1 00:00:04,900 --> 00:00:07,440 MARKUS KLUTE: Welcome back to 8.701. 2 00:00:07,440 --> 00:00:10,740 We continue discussion of nuclear decays 3 00:00:10,740 --> 00:00:12,820 with gamma decays. 4 00:00:12,820 --> 00:00:16,260 We have seen that we can understand nuclear stability 5 00:00:16,260 --> 00:00:17,670 or instability. 6 00:00:17,670 --> 00:00:20,550 We discussed alpha decays and beta decays. 7 00:00:20,550 --> 00:00:23,400 Now after the discussion of the shell model, 8 00:00:23,400 --> 00:00:29,730 it is apparent that transition from various nuclear states 9 00:00:29,730 --> 00:00:33,575 can be accomplished via the admission of a photon, a gamma 10 00:00:33,575 --> 00:00:35,340 ray. 11 00:00:35,340 --> 00:00:40,740 Gamma decays are specifically important in decay chains 12 00:00:40,740 --> 00:00:43,680 following an alpha decay or a beta decay 13 00:00:43,680 --> 00:00:46,630 where the remnant, the daughter nuclei, 14 00:00:46,630 --> 00:00:48,780 is left over in an excited state. 15 00:00:48,780 --> 00:00:54,120 Then the de-excitation follows with the admission of a photon. 16 00:00:54,120 --> 00:00:56,670 Practical consequences of this alpha example-- 17 00:00:56,670 --> 00:01:00,930 in fission processes where a significant amount of energies 18 00:01:00,930 --> 00:01:04,530 can be released with photons, radiotherapy 19 00:01:04,530 --> 00:01:08,430 where we try to remove cancer cells 20 00:01:08,430 --> 00:01:12,870 or kill cancer cells with gamma rays. 21 00:01:12,870 --> 00:01:15,930 Medical imaging works this way. 22 00:01:15,930 --> 00:01:19,710 And in general, you can use the emission of those photons 23 00:01:19,710 --> 00:01:23,130 to deduce the spin and the parity of excited states. 24 00:01:23,130 --> 00:01:26,110 If you go back very early to this lecture, 25 00:01:26,110 --> 00:01:28,470 we discussed the Wu experiment. 26 00:01:28,470 --> 00:01:31,290 And also there we used gamma rays 27 00:01:31,290 --> 00:01:36,730 in order to reduce the spin and the parity of the states 28 00:01:36,730 --> 00:01:37,230 involved. 29 00:01:39,990 --> 00:01:43,760 So nuclear spectroscopy, we haven't discussed the detectors 30 00:01:43,760 --> 00:01:44,260 yet. 31 00:01:44,260 --> 00:01:48,590 But what's shown here in this picture 32 00:01:48,590 --> 00:01:55,740 are two characteristic gamma ray spectra 33 00:01:55,740 --> 00:02:00,000 from the case of cobalt and cesium. 34 00:02:00,000 --> 00:02:03,060 And if you just focus on the blue line, for example, 35 00:02:03,060 --> 00:02:06,000 you see here this peak. 36 00:02:06,000 --> 00:02:09,800 This corresponds to the energy of a transition. 37 00:02:09,800 --> 00:02:12,380 But then photons, when they're emitted, 38 00:02:12,380 --> 00:02:14,210 they can go through Compton scattering. 39 00:02:14,210 --> 00:02:15,920 And they can lose through Compton 40 00:02:15,920 --> 00:02:17,310 scattering some of the energy. 41 00:02:17,310 --> 00:02:19,290 So you see this tail here. 42 00:02:19,290 --> 00:02:22,338 And then in this tail you see additional peaks. 43 00:02:22,338 --> 00:02:24,380 And those additional peaks can come from the fact 44 00:02:24,380 --> 00:02:31,280 that a photon can produce an electron-positron pair. 45 00:02:31,280 --> 00:02:33,590 And then you see one or two of those pairs 46 00:02:33,590 --> 00:02:38,270 in cases where one electron or positron, or both of them, 47 00:02:38,270 --> 00:02:40,680 have escaped the detection. 48 00:02:40,680 --> 00:02:43,370 So whenever you look at a nuclear decay, 49 00:02:43,370 --> 00:02:45,170 you find spectrums of the sort. 50 00:02:45,170 --> 00:02:50,900 And then there are various Compton scattering-- 51 00:02:50,900 --> 00:02:52,610 depends obviously on the material 52 00:02:52,610 --> 00:02:55,340 around and the composition and also 53 00:02:55,340 --> 00:02:58,460 this single and double escape kind of peaks, 54 00:02:58,460 --> 00:03:02,010 quite characteristic for the material you're looking at. 55 00:03:02,010 --> 00:03:05,480 So from this, on nuclear spectroscopy 56 00:03:05,480 --> 00:03:08,360 you can learn about the sample composition, the element 57 00:03:08,360 --> 00:03:12,836 composition of the probe. 58 00:03:12,836 --> 00:03:16,820 And interesting effect is the Mössbauer effect. 59 00:03:16,820 --> 00:03:18,560 Here again, I'll try to remind you 60 00:03:18,560 --> 00:03:21,330 of the discussion of special relativity. 61 00:03:21,330 --> 00:03:26,450 We looked at the energy of an emitted and an absorbed photon. 62 00:03:26,450 --> 00:03:35,450 And because in this emission process the leftover nuclei, 63 00:03:35,450 --> 00:03:36,960 there has to be a momentum balance. 64 00:03:36,960 --> 00:03:40,620 So there is a recoil on the leftover nuclei. 65 00:03:40,620 --> 00:03:42,290 So that means it starts moving. 66 00:03:42,290 --> 00:03:45,650 And because it's moving, we have to do a Doppler correction 67 00:03:45,650 --> 00:03:50,450 of the energy, meaning that the emitted photon energy is not 68 00:03:50,450 --> 00:03:53,510 equal to the energy needed in order to excite 69 00:03:53,510 --> 00:03:56,340 the nuclear state again. 70 00:03:56,340 --> 00:03:58,310 So this leads then to those energy spectrum. 71 00:03:58,310 --> 00:04:00,170 Here, they're the natural widths. 72 00:04:00,170 --> 00:04:05,060 And only in this overlapping region here you can reabsorb. 73 00:04:05,060 --> 00:04:10,340 Now the most Mössbauer effect now is a special variation 74 00:04:10,340 --> 00:04:12,380 of what I just described. 75 00:04:12,380 --> 00:04:19,790 In cases where the nucleon is part of a lattice, 76 00:04:19,790 --> 00:04:23,480 the lattice can absorb the recoiling energy. 77 00:04:23,480 --> 00:04:28,700 And it leads like to a situation of very, very heavy objects 78 00:04:28,700 --> 00:04:30,030 absorbing this recoil. 79 00:04:30,030 --> 00:04:33,290 And in those cases, you can have resonant effects, meaning 80 00:04:33,290 --> 00:04:35,840 that this emission line, then absorption lines, 81 00:04:35,840 --> 00:04:39,820 they lay over each other quite strongly.