --> 00:11:33,700 well, that's pretty cool. 212 00:11:33,700 --> 00:11:37,900 So these actually do matter, but not 213 00:11:37,900 --> 00:11:40,330 for all energies in all cases. 214 00:11:40,330 --> 00:11:44,920 So let's go back to the Janis database of cross-sections 215 00:11:44,920 --> 00:11:50,170 to see when inelastic scattering actually matters. 216 00:11:50,170 --> 00:11:51,895 Bring us back to normal size. 217 00:11:55,367 --> 00:11:57,200 And we'll look at some of the cross-sections 218 00:11:57,200 --> 00:12:01,490 to see when do we actually care about inelastic scattering? 219 00:12:01,490 --> 00:12:03,620 So we haven't selected a database yet. 220 00:12:03,620 --> 00:12:07,310 Let's say we're firing protons at things. 221 00:12:07,310 --> 00:12:10,190 And pick a database that actually 222 00:12:10,190 --> 00:12:12,710 has some elements listed. 223 00:12:12,710 --> 00:12:13,340 Not a lot. 224 00:12:13,340 --> 00:12:18,310 But iron, that works. 225 00:12:18,310 --> 00:12:21,490 So we can look at the difference between the elastic scattering 226 00:12:21,490 --> 00:12:26,410 cross-section and the anything cross-section. 227 00:12:26,410 --> 00:12:28,260 So the red curve here-- 228 00:12:28,260 --> 00:12:30,950 can I make it thicker easily? 229 00:12:30,950 --> 00:12:31,630 Probably. 230 00:12:31,630 --> 00:12:33,380 Yeah, I can make it thicker pretty easily. 231 00:12:33,380 --> 00:12:34,160 Easier to see. 232 00:12:41,095 --> 00:12:41,595 Plots. 233 00:12:44,930 --> 00:12:47,588 Wait. 234 00:12:47,588 --> 00:12:48,630 That's not what I wanted. 235 00:12:48,630 --> 00:12:50,588 I'm not going to mess around with this anymore. 236 00:12:50,588 --> 00:12:52,070 Do you guys see the two lines? 237 00:12:52,070 --> 00:12:55,310 OK, so this is the elastic scattering cross-section. 238 00:12:55,310 --> 00:12:57,198 Kind of funny to see it negative. 239 00:12:57,198 --> 00:12:59,240 But then there's the anything cross-section which 240 00:12:59,240 --> 00:13:01,930 picks up at around 3 MeV or so. 241 00:13:01,930 --> 00:13:05,540 And it usually takes somewhere between 1 and 10 MeV 242 00:13:05,540 --> 00:13:09,050 for inelastic scattering to quote unquote turn on, 243 00:13:09,050 --> 00:13:10,460 and that's because you have to be 244 00:13:10,460 --> 00:13:14,450 able to excite the nucleus to some next energy level. 245 00:13:14,450 --> 00:13:17,900 So sending in a proton at like 0.01 MeV 246 00:13:17,900 --> 00:13:20,900 is not going to excite any of the internal particles 247 00:13:20,900 --> 00:13:23,030 to a higher energy level. 248 00:13:23,030 --> 00:13:26,800 So if you want to see some pretty interesting cases, 249 00:13:26,800 --> 00:13:28,280 let's go to incident neutron data 250 00:13:28,280 --> 00:13:29,690 where we have a ton of this data. 251 00:13:32,377 --> 00:13:33,710 And I'll show you some examples. 252 00:13:37,550 --> 00:13:39,350 We've got lots more data for neutrons. 253 00:13:42,930 --> 00:13:45,320 So now we can look at some of these cross-sections. 254 00:13:49,600 --> 00:13:52,390 Like this z n prime. 255 00:13:52,390 --> 00:13:54,400 Let's take a look at what that looks like. 256 00:13:54,400 --> 00:13:56,680 That means a neutron comes in. 257 00:13:56,680 --> 00:13:58,030 Different neutron comes out. 258 00:13:58,030 --> 00:14:00,970 Notice that the scale only starts at 862 keV. 259 00:14:00,970 --> 00:14:05,030 So let's make it something else. 260 00:14:05,030 --> 00:14:05,540 Oh my. 261 00:14:05,540 --> 00:14:07,340 Look at that. 262 00:14:07,340 --> 00:14:11,720 Nothing going on until you reach almost 1 MeV, which means, 263 00:14:11,720 --> 00:14:13,400 hey, inelastic scattering doesn't really 264 00:14:13,400 --> 00:14:15,330 turn on until that. 265 00:14:15,330 --> 00:14:18,140 So I would say that this can matter, 266 00:14:18,140 --> 00:14:23,680 but for higher energy collisions. 267 00:14:23,680 --> 00:14:26,170 So yeah, it matters pretty much all the time. 268 00:14:26,170 --> 00:14:28,750 But higher energy collisions. 269 00:14:28,750 --> 00:14:29,950 And there's actually-- yeah. 270 00:14:29,950 --> 00:14:31,908 AUDIENCE: What does it say in the top left box? 271 00:14:31,908 --> 00:14:33,970 MICHAEL SHORT: Only for low energy electrons. 272 00:14:37,390 --> 00:14:40,060 That's the sort of compound reason that I and Chris said, 273 00:14:40,060 --> 00:14:42,790 one, is that you can't transfer much mass 274 00:14:42,790 --> 00:14:44,560 in an elastic collision, or I'm sorry, 275 00:14:44,560 --> 00:14:46,540 much energy in an elastic collision 276 00:14:46,540 --> 00:14:49,300 unless the masses are close enough to each other. 277 00:14:49,300 --> 00:14:52,330 And two, at higher energies, the electron 278 00:14:52,330 --> 00:14:55,240 radiates Bremsstrahlung much, much, much faster. 279 00:14:55,240 --> 00:14:58,900 As we saw at around 10 MeV, Bremsstrahlung 280 00:14:58,900 --> 00:15:01,720 and inelastic scattering give about equal contributions 281 00:15:01,720 --> 00:15:06,220 to the stopping power for high z materials like lead. 282 00:15:06,220 --> 00:15:09,340 So once you're down and let's say like the keV range, 283 00:15:09,340 --> 00:15:11,980 yeah, electron elastic collisions might matter. 284 00:15:14,510 --> 00:15:17,230 So we talked about those three. 285 00:15:17,230 --> 00:15:20,243 Now I think we can launch into the analytical technique. 286 00:15:20,243 --> 00:15:22,660 So for the rest of the lecture today, it's all going to be 287 00:15:22,660 --> 00:15:24,577 what can you do with the stuff that we've been 288 00:15:24,577 --> 00:15:26,620 learning since the first exam. 289 00:15:26,620 --> 00:15:28,000 I know it hasn't been long since, 290 00:15:28,000 --> 00:15:29,550 but we've actually learned a ton. 291 00:15:29,550 --> 00:15:31,973 And I want to show you what's actually possible. 292 00:15:31,973 --> 00:15:33,640 And this is not going to be with slides. 293 00:15:33,640 --> 00:15:35,650 It's all live from websites that I'd 294 00:15:35,650 --> 00:15:38,290 love for you guys to be able to follow along with or check out 295 00:15:38,290 --> 00:15:39,430 at home. 296 00:15:39,430 --> 00:15:42,160 So I'm going to show you an awesome resource 297 00:15:42,160 --> 00:15:45,550 through the MIT libraries and how to get there. 298 00:15:45,550 --> 00:15:49,300 If you go to vera.mit.edu, there's 299 00:15:49,300 --> 00:15:51,502 a great tool called the ASM Handbook. 300 00:15:51,502 --> 00:15:52,960 You can see I've been there before. 