Sec. 5.3] NEUTRO.XS 1 23 



Normally when a slow neutron is captured by a light or medium nucleus, 

 only one such process is highly probable. At high excitation energies, how- 

 ever, several processes may compete with probabilities of roughly the same 

 magnitude. 



The kinds of interactions that occur when nuclei are bombarded with 

 neutrons depends, superficially at least, on the kinetic energy of the incident 

 neutrons and on the particular kind of nucleus. In all cases neutron inter- 

 action leads to one of the following processes: 



1. Elastic scattering. The kinetic energy and momentum of the incident 

 neutron are shared by the recoil neutron and nucleus according to the laws 

 of conservation of energy and momentum; otherwise the struck nucleus is 

 left in its initial state. 



2. Radiative capture (n, 7). The struck nucleus retains the neutron and 

 emits a gamma ray. 



3. Neutron emission (n, n), (n, 2n), etc. After capture of a neutron, one or 

 more neutrons are boiled off. 



4. Charged-particle emission (n, p), (n, d), (n, a), etc. After capture of a 

 neutron one or more charged particles, or a combination of particles, are 

 emitted. 



5. Fission (n, f). A neutron is captured and the compound nucleus splits 

 into two large fragments (see Fission, Chap. 6). 



6. Other processes are reported in which very high-energy neutrons (~ 100 

 mev) totally disintegrate the nucleus or lead to the emission of a large number 

 of particles (spallation). 



All but elastic scattering are capture processes. The incident neutron 

 plus the struck nucleus (Z, A) form a compound nucleus (Z, A + 1) which 

 instantly (~ 10~ 10 to 10 -20 sec) disintegrates by one of the processes indicated 

 above. The residual nucleus is, except when elastic re-emission of the neu- 

 tron occurs, radioactive and subsequently decays at a relatively slow rate by 

 emission of those radiations characteristic of radioactive decay. 



5.3. Elastic Scattering of Neutrons. The precise manner in which neutrons 

 are scattered by nuclei depends in a complicated way on the nuclear forces. 

 Scattering into certain angles may be favored over others (anisotropic 

 scattering), and the cross section usually exhibits strong dependence on 

 neutron energy. At the present time evaluation of such effects of nuclear 

 interaction is possible only from experiment. For many purposes, however, 

 scattering may be considered isotropic in that all angles as measured from the 

 center of mass coordinate system are equally probable. 



If scattering is both isotropic and elastic, neutrons are then scattered by 

 nuclei in essentially the same manner as ideal gas molecules. The kinetic 

 energy and momentum of the incident neutron are shared by the recoil 

 neutron and nucleus as required by the conservation laws of energy and 



