Sec. 6.9] 



FISSION 



159 



The directional distribution of prompt neutrons has been shown by Wilson 

 [28] to be consistent with the assumption of isotropic evaporation from the 

 moving fission fragments. 



6.8. Delayed Neutrons. Delayed neutrons are emitted from fission 

 products during the course of radioactive decay. They are emitted, how- 

 ever, from only a few of the many possible fission products that may be 

 formed, and consequently are low in intensity compared with prompt 

 neutrons. The observed intensity under equilibrium conditions is 1.0 ± 0.2 

 per cent of the abundance of prompt neutrons [18]. 



A delayed neutron is emitted only after the initial fission fragment has 

 undergone beta decay. If, after beta emission, the residual nucleus still 

 possesses an excitation energy greater than the neutron binding energy, it 

 may decay by further beta transitions or by neutron emission; the latter 

 process usually reduces the nucleus to the ground state. Although the neu- 

 tron is emitted in an exceedingly short time after the nucleus is reduced to 

 the appropriate excited level, the observed half-time for neutron emission 

 following the primary fission process is controlled by the preceding beta 

 transition and, therefore, has the same half-life. Six such half-life periods 

 have been found for delayed neutrons as shown in Table 25. 



Table 25. Delayed Neutrons [29] 



* Relative to total neutron emission. 



These six periods are observed in the fission of both U 236 and Pu 239 and 

 presumably are associated with the same fission fragments from either 

 element. 



6.9. Alpha Particles. Alpha particles have been found to be associated 

 occasionally with fission of both U 235 and Pu 239 . Farwell et al. [11] observed 

 a continuous energy distribution with a maximum energy of about 16 mev 

 and were able to show that the alpha particles were emitted within 5 X 10 -6 

 sec after the primary fission process. Wollan et al. [20], using photographic 

 plates, found the maximum energy to be 22 mev (40 cm range in air at normal 

 temperature and pressure) and demonstrated that alpha particles were 

 emitted in the primary fission process and not from the fragments. It was 



