158 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 6 



Table 24. Yield of Hard Gamma Rays from Fission Products [17] 



energies greater than 2.17 mev from fission products indicate about eight 

 gamma-ray components, as shown in Table 24. 



6.7. Prompt Neutrons. On the basis of the existence rules for stable 

 isotopes (Sec. 1.1) it is apparent that the two fragments of medium atomic 

 weight formed in the fission of a heavy element must contain a considerable 

 excess of neutrons compared with stable nuclei with the same number of 

 protons. Some of the excess neutrons therefore are very weakly bound, and 

 in view of the high excitation energy given fission fragments in the process of 

 division, a portion of these neutrons boil off instantly. The average number 

 of prompt neutrons per fission has been extensively investigated, and the 

 values that have been reported vary from 1.5 to as many as 6. The most 

 reliable of these measurements gives values between two and three prompt 

 neutrons per fission, on the average. Although the reported experimental 

 results do not appear conclusive for accuracies greater than this, they tend to 

 indicate an average value of about 2.6. This value can be regarded only as a 

 tentative estimate until experiments demonstrate conclusively the magnitude 

 of the second significant figure. 



The observed energy distribution of prompt neutrons is found to be not 

 only continuous but also to be very nearly a Maxwellian distribution when the 

 translational component of the fission fragment is subtracted. The super- 

 position of the translational component of the fragment and the Maxwellian 

 distribution bears out the contention that the neutrons are emitted from the 

 fragments and not in the primary fission process. The form of the distribu- 

 tion is consistent with the assumption that at high excitation energies many 

 nuclear levels may be affected and because of their great density they present 

 an essentially continuous spectrum of possible transitions by neutron emission. 

 The most likely value of the mean energy per prompt neutron appears to be 

 approximately 2 mev, and therefore, the total average neutron energy is 4 to 6 

 mev per fission. 



