114 RADIATION BIOLOGY 



eventually onl.y after hundreds of collisions. As indicated in Sect. 2-5, 

 the (n,n) type of reaction, in which neutron capture is followed by the 

 release of a low-energy neutron, is quite common in "heavy" materials 

 and has the same effect as an inelastic scattering. On the whole, scatter- 

 ing constitutes the most important process in the passage of neutrons 

 through matter under most conditions. Therefore the flow of neutrons 

 takes primarily the character of a diffusion phenomenon. 



4-4a. Narrow-beam Penetration. The attenuation of a narrow beam 

 may be regarded as a basic phenomenon for neutrons as well as for X rays, 

 even though it is perhaps less characteristic for neutron than for X-ray 

 penetration. 



If the neutrons are monoenergetic, the attenuation follows the law of 

 Eq. (40): 



Nix) = N{Q)e->^- (46) 



Here N{x) indicates the neutron flux (particles per unit area per unit time) 

 after traversing a thickness x of material, and ju is the absorption coefficient for 

 neutrons of the given energy. 



The absorption coefficient represents the probability of collision of a neutron 

 per unit track length in the material. If the collisions are subdivided according 

 to their end effect, the absorption coefficient is regarded as the sum of components 

 corresponding to the probability of the various end effects. [Compare with the 

 analogous subdivision of ^ for X rays in Eq. (41).] However, comparatively 

 little detailed information is on hand regarding these various probabilities (see, 

 for example, Way et al., 1950; Adair^ 1950). 



The absorption coefficient shows a general trend to increase as the neutron 

 energy decreases. At lower energies the plots of n vs. energy for "medium" or 

 "heavy" materials generally show a series of sharp peaks corresponding to the 

 "resonance effect" mentioned in Sect. 2-5. 



Figure 1-75 shows typical data on the values of n for two chemical elements and 

 gives references to complete sources of data. 



The order of magnitude of jjl for high-energy neutrons relates simply to the size 

 of atomic nuclei and to the density of nuclei in a material, owing to the elementary 

 consideration that a collision results whenever a nucleus intercepts the path of a 

 neutron. This argument indicates that a neutron should travel without col- 

 lisions, on the average, for a distance of the order of centimeters in nongaseous 

 materials. Accordingly, the value of /x should be in the general range just below 

 1 cm~i or 1 cmVg (see Fig. 1-75). 



This argument is not serviceable in estimating the collision frequency of lower 

 energy neutrons. The basic experiments of atomic physics show that a beam of 

 atomic particles behaves unlike a stream of bullets in that the particles do not 

 appear to follow sharply defined tracks. In fact, the values of the absorption 

 coefficient ju often rise far above the estimate indicated above, as shown, for 

 example, in Fig. l-75c. 



The trend of variation of n for very low energies is inversely proportional to the 

 neutron velocity. This trend sets in at higher energies in light materials, Uke 



