Sec. 6.5] FISSION 153 



The distribution in charge of primary fission fragments is less easy to 

 establish, and conclusive experimental or theoretical results have not yet 

 been reported. It is likely however that there is no unique charge associated 

 with each mass but rather a number of possible values of charge distributed 

 about a mean value for each mass. 



The greatest portion of the total energy released in fission appears as 

 kinetic energy of the fragments. A rough estimate of the recoil energy may 

 be calculated from the electrostatic analogy of two charged particles. Assum- 

 ing the fission fragments to be two spheres with charges Z x and Z 2 and initially 

 in contact, the potential energy is then 



P eZiZo 



= = 1.47 X lO- 13 ^ + A 2 *) ergS 



where A\, Ao = atomic weights of fragments 



The recoil energy computed by this method is approximately 200 mev, about 

 25 per cent greater than the best experimental value. Measurements of the 

 average recoil energy have been made by many investigators [10,15,21,22, and 

 others] who have reported values varying from 120 to nearly 200 mev. The 

 total kinetic energy imparted to the fragments does not have a unique value 

 but exhibits a statistical variation about a mean value of about 160 mev 

 [21,23,24,25,26]. The distribution appears to be symmetrical about this 

 value and has a spread from 120 to nearly 200 mev. Each of the two 

 fragments from a single fission share the total kinetic energy inversely as 

 their respective masses, E\/E 2 = M2/M1. When the observed energies of 

 the fragments are plotted separately, two symmetrical distributions are 

 obtained with mean values of approximately 60 and 92 mev, corresponding 

 to the heavy and light components, respectively. The width of the low- 

 energy peak at one-half its maximum value is approximately 25 mev, and 

 for the high-energy component it is about 16 mev. The characteristics of 

 the energy-distribution curve appear to be nearly the same for Th 232 , U 235 , 

 U 238 , and Pu 239 . Although small differences for these isotopes have been 

 found, the variation in values reported for any one fissionable isotope appears 

 to be greater than the differences between the energy distributions for the 

 four isotopes above. 



6.5. Absorption and Range of Fission Fragments. The interaction of 

 fission fragments with matter involves the same processes as in the stopping 

 of lighter charged particles such as alpha particles, namely, ionization and 

 elastic nuclear collisions. Nevertheless, the characteristics of the rate of 

 energy loss and the range of fission fragments as functions of velocity are 

 strikingly different than for alpha particles owing to their great mass, high 

 charge number, and relatively low velocity. The range is small, the ioniza- 

 tion is many times more intense than that produced by alpha particles, and 