301 00:15:56,760 --> 00:15:58,890 There's the ASM handbooks online, 302 00:15:58,890 --> 00:16:00,660 and this is kind of that's everything 303 00:16:00,660 --> 00:16:03,180 to know about material science, metallurgy, 304 00:16:03,180 --> 00:16:05,520 and analytical techniques, absolutely everything 305 00:16:05,520 --> 00:16:08,700 from corrosion to fractography, to characterization, 306 00:16:08,700 --> 00:16:11,340 to structure of materials to where you can find 307 00:16:11,340 --> 00:16:14,490 every single alloy, to binary phase diagrams of how 308 00:16:14,490 --> 00:16:18,980 things mix, and we're going to head to one of these handbooks. 309 00:16:18,980 --> 00:16:22,280 Number nine or 10, materials characterization, 310 00:16:22,280 --> 00:16:24,920 because with the stuff that's on this board, 311 00:16:24,920 --> 00:16:27,980 you can understand how most materials characterization 312 00:16:27,980 --> 00:16:29,450 techniques work. 313 00:16:29,450 --> 00:16:31,290 And I want to show you a few of them. 314 00:16:31,290 --> 00:16:34,190 One of which-- no, two of which, we're going to demo 315 00:16:34,190 --> 00:16:36,850 out next Friday's recitation. 316 00:16:36,850 --> 00:16:38,600 So I think I told you guys in the syllabus 317 00:16:38,600 --> 00:16:40,310 and probably in person that we're 318 00:16:40,310 --> 00:16:41,990 going to try out some scanning electron 319 00:16:41,990 --> 00:16:45,950 microscopy and some energy dispersive X-ray or EDC 320 00:16:45,950 --> 00:16:47,280 analysis. 321 00:16:47,280 --> 00:16:49,640 So with the X-ray transition stuff you've learned, 322 00:16:49,640 --> 00:16:52,400 you actually know how to elementally analyze 323 00:16:52,400 --> 00:16:53,690 different materials. 324 00:16:53,690 --> 00:16:55,430 And with scanning electron microscope, 325 00:16:55,430 --> 00:16:59,360 you can get some idea about how electrons can make images much 326 00:16:59,360 --> 00:17:01,880 better than optical images. 327 00:17:01,880 --> 00:17:08,630 So let's head to electron optical methods, scanning 328 00:17:08,630 --> 00:17:15,310 electron microscopy, and show you what one of these things 329 00:17:15,310 --> 00:17:16,420 actually looks like. 330 00:17:16,420 --> 00:17:22,609 Let's take a look at an SEM, or scanning electron microscope. 331 00:17:22,609 --> 00:17:26,160 So up at the top, there is a device called the electron gun. 332 00:17:26,160 --> 00:17:28,500 For now, just imagine it's a source of electrons, 333 00:17:28,500 --> 00:17:30,000 but in a few minutes, we'll actually 334 00:17:30,000 --> 00:17:32,460 explain how it works using the principle 335 00:17:32,460 --> 00:17:35,790 of thermionic emission, which we talked about last Friday. 336 00:17:35,790 --> 00:17:38,110 You've got some electronic lenses, 337 00:17:38,110 --> 00:17:40,740 some focusing coils, that caused this beam to get 338 00:17:40,740 --> 00:17:43,840 focused further and further. 339 00:17:43,840 --> 00:17:46,800 So let's say you had this electron filament giving off 340 00:17:46,800 --> 00:17:49,050 electrons in all directions. 341 00:17:51,610 --> 00:17:54,400 When you see boxes with x's like this on an electron optics 342 00:17:54,400 --> 00:17:57,250 diagram, it usually means this is like a focusing coil 343 00:17:57,250 --> 00:17:58,510 of some sort. 344 00:17:58,510 --> 00:18:03,910 So that will cause the electrons to get bent and focused. 345 00:18:03,910 --> 00:18:09,430 There'll be another set of coils that focuses them further 346 00:18:09,430 --> 00:18:14,140 and some scanning coils that actually raster or xy 347 00:18:14,140 --> 00:18:18,430 scan this beam across the surface of a material. 348 00:18:18,430 --> 00:18:20,440 And so in this way, what you're actually 349 00:18:20,440 --> 00:18:22,870 doing is putting the electron beam at one 350 00:18:22,870 --> 00:18:26,270 part of your material and then with another detector, 351 00:18:26,270 --> 00:18:29,110 let's call it a secondary electron detector. 352 00:18:29,110 --> 00:18:32,290 Looking at the electrons produced from collisions 353 00:18:32,290 --> 00:18:36,190 with those other electrons that then get detected here, 354 00:18:36,190 --> 00:18:38,680 and the number of electrons produced at a point 355 00:18:38,680 --> 00:18:41,020 gives you the brightness of the image. 356 00:18:41,020 --> 00:18:43,540 That's kind of as simple as it is despite how 357 00:18:43,540 --> 00:18:45,670 complicated this diagram looks. 358 00:18:45,670 --> 00:18:47,500 There's an electron source. 359 00:18:47,500 --> 00:18:49,570 There's coils that scan it back and forth. 360 00:18:49,570 --> 00:18:52,660 Like has anyone ever seen the old cathode ray tube, 361 00:18:52,660 --> 00:18:55,620 CRT televisions? 362 00:18:55,620 --> 00:18:57,880 There's going to come a day when that answer is no. 363 00:18:57,880 --> 00:18:59,255 And I'm kind of worried for that, 364 00:18:59,255 --> 00:19:01,800 because that's the day I'll officially become old. 365 00:19:01,800 --> 00:19:04,853 But for now, everyone's seen a CRT, 366 00:19:04,853 --> 00:19:06,270 and the way that actually works is 367 00:19:06,270 --> 00:19:09,600 there's an electron gun that fires and scans left to right 368 00:19:09,600 --> 00:19:13,920 and up to down our rasters and produces that electron image. 369 00:19:13,920 --> 00:19:18,480 In an SEM, you use an electron gun, kind of similar, and then 370 00:19:18,480 --> 00:19:21,930 collect the electrons generated in the specimen, what's 371 00:19:21,930 --> 00:19:23,880 called secondary electrons. 372 00:19:23,880 --> 00:19:25,860 And the number that you see gives you 373 00:19:25,860 --> 00:19:27,810 the brightness of the image. 374 00:19:27,810 --> 00:19:29,940 The cool thing is this actually allows 375 00:19:29,940 --> 00:19:34,560 you to look at both secondary electron contrast 376 00:19:34,560 --> 00:19:38,680 and topology of a sample. 377 00:19:38,680 --> 00:19:44,530 So let's say this was your secondary electron detector. 378 00:19:44,530 --> 00:19:49,680 And you had an electron beam scanning across your sample 379 00:19:49,680 --> 00:19:51,710 to some of those peaks and valleys. 380 00:19:51,710 --> 00:19:54,810 And I'll probably draw one right here for a good reason. 381 00:19:54,810 --> 00:19:58,050 Let's say the electrons hit right here, 382 00:19:58,050 --> 00:20:03,330 and you send out a wave of secondary electrons. 383 00:20:03,330 --> 00:20:06,450 The material partly determines how many electrons come off, 384 00:20:06,450 --> 00:20:08,550 but also, so does the geometry. 385 00:20:08,550 --> 00:20:11,160 There will usually be a little cage 386 00:20:11,160 --> 00:20:14,220 with some sort of a positive voltage on it 387 00:20:14,220 --> 00:20:16,980 to attract those secondary electrons. 388 00:20:16,980 --> 00:20:19,740 And some of them will curve into the detector and become part 389 00:20:19,740 --> 00:20:24,000 of your signal, but some of them won't. 390 00:20:24,000 --> 00:20:27,390 Meanwhile, if you have this beam right here producing 391 00:20:27,390 --> 00:20:30,450 secondary electrons, pretty much all of them 392 00:20:30,450 --> 00:20:32,640 go slamming into your detector. 