NEUTRON INTERACTIONS WITH HYDROGEN AND TISSUE 43 



a neutron and a proton. In 1935, Lea observed the inverse (n, y) 

 process of neutron capture followed by gamma-ray emission, and the 

 gamma-ray energy was subsequently measured as 2.2 mev. This means 

 that every neutron impinging on tissue and ultimately captured by a 

 proton releases 2.2 mev by a nuclear process in addition to the elastic- 

 scattering losses already discussed. 



For fast neutrons the cross section (o"n,^) for nuclear capture and 

 gamma-ray emission may be given by a^.y = 7r^"^/(r), where ttR^ is 

 the geometrical area of the struck nucleus, ^ is an empirical factor 

 called the sticking factor, and/(r) is the relative probability of gamma 

 emission compared to emission of other charged particles. The sticking 

 factor, which is the probability that a neutron, once having hit a nucleus, 

 will stick, must by definition be less than or equal to 1. Also, the relative 

 probability for gamma emission must be less than or equal to 1, so for 

 a fast neutron, the (/i, 7) cross section must be less than or equal to the 

 geometrical cross section, which, as we have seen, is 0.07 barn for hydiogen. 



At low energies the cross section for capture is inversely proportional 

 to the neutron velocity. This proportionality, called the l/v law, holds 

 in the case of light nuclei up to appreciable energies, as illustrated by 

 the B^^ (n, 7) reaction, where it is followed up to 50 kev. In the thermal- 

 neutron region the capture cross section for hydrogen is 0.30 barn, in 

 good agreement with theory. This value is low compared to the total 

 thermal scattering cross section of 59 barns, but is about 100 times 

 greater than the capture cross sections for other near-by elements such 

 as deuterium and carbon. As a consequence, heavy water and carbon 

 are used as moderators in nuclear piles; ordinary water removes too 

 many neutrons from circulation. 



Thermal neutrons can also be captured by nitrogen; in addition to 

 (w, 7) capture, N^* can also capture a neutron and emit an 0.58-mev 

 proton to form radioactive C^'^. Such an (n, p) reaction produced by 

 thermal neutrons is possible only for light nuclei and takes place in only 

 a few cases. Both (n, p) and (n, 7) processes contribute to the total 

 nitrogen thermal-neutron capture cross section of 2.15 barns (1).* Very 

 little additional energy is contributed to the process by the radioactive 

 C^*, since its half life is 5700 years (2) and its maximum beta-ray energy 

 is 154 kev. 



* It is pointed out by Zirkle in his discussion that the relative effectiveness in 

 small animals of the (n, p) reaction on N^* and the (n, 7) reaction on H could not 

 be determined only from the relative energies of the proton and gamma ray, because 

 the 0.58-mev proton is completely absorbed in a short distance, whereas the 2.2- 

 mev gamma ray may well pass out of the tissue before it has lost a large part of its 

 energy. 



