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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1964 



most commonly used isotope is cobalt-57, which decays by the emis- 

 sion of beta rays to iron-57, with a half -life of 280 days. 



Following the beta emission, the iron-5T nucleus is in an excited 

 state which lasts for about a tenth of a microsecond, a reasonably long 

 time as nuclear lifetimes go. It now emits a gamma ray of about 14 

 kilo-electron-volt energy, which corresponds to a frequency of 3 X 10^^ 

 oscillations per second. This gamma ray may be visualized as a wave 

 packet containing about 10^^ waves altogether. 



If this packet encounters another iron nucleus likewise bound in a 

 crystal lattice, it is able to raise the nucleus up to its 14 kev. excited 

 state, and is absorbed in the process. The nucleus behaves as a very 

 delicate frequency-measuring device. If the frequency of the incom- 

 ing wave varies by only one part in 10^^, the probability of absorption 

 will be reduced by a large factor. 



Thus we have a wave whose frequency is very sharply defined, and 

 we also have a measuring device which is equally sensitive to changes 

 in frequency. 



So sensitive is tliis system that if the source is moving only a few 

 millimeters per second with respect to the absorber, the resonance is 

 wiped out, due to the Doppler shift in the emitted frequency. The 

 standard method of doing a Mossbauer effect experiment is to have a 

 radioactive source mounted on some device (such as a lathe carriage) 

 which can move it at a known speed toward or away from a scintilla- 

 tion counter which counts the number of photons transmitted through 

 an absorber (fig. 4). If the source is iron-57, the absorber is usually 

 of iron enriched in the isotope iron-57. By plotting the number of 

 photons counted in a given time at various source velocities, the reso- 

 nance curve shown in figure 5 may be obtained. 



The measurement of this type of resonance curve is the basis for all 

 of the recent experiments which have been performed to test the theory 

 of relativity. 



The theory of the Doppler shift informs us that the frequency of 

 the emitted radiation is increased when the source moves toward the 

 absorber, and is decreased when the source moves away from the 



RADIOACTIVE 

 SOURCE 



ABSORBER 



TT-n 



SCINTILLATION 

 COUNTER 



Figure 4. — The basic apparatus for a Mossbauer effect experiment. 