393 00:20:32,640 --> 00:20:34,620 And that's what actually allows the electron 394 00:20:34,620 --> 00:20:36,960 microscope to get topology. 395 00:20:36,960 --> 00:20:40,470 That's why images in the SEM look fairly 3D. 396 00:20:40,470 --> 00:20:45,490 So I want to show you a few examples from my own boredom 397 00:20:45,490 --> 00:20:49,450 when I was doing a lot of science. 398 00:20:49,450 --> 00:20:50,950 There we go. 399 00:20:50,950 --> 00:20:53,510 I have a whole gallery of SEM images 400 00:20:53,510 --> 00:20:55,730 when I was supposed to be doing something better. 401 00:20:55,730 --> 00:20:56,300 Oh no. 402 00:21:00,610 --> 00:21:05,490 404. 403 00:21:05,490 --> 00:21:07,450 My website's broken. 404 00:21:07,450 --> 00:21:07,950 Oh yeah. 405 00:21:07,950 --> 00:21:10,033 This is also what you do when you're bored, right? 406 00:21:10,033 --> 00:21:13,690 Make your own 404 page. 407 00:21:13,690 --> 00:21:15,610 My SEM galleries are dead. 408 00:21:15,610 --> 00:21:16,510 Well, that's OK. 409 00:21:16,510 --> 00:21:18,370 I have other images ready to show you guys. 410 00:21:22,210 --> 00:21:25,380 So this is a neat-- this is a paper that I published out 411 00:21:25,380 --> 00:21:28,750 of my PhD work that shows the real difference 412 00:21:28,750 --> 00:21:32,530 between optical and electron microscopy. 413 00:21:32,530 --> 00:21:36,190 Part of it is the limit of your resolution 414 00:21:36,190 --> 00:21:39,610 depends on the wavelength or de Broglie wavelength of the thing 415 00:21:39,610 --> 00:21:41,470 you're using to make the image. 416 00:21:41,470 --> 00:21:43,960 So an optical microscope, in this case, 417 00:21:43,960 --> 00:21:47,290 you can't get better resolution than about half a micron, 418 00:21:47,290 --> 00:21:49,150 because even the blue wavelengths of light 419 00:21:49,150 --> 00:21:52,510 are getting down into about the 450 nanometer regime. 420 00:21:52,510 --> 00:21:55,210 And it's very difficult without interference techniques 421 00:21:55,210 --> 00:21:58,720 or other fancy things to beat that diffraction limit, 422 00:21:58,720 --> 00:22:02,570 to beat the sort of wavelength limit of optical microscopy. 423 00:22:02,570 --> 00:22:05,650 So this is a 500x optical microscope image, 424 00:22:05,650 --> 00:22:07,690 and you can see these little fingers-- 425 00:22:07,690 --> 00:22:09,610 in this case, it's liquid lead bismuth 426 00:22:09,610 --> 00:22:11,920 penetrating into a stainless steel 427 00:22:11,920 --> 00:22:14,170 that we were doing corrosion experiments on. 428 00:22:14,170 --> 00:22:15,700 And that's as good as the image can 429 00:22:15,700 --> 00:22:19,220 get in an optical microscope. 430 00:22:19,220 --> 00:22:22,040 Switch down to an SEM, and then all of a sudden 431 00:22:22,040 --> 00:22:24,740 the picture becomes much, much, much more clear. 432 00:22:24,740 --> 00:22:27,920 You can start to see things-- the best SEM we have in our lab 433 00:22:27,920 --> 00:22:32,600 has an ultimate resolution of about 1 nanometer. 434 00:22:32,600 --> 00:22:35,510 Now, resolution is kind of a funny thing. 435 00:22:35,510 --> 00:22:37,760 It's neat to tell you what that means. 436 00:22:37,760 --> 00:22:42,530 It doesn't mean that if you have a pattern of lines 437 00:22:42,530 --> 00:22:47,540 that are exactly one nanometer thick, that you will see them 438 00:22:47,540 --> 00:22:50,060 as lines 1 nanometer thick. 439 00:22:50,060 --> 00:22:51,950 It means that if you then plot, let's 440 00:22:51,950 --> 00:22:58,710 say, your signal or your brightness versus x, 441 00:22:58,710 --> 00:23:02,820 you'll have some barely distinguishable and fuzzy 442 00:23:02,820 --> 00:23:06,600 lines, just enough for you to say those are two optically 443 00:23:06,600 --> 00:23:07,980 distinct features. 444 00:23:07,980 --> 00:23:14,340 So what you'll actually see in a 1 nanometer microscope 445 00:23:14,340 --> 00:23:17,850 is maybe something like this. 446 00:23:17,850 --> 00:23:20,790 That's technically resolved at the level of 1 nanometer. 447 00:23:20,790 --> 00:23:23,250 So the best you can do for crisp objects in this thing 448 00:23:23,250 --> 00:23:25,260 is about 20 nanometers. 449 00:23:25,260 --> 00:23:26,430 Not bad. 450 00:23:26,430 --> 00:23:30,250 It's like something that's a few thousand atoms on a side. 451 00:23:30,250 --> 00:23:32,420 Pretty cool. 452 00:23:32,420 --> 00:23:36,260 And so what you can see in here is liquid lead bismuth 453 00:23:36,260 --> 00:23:38,900 penetrating into this stainless steel, 454 00:23:38,900 --> 00:23:40,970 and you notice a few different things. 455 00:23:40,970 --> 00:23:45,770 This image was taken in backscatter electron mode. 456 00:23:45,770 --> 00:23:49,160 Back scattering is-- we've talked about this before. 457 00:23:49,160 --> 00:23:54,140 When you have a scattering event where theta equals pi, 458 00:23:54,140 --> 00:23:55,820 we call that backscatter. 459 00:24:04,950 --> 00:24:10,410 Let's kind of split this into regular and backscatter. 460 00:24:14,230 --> 00:24:17,560 For a backscattering, the cross-section for this 461 00:24:17,560 --> 00:24:21,580 is proportional to z squared, another one 462 00:24:21,580 --> 00:24:25,000 of those extremely z dependent cross-sections, which 463 00:24:25,000 --> 00:24:28,540 means that the larger the z, the higher the atomic number 464 00:24:28,540 --> 00:24:31,090 the more backscatter contrast you get, 465 00:24:31,090 --> 00:24:34,240 so if you want to figure out where the little lead whiskers 466 00:24:34,240 --> 00:24:36,520 are penetrating into the stainless steel, 467 00:24:36,520 --> 00:24:41,650 since lead has a z of like 82, and iron has a z of like 26, 468 00:24:41,650 --> 00:24:43,435 it shows up like night and day. 469 00:24:43,435 --> 00:24:45,340 Do you have a question, Julia? 470 00:24:45,340 --> 00:24:46,347 OK. 471 00:24:46,347 --> 00:24:48,680 Yeah, so this is something we'll actually be able to do. 472 00:24:48,680 --> 00:24:50,840 So for the two folks I asked to bring in samples, 473 00:24:50,840 --> 00:24:53,270 if you want to bring in something with very 474 00:24:53,270 --> 00:24:55,310 different elements in it, we should 475 00:24:55,310 --> 00:24:59,350 be able to see it in backscatter contrast very, very clearly. 476 00:24:59,350 --> 00:25:03,950 And in the image of the SEM, I'll go back to that-- 477 00:25:03,950 --> 00:25:07,190 which one of these pages is it? 478 00:25:07,190 --> 00:25:10,520 Notice here that there is a backscatter detector. 479 00:25:10,520 --> 00:25:12,740 So it will detect which of those electrons 480 00:25:12,740 --> 00:25:16,110 scatter back at almost 180 degrees. 