282 BELL SYSTEM TECHNICAL JOURNAL 



against them. They may also be excited by projecting photons 

 against them, and this is sometimes done. Both of these ways are 

 completely out of the question as yet with any artificial radioactive 

 substance, for the greatest amount yet produced of any of these is so 

 small that if its atoms were placed in a target, the hits made upon 

 them by electrons or photons projected in streams of any feasible 

 strength would not be numerous enough to produce detectable X-rays. 

 If, however, the necessary photons proceed from the nuclei of the 

 atoms themselves, then the whole situation is changed, because now 

 the efficiency of excitation is so great. Such is the case with many of 

 the natural radioactive substances, and now also (it appears) with the 

 "77-hour tellurium." Excited presumably by photons proceeding 

 from their nuclei, ^^ the atoms emit X-rays, and these have been found 

 (by Abelson in Berkeley, by Feather and Bretscher in England) to be 

 the characteristic rays of the K-series of iodine. " Iodine" here is not 

 a misprint for tellurium! When the nucleus is radioactive by virtue 

 of the emission of an electron, the photon (if any) leaves after the 

 electron is gone, by which time the atom is already an atom of the 

 daughter-substance.^^ 



Now we take up the yield of the fission -process: how does it depend 

 on the energy of the incident neutrons? 



Here uranium sets itself apart from the two other fissurable elements. 

 Thorium and protactinium respond to fast neutrons only, uranium 

 both to slow and to fast (but not to intermediate) neutrons. It is, 

 however, believed that with uranium, one isotope is sensitive only to 

 fast and another both to fast and to slow (or possibly only to slow). 

 There are good theoretical grounds for this belief, and also for choosing 

 the respective isotopes; but as yet there is not the certainty to be 

 expected from some future and probably imminent experiment on 

 separated isotopes. ^^ Accepting nevertheless the current belief, we sup- 



^1 Another mode of excitation is now known: an electron may fall into a nucleus, 

 and by quitting its place in the orbital electron-family create the condition for the 

 emission of an X-ray photon. Whether this or the other or both be the mode of 

 excitation of the 77-hour tellurium is not yet certainly known. 



12 As this is likely to cause confusion, I emphasize that when the chemical separa- 

 tion is made, the atoms which have not yet emitted nuclear electrons are still tellurium 

 atoms, and when they manifest themselves by that emission it appears among the 

 tellurium; from then on they are iodine atoms among the tellurium, but no longer 

 manifest themselves except through these X-rays. One may wonder whether the 

 "transuranic elements," to which the 77-hour body was formerly thought to belong, 

 would have been discredited if this measurement on the X-rays had been made 

 earlier. Well, the measurement was earlier made (though not so precisely) and the 

 rays were interpreted as characteristic X-rays of the L-series of a transuranic element. 

 It is hard to make a guess as to whether further and better measurements would 

 have destroyed this possibility. 



" As these pages start for the press I am authorized to say that the separation has 

 been achieved h\ Nier and the experiment performed by Dunning, Booth and von 

 Grosse. The "light fraction," consisting of U^^ with a small proportion of the very 

 rare isotope U^'^ is definitely sensitive to slow neutrons; U^^' is definitely not. 



