152 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS 



[Chap. 6 



Fig. 45, the masses tend to group into symmetrical distributions about a 

 light and heavy component. The most probable fragments from U 236 , for 

 example, are those with masses 95 and 140, corresponding to the maxima of 

 the empirical yield curve. These values appear to be nearly the same also 

 for U 239 and Th 233 . The total variation in mass ranges from approximately 



70 



80 



90 



130 



140 



150 



160 



100 110 120 



MASS NUMBER 

 Fig. 45. Yield of fission products from U 235 as a function of mass number. [From J. Am. 

 Cliem. Soc. 68, 2435 (1946-).] 



70 to 160 although a very high percentage of the yield is confined to the two 

 mass groups 80 to 110 and 128 to 154. In all instances the sum of the 

 atomic weights of the two fragments is somewhat smaller than the initial 

 nucleus owing to the loss of two to three prompt neutrons. No satisfactory 

 theory has as yet been provided which accounts for the asymmetry in fission 

 yield. 