481 00:25:16,110 --> 00:25:20,090 And that's at about z squared proportionality, super useful 482 00:25:20,090 --> 00:25:23,110 tool, because if you want to see, for example, where 483 00:25:23,110 --> 00:25:24,860 the circuit board traces are, and you want 484 00:25:24,860 --> 00:25:27,420 to look at aluminum versus oxygen contrast, 485 00:25:27,420 --> 00:25:29,210 that'll help you really well. 486 00:25:29,210 --> 00:25:31,160 If you want to see where is lead penetrating 487 00:25:31,160 --> 00:25:35,572 into stainless steel, it shines up clear as day, 488 00:25:35,572 --> 00:25:38,020 which is pretty fun. 489 00:25:38,020 --> 00:25:39,700 The other thing the electrons will 490 00:25:39,700 --> 00:25:41,410 do when they enter into a material 491 00:25:41,410 --> 00:25:44,260 is excite lots of things. 492 00:25:44,260 --> 00:25:47,470 So anything from X-rays to Auger electrons. 493 00:25:47,470 --> 00:25:50,440 So now I'd like to bring up Auger electron spectroscopy. 494 00:25:52,960 --> 00:25:56,380 Electron or X-ray spectroscopic methods. 495 00:25:56,380 --> 00:25:58,030 Auger electron spectroscopy, it's 496 00:25:58,030 --> 00:26:00,220 not just a thing to trip you up on the exam 497 00:26:00,220 --> 00:26:04,360 or a little minutia from radioactive decay. 498 00:26:04,360 --> 00:26:09,020 It's actually incredibly useful, because 499 00:26:09,020 --> 00:26:12,080 of where the Auger electrons are generated 500 00:26:12,080 --> 00:26:15,650 and what they tell you about the material. 501 00:26:15,650 --> 00:26:18,790 So as a quick refresher, normally you could have, 502 00:26:18,790 --> 00:26:23,140 let's say, if a photon comes in and injects a photo electron 503 00:26:23,140 --> 00:26:26,620 as another electron comes to fill that hole, 504 00:26:26,620 --> 00:26:30,160 either an X-ray will be emitted or an Auger electron 505 00:26:30,160 --> 00:26:31,270 will be emitted. 506 00:26:31,270 --> 00:26:32,920 And it's those Auger electrons, they're 507 00:26:32,920 --> 00:26:34,540 outer binding energy electrons. 508 00:26:34,540 --> 00:26:38,770 They have very low binding energy, which means-- 509 00:26:38,770 --> 00:26:39,280 let's see. 510 00:26:39,280 --> 00:26:41,752 I keep running out of room. 511 00:26:41,752 --> 00:26:43,210 You know, I'm not going to draw it. 512 00:26:43,210 --> 00:26:46,090 I'm going to show you, because I know there is a diagram of what 513 00:26:46,090 --> 00:26:47,890 I want to show you here. 514 00:26:47,890 --> 00:26:50,800 If you want to see where the Auger electrons are actually 515 00:26:50,800 --> 00:26:52,630 produced in the material-- 516 00:26:52,630 --> 00:26:55,090 here we go. 517 00:26:55,090 --> 00:26:58,210 Since they're such low energy, the only Auger electrons 518 00:26:58,210 --> 00:27:02,620 that actually get out would be in this outer few mono layers. 519 00:27:02,620 --> 00:27:04,510 In fact, there's some Auger electron energies 520 00:27:04,510 --> 00:27:07,180 that can only get out one or two atomic mono 521 00:27:07,180 --> 00:27:08,780 layers from a material. 522 00:27:08,780 --> 00:27:10,840 So it's one of the best surface analysis 523 00:27:10,840 --> 00:27:12,460 techniques that we have. 524 00:27:12,460 --> 00:27:15,730 You can both use Auger electrons to make an electron image, 525 00:27:15,730 --> 00:27:18,100 like any other SEM. 526 00:27:18,100 --> 00:27:22,930 And you can collect them and measure their energy 527 00:27:22,930 --> 00:27:26,490 to figure out which elements they came from. 528 00:27:26,490 --> 00:27:29,070 And this kind of teardrop shape is a-- 529 00:27:29,070 --> 00:27:31,740 one, it's a great synthesis of all the information 530 00:27:31,740 --> 00:27:34,560 you need to know in the SEM that we'll see on Friday. 531 00:27:34,560 --> 00:27:39,060 And two, its why people screw up SEM analysis a lot. 532 00:27:39,060 --> 00:27:43,660 A lot of the X-ray excitation happens down here. 533 00:27:43,660 --> 00:27:47,100 Why do you think that the X-ray exaltation would happen 534 00:27:47,100 --> 00:27:49,873 near the end of the path of the electron beam 535 00:27:49,873 --> 00:27:51,540 from what you know about stopping power? 536 00:27:58,290 --> 00:28:03,150 Or, if I asked you to draw a graph of let's say 537 00:28:03,150 --> 00:28:10,800 energy versus stopping power for ionization, what would 538 00:28:10,800 --> 00:28:11,400 it look like? 539 00:28:16,110 --> 00:28:16,910 Yeah. 540 00:28:16,910 --> 00:28:19,779 AUDIENCE: It comes up like a peak at low energy. 541 00:28:19,779 --> 00:28:20,612 MICHAEL SHORT: Yeah. 542 00:28:20,612 --> 00:28:22,450 AUDIENCE: And then drops back down. 543 00:28:22,450 --> 00:28:23,283 MICHAEL SHORT: Yeah. 544 00:28:23,283 --> 00:28:26,322 AUDIENCE: And as the energy goes out, it sort of flattens out. 545 00:28:26,322 --> 00:28:28,030 MICHAEL SHORT: Yep, sort of flattens out, 546 00:28:28,030 --> 00:28:30,940 and then eventually starts picking up again but not very 547 00:28:30,940 --> 00:28:31,970 much. 548 00:28:31,970 --> 00:28:35,632 So as the energy of whatever you're going into-- 549 00:28:35,632 --> 00:28:37,090 I'm sorry, whatever particle you're 550 00:28:37,090 --> 00:28:40,120 sending in it gets lower, it's stopping power increases, 551 00:28:40,120 --> 00:28:42,310 and you have a much higher chance of this ionization 552 00:28:42,310 --> 00:28:44,740 happening, especially in the case of electrons. 553 00:28:44,740 --> 00:28:48,923 They usually come in at between 10 and 40 kV. 554 00:28:48,923 --> 00:28:50,340 And so near the end of their range 555 00:28:50,340 --> 00:28:52,870 is where they produce a lot of the X-rays. 556 00:28:52,870 --> 00:28:54,520 Now there's a lot of other nuances 557 00:28:54,520 --> 00:28:57,790 to say, well, which X-rays were produced here 558 00:28:57,790 --> 00:29:01,930 and what elements are they from. 559 00:29:01,930 --> 00:29:06,350 Let's say you had the same material here or here. 560 00:29:06,350 --> 00:29:08,600 Fewer X-rays will get out of the bottom region 561 00:29:08,600 --> 00:29:10,330 than they will from the top region. 562 00:29:10,330 --> 00:29:12,080 So if you happen to be analyzing something 563 00:29:12,080 --> 00:29:15,080 that has a gradient in composition 564 00:29:15,080 --> 00:29:18,110 or a change in composition from the top to the bottom, 565 00:29:18,110 --> 00:29:21,830 you might be like, oh, well, I have a few nanometers 566 00:29:21,830 --> 00:29:24,050 of oxygen on silicon. 567 00:29:24,050 --> 00:29:26,480 Why aren't I seeing any oxygen X-rays? 568 00:29:26,480 --> 00:29:30,380 Because you're probably generating them down here. 569 00:29:30,380 --> 00:29:32,150 That's one of those things to note. 570 00:29:32,150 --> 00:29:34,040 So sometimes you'll see and elect 571 00:29:34,040 --> 00:29:38,362 an elemental map of things that shows X-rays 572 00:29:38,362 --> 00:29:40,070 of certain element coming from somewhere, 573 00:29:40,070 --> 00:29:41,930 and you can't see it at all in the image. 