NUCLEAR FISSION 281 



that certainly it is not always and possibly it is never an initial frag- 

 ment.) Others were unknown till 1939. 



So numerous are these and the other fission-products still unrecog- 

 nized, that the "decay-curve" for a piece of bombarded uranium or 

 for the deposit on a nearby collector, due to all of them conjointly, 

 looks like the resultant of contributions practically limitless in number 

 and with a random distribution of half-periods. Not only is this also 

 true when neutrons impinge on thorium, but the curves for the two 

 elements cannot be told apart! Only after chemical separations have 

 been made can individual half-periods be sorted out from among the 

 welter; and if there are some characteristic differences between the 

 results of the fission of uranium and those of the fission of thorium, 

 they have not yet been proved. 



Special interest attaches to the fission-products which are gaseous. 

 They can be separated physically from the rest: the fission-products 

 are received or dissolved into water (or indeed the uranium may be 

 exposed to neutrons while in aqueous solution) and through the water 

 a stream of air is bubbled, which takes along these particular ones to 

 distant points in the system of tubing where they and their descendants 

 can be studied. They cannot themselves be identified, but among 

 their descendants are found (radioactive) isotopes of srRb and ssCs; 

 therefore the gases comprise unstable isotopes of krypton (seKr) and 

 xenon (54Xe). Could these be initial fragments of various types of 

 fission? If so, their mates are seBa and ssSr. Now, barium and 

 strontium are found indeed among the fission-products, which seems 

 to sustain this idea. But barium and strontium may also be the 

 immediate descendants of the caesium and the rubidium aforesaid. 

 This alternative idea is testable; and according to Hahn and Strass- 

 mann, two among the three barium isotopes (Ba^^^ being one of the 

 two) are surely descendants of caesium, while the third may be an 

 initial fragment. 



Many other such "genetic" relationships have been published, but 

 it would be lengthy and might be premature to quote them. I will 

 mention at least that several sequences have been traced by Abelson 

 in greater or less detail among the many fission-products which are 

 isotopes of the three consecutive elements oiSb, 52Te, and ssl- A 

 special interest attaches to one of these bodies, the " 77-hour tellurium " ; 

 for it has been identified as tellurium not only by its chemical proper- 

 ties but also by its X-rays. Let us pause to consider this. 



The ordinary way of evoking an X-ray spectrum is to use the element 

 in question as the target, or a constituent of the target, of an X-ray 

 tube. This means that the atoms are excited by projecting electrons 



