116 



RADIATION BIOLOGY 



because of their higher absorption coefficient. Therefore the average energy of 

 narrow-beam neutrons transmitted by an absorbing layer of matter is an increas- 

 ing function of the layer thickness. 



4-4b. Scattered and Secondary Neutrons. The accumulation of scat- 

 tered neutrons in a material influences the over-all penetration even more 

 than the accumulation of scattered X rays because the neutrons survive 

 a much larger number of scattering processes. Secondary neutrons are 

 not formed very frequently except under conditions in which (n,2n) proc- 

 esses are favored or in the presence of uranium or other fissionable mate- 

 rials (see Sect. l-2d). 



Since neutrons are scattered in all directions more evenly than high- 

 energy X rays, scattered neutrons accumulate to a far greater extent near 

 the place where they first enter a material or where they originate. Back- 

 scattering of neutrons from the surface of a thick layer of material is ciuite 

 important and may approach 100 per cent. The trend of the build-up 

 factor at great depths in a material depends on the same considerations as 

 in the case of X rays (see Sect. 4-3b). 



The mathematical methods which have been developed to calculate 

 the distribution and diffusion of neutrons have been described in the 

 literature, for example, by Marshak et al. (1949). The results of many 

 detailed applications are presumably still withheld for security reasons. 



As indicated above, the degree of accumulation of scattered neutrons, 

 i.e., the value of the build-up factor, depends on the duration of survival 

 of a neutron before eventual capture. The probability of outright cap- 

 ture varies very greatl}' from one material to another. Therefore the 

 presence of traces of highly capturing elements may have a great influence 

 on the neutron propagation through a material. Table 1-11 gives data 



Table 1-11. Capture Component of the Narrow-beam Absorption Coefficients 



FOR 0.025-ev Neutrons 

 (For complete data see Way et al., 1950) 



on the contribution of outright capture to the absorption coefficients of a 

 few elements. 



4-4c. Energy Distribution. Much of the energy released by neutrons within a 

 material is dissipated by heavy charged particles. The tracks of these particles 

 are quite short in nongaseous materials, less than 1 mm except when the neutron 

 energy exceeds 10 Mev and still shorter at lower energies. Therefore many of 

 the complications which influence the spatial distribution of X-ray energy have 

 little influence in the case of neutrons. The eventual distribution in space of 

 neutron energy parallels rather closely the distribution of the energy transfers 

 from the neutrons to heavy charged particles, except for the effect of the second- 

 ary 7 rays, which will be discussed separately. 