574 00:29:41,930 --> 00:29:43,460 That's because they might be underneath what 575 00:29:43,460 --> 00:29:44,600 you can see in the image. 576 00:29:44,600 --> 00:29:46,945 It's kind of tricky like that. 577 00:29:46,945 --> 00:29:49,070 I'll show you some examples of what those maps look 578 00:29:49,070 --> 00:29:53,850 like also from this paper. 579 00:29:53,850 --> 00:29:56,820 So from the electron image, we sort of 580 00:29:56,820 --> 00:29:59,310 concluded, all right, lead is probably penetrating 581 00:29:59,310 --> 00:30:01,050 into the stainless steel. 582 00:30:01,050 --> 00:30:02,700 How do we know for sure? 583 00:30:02,700 --> 00:30:05,970 You can make EDX or elemental dispersive-- 584 00:30:05,970 --> 00:30:08,670 I'm sorry, energy dispersive X-ray maps 585 00:30:08,670 --> 00:30:11,310 by focusing the electron beam at one point, 586 00:30:11,310 --> 00:30:12,810 collecting all the different X-rays 587 00:30:12,810 --> 00:30:15,870 and then moving from one point to another 588 00:30:15,870 --> 00:30:18,630 to see when do you get characteristic X-rays from each 589 00:30:18,630 --> 00:30:19,870 of those elements. 590 00:30:19,870 --> 00:30:21,360 So you can actually prove to say, 591 00:30:21,360 --> 00:30:26,550 yes, those little fingers are indeed bismuth and lead, 592 00:30:26,550 --> 00:30:29,220 and you can see that, in this case, where the lead in bismuth 593 00:30:29,220 --> 00:30:31,140 is, the iron is not. 594 00:30:31,140 --> 00:30:32,520 But the curious thing we found is 595 00:30:32,520 --> 00:30:35,580 that in this whole band right here, most of the chromium 596 00:30:35,580 --> 00:30:36,870 disappeared. 597 00:30:36,870 --> 00:30:39,540 So it turns out that the corrosion mechanism 598 00:30:39,540 --> 00:30:41,340 was chromium dissolution. 599 00:30:41,340 --> 00:30:44,730 And we would not have been able to know that without this EDX 600 00:30:44,730 --> 00:30:47,370 mapping, and without understanding how the EDX 601 00:30:47,370 --> 00:30:50,550 maps are made from the electrons interacting with matter 602 00:30:50,550 --> 00:30:52,820 and producing characteristic X-rays, 603 00:30:52,820 --> 00:30:55,190 wouldn't have been able to prove this. 604 00:30:55,190 --> 00:30:56,920 Yet another example where the basic stuff 605 00:30:56,920 --> 00:31:01,090 you're learning in 22.01 is the theoretical underpinning 606 00:31:01,090 --> 00:31:04,190 of the techniques that we use all the time 607 00:31:04,190 --> 00:31:09,106 in material science, which I thought was pretty cool. 608 00:31:09,106 --> 00:31:11,080 I've got more examples of that that 609 00:31:11,080 --> 00:31:13,570 are even more striking, because I let 610 00:31:13,570 --> 00:31:16,840 it collect for a little longer. 611 00:31:16,840 --> 00:31:20,200 You can actually see right here that where the bismuth is, 612 00:31:20,200 --> 00:31:23,340 the iron isn't, but the iron's not dissolving. 613 00:31:23,340 --> 00:31:24,120 The chromium is. 614 00:31:24,120 --> 00:31:25,590 It's just the lead and bismuth are 615 00:31:25,590 --> 00:31:28,080 kind of sucking the chromium out of the metal right there, 616 00:31:28,080 --> 00:31:31,860 and that's what making the stainless steel less stainless. 617 00:31:31,860 --> 00:31:33,700 It's pretty neat. 618 00:31:33,700 --> 00:31:37,540 Then on to EDX analysis, what sort of information 619 00:31:37,540 --> 00:31:41,960 are we looking at every one of these pixels? 620 00:31:41,960 --> 00:31:45,400 I have a couple of other example X-ray spectra. 621 00:31:45,400 --> 00:31:48,090 So now we're in a position to understand 622 00:31:48,090 --> 00:31:51,420 why one of these X-ray spectra looks the way it does. 623 00:31:51,420 --> 00:31:54,580 In this case, we're firing electrons at a material. 624 00:31:54,580 --> 00:31:55,260 Let's see. 625 00:31:55,260 --> 00:31:57,600 Where is our material we're firing at? 626 00:31:57,600 --> 00:31:59,250 Right here. 627 00:31:59,250 --> 00:32:02,010 So we're firing in electrons. 628 00:32:02,010 --> 00:32:05,280 And in some cases, let's say we had an iron atom. 629 00:32:08,190 --> 00:32:13,998 That electron can eject another electron. 630 00:32:13,998 --> 00:32:15,540 And then one of those other electrons 631 00:32:15,540 --> 00:32:18,270 will fall down in that shell, giving off 632 00:32:18,270 --> 00:32:20,100 a characteristic X-ray. 633 00:32:20,100 --> 00:32:23,250 In this case, since it's from the third to the second shell, 634 00:32:23,250 --> 00:32:26,550 that would be what we call an L X-ray or a something 635 00:32:26,550 --> 00:32:29,310 to level two transition. 636 00:32:29,310 --> 00:32:32,010 And every element has got its characteristic X-ray 637 00:32:32,010 --> 00:32:36,840 transitions, like we saw on the NIST X-ray transition database. 638 00:32:36,840 --> 00:32:38,820 And since we know what all of those are, 639 00:32:38,820 --> 00:32:40,200 we know where to expect them. 640 00:32:40,200 --> 00:32:43,320 So we know where we expect to see chromium's X-rays 641 00:32:43,320 --> 00:32:45,420 and iron's X-rays. 642 00:32:45,420 --> 00:32:48,190 Gold's kind of an interesting one. 643 00:32:48,190 --> 00:32:52,150 There's two things about doing analysis with gold. 644 00:32:52,150 --> 00:32:54,400 A lot of times you have to coat your materials in gold 645 00:32:54,400 --> 00:32:57,830 to boost their secondary electron contrast. 646 00:32:57,830 --> 00:33:01,820 But also gold, I think it's its L line or M line, 647 00:33:01,820 --> 00:33:06,643 I forget which one, is the same as argon's K line. 648 00:33:06,643 --> 00:33:08,810 And we have an expression in the electron microscopy 649 00:33:08,810 --> 00:33:11,960 world, the probability of finding argon in your sample 650 00:33:11,960 --> 00:33:15,420 decreases with experience. 651 00:33:15,420 --> 00:33:17,135 Takes a second to parse that. 652 00:33:17,135 --> 00:33:19,260 Chances are, if you're looking at a solid material. 653 00:33:19,260 --> 00:33:20,790 You don't have argon in it. 654 00:33:20,790 --> 00:33:23,940 But there are extra lines that overlap with each other, 655 00:33:23,940 --> 00:33:27,330 like the L line for gold and the K line for argon 656 00:33:27,330 --> 00:33:31,110 are at pretty much the same energy, certainly 657 00:33:31,110 --> 00:33:34,920 similar enough that it's within the resolution 658 00:33:34,920 --> 00:33:38,908 or like full width of half maximum of these two peaks. 659 00:33:38,908 --> 00:33:40,950 So remember how we were analyzing the uncertainty 660 00:33:40,950 --> 00:33:44,190 of our banana spectra with the FWHM or full width 661 00:33:44,190 --> 00:33:45,432 at half maximum? 