44 



NEUTRONS AND THEIR SPECIAL EFFECTS 



The predominant part played by hydrogen in the interaction of neu- 

 trons with tissue is amply confirmed by Table 1, which shows the 

 approximate percentage composition of the human body, together with 

 the total neutron (scattering and capture) cross section for the element 

 where available for both thermal and 10-ev neutrons. In general, the 



TABLE 1 



Approximate Composition of Hum.\n Body and Approximate Thermal- 

 Neutron Cross Sections 



total cross sections remain about the same between 10 and 100 ev. 

 Further, in the last column the gram-atom per cent of H and N has been 

 multiplied by the thermal-neutron capture cross section for each of these 

 elements, thus providing an index of their relative importance in neutron 

 capture. It can be seen that nitrogen accounts for only about 11 per 

 cent of the neutrons. 



A few more approximations will fill out and complete our picture of 

 the neutron, which, as we have seen so far, enters tissue with energies in 

 the million-volt range and is slowed down predominantly by collision 

 with protons until it is finally captured, usually to form a deuteron with 

 the emission of a 2.2-mev gamma ray. From its mean life of about 10~* 

 sec and average velocity of 2200 meters per sec we can calculate that the 

 average thermal neutron traverses a total tortuous path of 22 cm; and 

 further, from elementary diffusion theory, we learn that it makes about 

 one collision per centimeter of path. 



