54 NEUTRONS AND THEIR SPECIAL EFFECTS 



(6) What is the probabihty for the conversion of a gamma ray in these proc- 

 esses? The chemical effects resulting from this process may well be more 

 important than the mechanical effects of the recoil. 



Solomon: 



With middle- and high-atomic-number atoms, frequeiit multiple processes 

 yielding more than one photon are likely; with low-atomic-number atoms, a 

 single photon is more probable. 



Morrison: 



Very recent work at Chalk River indicates that, at least with N^^, capture 

 results in a mixture of gammas, principally 4 gammas from two cascades, all 

 between 4 and 6 me v.* 



Platzman : 



Magee is certainly correct in calling attention to the fact that further com- 

 plexity in the spectrum of recoil energies associated with capture of thermal 

 neutrons is contributed by internal conversion of the capture gamma rays. 

 There is no reason, of course, why capture gamma rays should exhibit internal 

 conversion phenomena at all different from those of ganmia rays of any other 

 nuclear origin. [Except for the high conversion associated with the slow radia- 

 tion of isomers. A rather special circumstance — that capture gamma rays 

 commonly involve the consecutive emission of several "cascade" photons from 

 the same nucleus — will not introduce any special effect, because, just for a 

 gamma-ray transition which has appreciable conversion coefficient, the average 

 time elapsing before emission of the photon will be longer than that required 

 for refilling a depleted inner electronic shell of the atom; the inner part of the 

 atom, which is the only portion relevant for the internal conversion, reverts to 

 normalcy after each stage of the cascade. Quite obviously, complexity of the 

 gamma-ray spectrum, and angular correlation between some of the successive 

 photons, will lead to a most intricate spectrum of recoil energies. Ed.] 



Only very little information on the spectra of capture gamma rays has at the 

 present time been attained. The few cases which have been studied show \'ery 

 complex capture gamma-ray spectra, apparently even for atoms of fairly low 

 atomic weight. The only instance in which internal conversion of capture 

 gamma rays has been studied involves bromine, f and here the effect was indeed 

 found. This experiment is, incidentally, a difficult one. The results indicate 

 at least 0.15-0.40 conversion electron per neutron captured, but they cannot be 

 interpreted adequately because the spectrum of the capture gamma rays is not 

 fully known. On general grounds, one must expect that the average conversion 

 coefficient of all capture gamma rays for any one nucleus will in most cases be 

 small, because the nucleus, in its transformation from the initial, highly excited 

 state following neutron capture to its ground state, will pursue a path from one 



* S. Wexler and T. H. Davies, Report BNL-C-7, p. 82, 1948. 

 t B. B. Kinsey et al, Phys. Rev., 77: 723(L), 1950. 