662 00:33:45,432 --> 00:33:46,890 Same thing here, and you can really 663 00:33:46,890 --> 00:33:49,710 see that the energy resolution of this detector 664 00:33:49,710 --> 00:33:51,740 is not the best. 665 00:33:51,740 --> 00:33:53,450 So if you see a peak, it might be 666 00:33:53,450 --> 00:33:58,672 due to two or more peaks crowding in that right there. 667 00:33:58,672 --> 00:34:01,130 And with a lot of correction factors that I won't get into, 668 00:34:01,130 --> 00:34:02,930 you can then use this information 669 00:34:02,930 --> 00:34:04,760 to integrate the area under these peaks 670 00:34:04,760 --> 00:34:07,920 and get elemental analysis. 671 00:34:07,920 --> 00:34:11,300 You can say how much chromium or how much iron and silicon is 672 00:34:11,300 --> 00:34:14,400 in one of these samples. 673 00:34:14,400 --> 00:34:16,545 What's this stuff here on the bottom? 674 00:34:16,545 --> 00:34:17,170 Anyone tell me? 675 00:34:21,530 --> 00:34:24,484 That continuum of observed X-ray energies? 676 00:34:27,906 --> 00:34:28,810 AUDIENCE: Compton. 677 00:34:28,810 --> 00:34:31,360 MICHAEL SHORT: Compton scattering is a photon effect, 678 00:34:31,360 --> 00:34:34,250 so that would be-- 679 00:34:34,250 --> 00:34:38,500 if this were a photon analysis spectrum, then 680 00:34:38,500 --> 00:34:41,170 you would see something like this but of a different shape. 681 00:34:41,170 --> 00:34:43,719 You'd have that Compton bowl with an edge. 682 00:34:43,719 --> 00:34:45,429 But this is, in this case, electrons 683 00:34:45,429 --> 00:34:47,630 interacting with material. 684 00:34:47,630 --> 00:34:49,896 What do you think is causing that broad background? 685 00:34:53,030 --> 00:34:55,880 Well, what are the different ways in which electrons 686 00:34:55,880 --> 00:34:58,460 can interact with matter? 687 00:34:58,460 --> 00:35:00,923 You're seeing the ionizations here. 688 00:35:00,923 --> 00:35:02,840 We're not really seeing Rutherford scattering. 689 00:35:02,840 --> 00:35:05,725 What's left? 690 00:35:05,725 --> 00:35:07,120 AUDIENCE: Bremsstrahlung? 691 00:35:07,120 --> 00:35:08,370 MICHAEL SHORT: Bremsstrahlung. 692 00:35:08,370 --> 00:35:09,920 Yep, that's exactly it. 693 00:35:09,920 --> 00:35:12,610 So the observed Bremsstrahlung spectrum 694 00:35:12,610 --> 00:35:15,550 follows this sort of characteristic peak 695 00:35:15,550 --> 00:35:18,350 early and then tail off curve. 696 00:35:18,350 --> 00:35:20,530 What's the actual Bremsstrahlung spectrum 697 00:35:20,530 --> 00:35:21,595 that we're not sensing? 698 00:35:25,560 --> 00:35:26,790 What would it look like? 699 00:35:29,610 --> 00:35:33,000 Always running out of room. 700 00:35:33,000 --> 00:35:35,040 If this is what we're actually observing, 701 00:35:35,040 --> 00:35:40,730 let's say we have a few peeks, that would be intensity, 702 00:35:40,730 --> 00:35:45,183 and that would be energy, what's really going on physically 703 00:35:45,183 --> 00:35:46,100 that we're not seeing? 704 00:35:50,180 --> 00:35:51,254 Yeah. 705 00:35:51,254 --> 00:35:56,670 AUDIENCE: Isn't it sort of like almost like exponential decay. 706 00:35:56,670 --> 00:36:00,260 So it starts out with very high intensity and goes down. 707 00:36:00,260 --> 00:36:02,330 MICHAEL SHORT: That's right. 708 00:36:02,330 --> 00:36:07,144 You actually should get more low energy Bremsstrahlung. 709 00:36:09,750 --> 00:36:11,610 One of some of the reasons you don't is 710 00:36:11,610 --> 00:36:14,940 that the lower energy, those X-rays come out, 711 00:36:14,940 --> 00:36:19,350 the more they get self-absorbed in the material in the few gas 712 00:36:19,350 --> 00:36:22,710 molecules in the SEM and in the window of the detector. 713 00:36:22,710 --> 00:36:24,210 So just because this is what you see 714 00:36:24,210 --> 00:36:25,710 doesn't mean this is what's actually 715 00:36:25,710 --> 00:36:28,560 going on in your material. 716 00:36:28,560 --> 00:36:32,840 If we think back then to where the electrons and X-rays are 717 00:36:32,840 --> 00:36:36,253 generated, the X-rays that are generated down here, 718 00:36:36,253 --> 00:36:38,420 the lower energy ones are going to be shielded more. 719 00:36:38,420 --> 00:36:41,300 And this kind of messes up your elemental analysis, 720 00:36:41,300 --> 00:36:45,500 because if the X-rays produced here, 721 00:36:45,500 --> 00:36:47,690 proportionally more of the low energy 722 00:36:47,690 --> 00:36:50,300 ones will get out than the ones produced here. 723 00:36:50,300 --> 00:36:53,030 So as you change your-- 724 00:36:53,030 --> 00:36:55,070 as you change your electron beam energy, 725 00:36:55,070 --> 00:36:58,040 you might see your elemental composition appear to change 726 00:36:58,040 --> 00:36:59,858 when you know it's really not. 727 00:36:59,858 --> 00:37:02,150 And that's because where the X-rays are being generated 728 00:37:02,150 --> 00:37:05,180 change, and proportionately, more of the low energy 729 00:37:05,180 --> 00:37:08,120 ones get self shielded by your material. 730 00:37:08,120 --> 00:37:10,010 So you actually have to correct for that 731 00:37:10,010 --> 00:37:13,070 and input your beam energy into the EDX analyzer 732 00:37:13,070 --> 00:37:16,660 so it knows how to correct for this. 733 00:37:16,660 --> 00:37:18,790 But with the understanding I've been giving you 734 00:37:18,790 --> 00:37:21,400 guys in this class you can understand like well, 735 00:37:21,400 --> 00:37:22,700 why can you get screwed up? 736 00:37:22,700 --> 00:37:26,820 Why do we have to have all these correction factors? 737 00:37:26,820 --> 00:37:29,090 I think it's pretty neat. 738 00:37:29,090 --> 00:37:33,600 Then let's get on to some of the other methods, like X-ray photo 739 00:37:33,600 --> 00:37:36,020 electron spectroscopy or XPS. 740 00:37:36,020 --> 00:37:37,770 This is something I hinted to a little bit 741 00:37:37,770 --> 00:37:41,430 earlier that actually uses the photoelectric effect, 742 00:37:41,430 --> 00:37:44,130 because it's a photo electron spectroscopy method. 743 00:37:47,410 --> 00:37:49,900 This one's incredibly useful because not only does it 744 00:37:49,900 --> 00:37:51,670 tell you what elements are there, 745 00:37:51,670 --> 00:37:53,650 but in what binding state they are, 746 00:37:53,650 --> 00:37:55,420 because photo electron spectrometers 747 00:37:55,420 --> 00:37:57,760 can be incredibly precise. 748 00:37:57,760 --> 00:38:01,810 The energy equation should look pretty familiar to you 749 00:38:01,810 --> 00:38:03,700 it's whatever photo electron you get 750 00:38:03,700 --> 00:38:05,290 is the gamma ray energy that comes 751 00:38:05,290 --> 00:38:08,980 in minus the binding energy of that electron 752 00:38:08,980 --> 00:38:10,600 and the work function. 