PRINCIPLES OF RADIOLOGICAL PHYSICS 



115 



lithium and boron, in which an {n,a) reaction proceeds without difficulty (see 

 Sect. 2-1 d). 



The chances of neutron collision with the nuclei of adjacent atoms are quite 

 independent, except at energies well below 1 ev, where diffraction effects take 

 place in crystals and, to some extent, in noncrystalline materials. Therefore the 



0.3 - 



E 0.2 



0. 



0.001 001 



(a) 



E 

 u 



0.1 



10 



0.01 



(6) 



0.1 



10 



Mev 



6x 



100 



5x10-2^ 

 I- a? 



■-' UJ 3 



en c 



2 CO cvj 

 C/5 



O E 

 I q: <-> 



100 300 



Oh- c 

 O UJ cvi 



en E 



N 



E 

 u 



^ 



Ol 



10 



7 . 



4 



2 



to 



7 

 4 

 2 



10 

 7 

 4 

 2 

 10 

 7 

 4 

 2 



10- 

 100 1000 



22 



-23 



o 



Ll) 



in 



o 



C 



cn t\j 

 in 



o 

 q: 

 o 



24 



ev 



0.001 OOl 



{c) En . 



Fig. 1-75. Narrow-beam absorption cross sections of carbon (a) and (6) and cad- 

 mium (c) for neutrons of various energies. (Adair, 1950.) 



absorption coefficient for any material is found by combining the coefficients for 

 its constituent elements as is done for X rays according to Eq. (43) : 



M 



PiMi + P2M2 + 



In fact, the absorption coefficient may be analyzed in greater detail by considering 

 separately the probability of collision with the nuclei of different isotopes of each 

 element. However, most data are available only for the naturally occurring 

 mixture of isotopes of each element rather than for separate isotopes. 



A beam containing neutrons of different energies penetrates under narrow- 

 beam conditions according to the same pattern as a non-monochromatic beam of 

 X rays (see Sect. 4-3b). The slower neutrons are removed from the beam first 



