Sec. 6.2] 



FISSION 



149 



with a total kinetic energy of about 160 mev. The high excitation energy of 

 the fragments following fission is subsequently lost by the emission of prompt 

 (instantaneous) neutrons and gamma rays and through radioactive chains 

 leading finally to known stable nuclei. 



6.2. Fission Probability. The probability of fission induced by an inci- 

 dent neutron depends (1) on the probability of neutron capture to form a 

 compound nucleus and (2) on the relative probabilities for deexcitation of the 

 compound nucleus by radiation, neutron emission, and fission. Both the 

 neutron binding energy and the critical energy for fission in the heaviest 

 nuclei lie in the energy range of 5 to 7 mev. When the binding energy plus 

 the kinetic energy of the captured neutron is less than the critical fission 



r. 



NEUTRON BINDING 

 (ODD A) 

 FISSION ENER8Y 



o y 



u. m 



NEUTRON BINDING 

 (EVEN A) 



SROUNO LEVEL — 



> 

 O 



or 



ill 



ELECTROSTATIC FIELD 



Fig. 43. 



DISTANCE FROM CENTER OF NUCLEUS 

 Diagrammatic representation of potential fields of nucleus. 



energy, gamma-ray and neutron emission are more probable than fission, 

 but if it is greater, fission of the compound nucleus is most probable. Thus, 

 neutrons with only thermal kinetic energies induce fission in the nuclei U 23 " 

 and Pu 239 with greater probability than for radiative capture since the neutron 

 binding energies of 5.4 and 6.4 mev are greater than the respective critical 

 energies of 5.2 and 5 mev. On the other hand, the probability of fission in 

 U 23S and Th 232 becomes important only for fast neutrons since the critical 

 energies are of the order of 1 mev greater than the neutron binding energy. 

 In general, the fission barrier height in the heaviest nuclei with odd atomic 

 weight is lower than the neutron binding energy, thus allowing fission to be 

 induced by thermal neutrons. 



The absolute fission cross section depends, as in all other nuclear reactions, 

 on the neutron energy. When fission can be induced by thermal neutrons, 

 well-defined resonance peaks may be involved at certain energies. The 

 fission cross section at exact resonance is then many times greater than its 

 off-resonance value. The cross section in the thermal-energy region when 