753 00:38:10,600 --> 00:38:13,210 And so you can very, very simply figure out 754 00:38:13,210 --> 00:38:15,580 for a given element and a given electron 755 00:38:15,580 --> 00:38:19,720 shell what photo electron energy's do you expect. 756 00:38:19,720 --> 00:38:22,030 So you can collect them. 757 00:38:22,030 --> 00:38:25,100 So I'll show you another example from this paper, where 758 00:38:25,100 --> 00:38:26,700 we started to do that. 759 00:38:26,700 --> 00:38:28,970 We wanted to answer the question, what 760 00:38:28,970 --> 00:38:32,390 are the oxides forming on the stainless steel 761 00:38:32,390 --> 00:38:34,130 when lead corrodes it? 762 00:38:34,130 --> 00:38:36,140 And just telling you which elements 763 00:38:36,140 --> 00:38:38,150 are there and in what proportion doesn't give 764 00:38:38,150 --> 00:38:39,980 the answer, because what if there's 765 00:38:39,980 --> 00:38:41,810 multiple phases of each oxide? 766 00:38:41,810 --> 00:38:53,490 Like, for example, iron can take forms like FeO, Fe2O3, Fe3O4, 767 00:38:53,490 --> 00:38:57,108 and this FeO can actually have a range of stoichiometries. 768 00:38:57,108 --> 00:38:57,900 So how do you know? 769 00:38:57,900 --> 00:38:58,650 You don't know. 770 00:38:58,650 --> 00:39:02,237 There could be like scores of phases of this iron oxide. 771 00:39:02,237 --> 00:39:04,320 The question is how you know which ones are there. 772 00:39:04,320 --> 00:39:07,350 The photo electrons will tell you. 773 00:39:07,350 --> 00:39:12,870 So what you can do first is fire monochromatic X-rays, so single 774 00:39:12,870 --> 00:39:17,520 energy X-rays, in this case from aluminum at your material 775 00:39:17,520 --> 00:39:21,330 and see which photo electrons of which energy come off. 776 00:39:21,330 --> 00:39:24,090 And you can tell which atomic shell 777 00:39:24,090 --> 00:39:26,460 they're from and which elements they should 778 00:39:26,460 --> 00:39:28,530 be to a very high precision. 779 00:39:28,530 --> 00:39:33,390 In this case, this is done to 100 milli MeV or 0.1 MeV 780 00:39:33,390 --> 00:39:36,360 precision, 0.1 eV precision. 781 00:39:36,360 --> 00:39:38,790 So we can tell not only what elements are there, 782 00:39:38,790 --> 00:39:41,430 but what shells they came from. 783 00:39:41,430 --> 00:39:43,170 Then you can get even crazier. 784 00:39:43,170 --> 00:39:45,330 You can scan very slowly over one 785 00:39:45,330 --> 00:39:50,670 of these peaks with 0.001 eV precision 786 00:39:50,670 --> 00:39:52,680 and start to see something pretty cool. 787 00:39:52,680 --> 00:39:56,008 If you look at the carbon 1s electrons, 788 00:39:56,008 --> 00:39:58,050 you can see that there are actually three of them 789 00:39:58,050 --> 00:40:01,470 only a couple eV apart, and this corresponds 790 00:40:01,470 --> 00:40:06,220 to different binding states of molecules with that carbon. 791 00:40:06,220 --> 00:40:08,410 For an even more subtle example, but ended up 792 00:40:08,410 --> 00:40:11,170 being incredibly important for us, 793 00:40:11,170 --> 00:40:13,750 here's one of the chromium 2p shell peaks. 794 00:40:13,750 --> 00:40:15,340 You can actually see there's three 795 00:40:15,340 --> 00:40:18,400 of them superimposed give that funny 796 00:40:18,400 --> 00:40:20,148 looking peak shape right there. 797 00:40:20,148 --> 00:40:21,940 What that actually tells us is that there's 798 00:40:21,940 --> 00:40:26,290 chromium in three different binding states in that oxide. 799 00:40:26,290 --> 00:40:29,080 And the ones we figured out must be there, 800 00:40:29,080 --> 00:40:35,840 we saw the ones corresponding to Cr2O3, FeCr2O4, 801 00:40:35,840 --> 00:40:38,810 and I forget which other one, but we have them tabulated. 802 00:40:41,440 --> 00:40:43,570 There we go. 803 00:40:43,570 --> 00:40:44,270 Oh wow. 804 00:40:44,270 --> 00:40:49,270 Fe 2.4 Cr 0.64, known crystallographic phases 805 00:40:49,270 --> 00:40:51,130 of these oxides. 806 00:40:51,130 --> 00:40:53,350 So you can look at the peaks found 807 00:40:53,350 --> 00:40:56,560 to have resolution of like 100th of an electron volt, 808 00:40:56,560 --> 00:40:59,560 compared to reference values taken on pure compounds 809 00:40:59,560 --> 00:41:03,850 and materials to figure out what actual oxides do you have. 810 00:41:03,850 --> 00:41:06,415 That can help tell you things like how protective are they, 811 00:41:06,415 --> 00:41:08,290 how fast are they going to grow, and are they 812 00:41:08,290 --> 00:41:09,820 going to be a problem if you want 813 00:41:09,820 --> 00:41:12,430 to use this new stainless steel that we developed 814 00:41:12,430 --> 00:41:14,763 in a lead bismuth reactor. 815 00:41:14,763 --> 00:41:16,680 The biggest problem with lead bismuth reactors 816 00:41:16,680 --> 00:41:18,675 is lead corrodes like everything. 817 00:41:18,675 --> 00:41:20,550 And so the whole point of my graduate studies 818 00:41:20,550 --> 00:41:23,430 was design an alloy that doesn't corrode and lead 819 00:41:23,430 --> 00:41:25,780 and make ab alloy composite out of it. 820 00:41:25,780 --> 00:41:28,500 But you can't prove that it works unless you not only know 821 00:41:28,500 --> 00:41:32,670 how fast it corrodes, but how it corrodes, which oxides form, 822 00:41:32,670 --> 00:41:34,852 and in what order. 823 00:41:34,852 --> 00:41:36,310 That's the last part I haven't told 824 00:41:36,310 --> 00:41:38,650 you about yet is what order. 825 00:41:38,650 --> 00:41:44,910 So I want to switch to another technique called secondary ion 826 00:41:44,910 --> 00:41:47,430 mass spectroscopy or SIMS. 827 00:41:47,430 --> 00:41:50,760 In this case, you start off with firing ions 828 00:41:50,760 --> 00:41:54,840 at a material, which will then eject or sputter away 829 00:41:54,840 --> 00:41:57,130 secondary ions. 830 00:41:57,130 --> 00:42:00,570 In this case, this process of sputtering-- 831 00:42:00,570 --> 00:42:02,350 let's say this is your material here. 832 00:42:05,010 --> 00:42:08,950 You send in something like oxygen ions, which 833 00:42:08,950 --> 00:42:12,370 might be like O2 minus with a mass of 32, 834 00:42:12,370 --> 00:42:17,300 and then you blast off or sputter away. 835 00:42:19,880 --> 00:42:22,640 A few atoms at a time from that surface, 836 00:42:22,640 --> 00:42:25,287 and they'll come off the various masses and charges, 837 00:42:25,287 --> 00:42:26,870 and in this case, the sputtering could 838 00:42:26,870 --> 00:42:29,720 be due to Rutherford scattering, because you might directly 839 00:42:29,720 --> 00:42:32,510 ballistically slam and ion out of the surface. 840 00:42:32,510 --> 00:42:36,080 Then every one of these ions has a different mass 841 00:42:36,080 --> 00:42:38,070 and a different charge. 