RECENT EVENTS IN RELATIVITY — ROTHMAN 



391 



Co 



57 



i 



14Kev 



Gamma 



Ray 



Fe 



57 



57 



Figure 3. — Radioactive cobalt-S7 decays by beta emission to an excited state of iron-S7, 

 which quickly emits a 14-kev gamma ray. If this gamma ray photon encounters 

 another nucleus of iron-S7, it may be absorbed. 



that when a nucleus has been raised to an excited state, it decays to the 

 normal ground state by emitting a gamma ray of a definite frequency. 

 If this gamma ray now encounters another nucleus of the same kind 

 as the first, this second nucleus may now be raised to its excited state 

 by absorbing the energy of the gamma ray (fig. 3). 



This is a resonance effect. If the gamma ray differs in frequency 

 by as little as one part in 10" of the resonance frequency, the absorp- 

 tion will be greatly reduced — the amount of reduction depends upon 

 the "width," or energy spread, of the excited level. Theoretically, at 

 least, the absorption can be measured by counting the gamma rays 

 from an appropriate radioactive source first with and then without 

 the proper absorber between the source and counter. 



Unfortunately, when a nucleus emits a gamma ray, some of the 

 energy of the excited state goes into recoil motion of the nucleus. This 

 means that the gamma ray frequency is reduced considerably, so that 

 when it hits a nucleus which might be receptive to it, it is far-off 

 resonance, and there is no absorption at all. In the past, people have 

 managed to compensate for this recoil motion by heating the source 

 or by whirling it around in a centrifuge. 



In 1958, R. L. Mossbauer, a young German physicist, discovered 

 that in a few favorable cases one could obtain gamma rays with prac- 

 tically no recoil at aU (Mossbauer, 1958 ; Benedetti, 1960) . There are 

 a few radioactive elements which emit rather low energy gamma rays 

 and which are so strongly bound in their crystal lattice that the recoil 

 energy is taken up by the crystal as a whole rather than by the individ- 

 ual radiating nucleus. 



Immediately, a number of physicists realized that the Mossbauer 

 effect provided a source of radiation of unparalleled precision, as far 

 as the energy (or frequency) of the radiation was concerned. The 