842 00:42:38,070 --> 00:42:40,100 And by sending it through a mass spectrometer, 843 00:42:40,100 --> 00:42:42,680 something that separates these materials by their mass 844 00:42:42,680 --> 00:42:45,290 to charge ratio, because the higher the charge, 845 00:42:45,290 --> 00:42:48,330 the more deflected an ion will be. 846 00:42:48,330 --> 00:42:51,588 But the higher the mass, the less deflected it will be. 847 00:42:51,588 --> 00:42:53,005 That should sound really familiar. 848 00:42:55,830 --> 00:43:00,710 In our idea here where how these ionization collisions happen, 849 00:43:00,710 --> 00:43:02,750 if you remember the higher the charge, 850 00:43:02,750 --> 00:43:05,270 the stronger the Coulomb force-- 851 00:43:05,270 --> 00:43:11,590 that q1, q2 over r squared. 852 00:43:11,590 --> 00:43:14,330 I think there was a constant in there. 853 00:43:14,330 --> 00:43:17,210 So the higher the charge, the higher those q's, 854 00:43:17,210 --> 00:43:19,340 and the stronger the Coulomb forces. 855 00:43:19,340 --> 00:43:23,480 But the larger the masses, the less momentum it can impart. 856 00:43:23,480 --> 00:43:26,000 And so the deflection will be weaker. 857 00:43:26,000 --> 00:43:28,340 Exact same thing's happening here. 858 00:43:28,340 --> 00:43:30,890 And you can separate out atoms not only 859 00:43:30,890 --> 00:43:33,560 by their charge and their mass, but specifically 860 00:43:33,560 --> 00:43:35,078 by their isotope. 861 00:43:35,078 --> 00:43:36,620 So this is one of those ways that you 862 00:43:36,620 --> 00:43:40,640 can figure out and make an isotopic map of a material 863 00:43:40,640 --> 00:43:42,140 in three dimensions. 864 00:43:42,140 --> 00:43:44,900 You can scan your ion beam across the material 865 00:43:44,900 --> 00:43:47,180 and collect the ions at every point. 866 00:43:47,180 --> 00:43:52,160 And as you sputter, you slowly wear away 867 00:43:52,160 --> 00:43:54,400 layers of this material. 868 00:43:54,400 --> 00:43:56,900 And so you can actually reconstruct a 3D map 869 00:43:56,900 --> 00:44:01,070 with almost nanometer precision of every single isotope that 870 00:44:01,070 --> 00:44:05,280 was at every location, which is quite cool. 871 00:44:05,280 --> 00:44:07,560 As you can see, these master charge ratios 872 00:44:07,560 --> 00:44:10,830 can depend on which isotope of silicon you have, 873 00:44:10,830 --> 00:44:14,230 what sort of cluster, what molecule, what charge you have. 874 00:44:14,230 --> 00:44:16,530 And you can do some pretty crazy analysis of things 875 00:44:16,530 --> 00:44:18,420 to even figure out what sort of compounds 876 00:44:18,420 --> 00:44:21,870 exist on surfaces, because sometimes you sputter off 877 00:44:21,870 --> 00:44:23,220 whole molecules. 878 00:44:23,220 --> 00:44:26,170 They're going to have their own mass to charge ratio. 879 00:44:26,170 --> 00:44:31,260 And that's what we did for this lead bismuth work. 880 00:44:31,260 --> 00:44:33,750 Lots of XPS spectra to jump through. 881 00:44:33,750 --> 00:44:37,170 We wanted to find out which oxides were forming 882 00:44:37,170 --> 00:44:40,110 and in what order. 883 00:44:40,110 --> 00:44:41,820 Which one's the best one I want to show? 884 00:44:44,277 --> 00:44:45,110 Think it's this one. 885 00:44:48,260 --> 00:44:52,330 So in this case, we used ion sputtering 886 00:44:52,330 --> 00:44:55,810 to sputter away surface layers to a depth of a few hundred 887 00:44:55,810 --> 00:44:59,800 nanometers, and we're actually able to show that the chromium 888 00:44:59,800 --> 00:45:03,640 oxide, right here, was on the outside of the sample, 889 00:45:03,640 --> 00:45:07,540 followed by silicon oxide, followed by iron metal. 890 00:45:07,540 --> 00:45:10,330 So in this way, we were able to figure out 891 00:45:10,330 --> 00:45:12,280 using XPS, the nature of the oxides 892 00:45:12,280 --> 00:45:15,160 and using SIMS, the order of the oxides, 893 00:45:15,160 --> 00:45:18,100 so not only how fast were they growing to nanometer precision, 894 00:45:18,100 --> 00:45:20,920 but in what order did they form. 895 00:45:20,920 --> 00:45:22,610 And that helped us figure out this sort 896 00:45:22,610 --> 00:45:27,412 of synergistic chromium and silicon oxidation mechanism 897 00:45:27,412 --> 00:45:29,870 that helps really protect the layers of the stainless steel 898 00:45:29,870 --> 00:45:34,350 and explain why it's corrosion resistant lead bismuth, 899 00:45:34,350 --> 00:45:38,970 all using principles from 22.01. 900 00:45:38,970 --> 00:45:41,607 So it's about two of five of. 901 00:45:41,607 --> 00:45:43,440 So I wanted to stop and see if you guys have 902 00:45:43,440 --> 00:45:45,500 any questions on these analytical techniques, 903 00:45:45,500 --> 00:45:47,250 knowing that we're actually going to go do 904 00:45:47,250 --> 00:45:50,690 a couple of these next Friday. 905 00:45:50,690 --> 00:45:53,508 Has anyone used any of these before? 906 00:45:53,508 --> 00:45:54,800 Yeah, which ones have you used? 907 00:45:54,800 --> 00:46:01,260 AUDIENCE: SEM and XPS, XPS [INAUDIBLE] 908 00:46:01,260 --> 00:46:03,110 MICHAEL SHORT: SEM and XPS? 909 00:46:03,110 --> 00:46:04,108 OK, cool. 910 00:46:04,108 --> 00:46:05,650 Yeah, we've got all these instruments 911 00:46:05,650 --> 00:46:07,630 I think except for SIMS here at MIT. 912 00:46:11,960 --> 00:46:12,470 Yeah. 913 00:46:12,470 --> 00:46:12,970 Yeah. 914 00:46:12,970 --> 00:46:14,845 AUDIENCE: Sorry, what is that second equation 915 00:46:14,845 --> 00:46:18,585 on the energy for Compton scattering? 916 00:46:18,585 --> 00:46:21,210 MICHAEL SHORT: This would be the energy of the Compton electron 917 00:46:21,210 --> 00:46:25,440 that comes out when a photon scatters off of it. 918 00:46:25,440 --> 00:46:28,020 So the photon will end up losing some energy, 919 00:46:28,020 --> 00:46:32,165 and the Compton electron will pick up that energy. 920 00:46:32,165 --> 00:46:33,040 AUDIENCE: [INAUDIBLE] 921 00:46:33,040 --> 00:46:33,370 MICHAEL SHORT: Sorry? 922 00:46:33,370 --> 00:46:35,530 AUDIENCE: What's the denominator of that? 923 00:46:35,530 --> 00:46:37,810 MICHAEL SHORT: It's a 1 plus alpha times 1 924 00:46:37,810 --> 00:46:40,030 minus cosine theta, where alpha-- 925 00:46:42,745 --> 00:46:45,330 I'll mention what alpha is. 926 00:46:45,330 --> 00:46:50,230 It's a ratio of the photon energy to the electron rest 927 00:46:50,230 --> 00:46:51,070 mass energy. 928 00:46:57,400 --> 00:46:59,740 This is kind of a nice-- on these two boards right here, 929 00:46:59,740 --> 00:47:01,960 it's kind of a nice summary of the stuff we've 930 00:47:01,960 --> 00:47:03,912 been doing over the last three weeks or so, 931 00:47:03,912 --> 00:47:05,620 and then all the stuff I showed you today 932 00:47:05,620 --> 00:47:07,890 is what you can do with it.