THE USE OF ACCELERATORS OF CHARGED PARTICLES 



413 



be found suitable. In this method the bore-hole wall is irradiated by a powerful 

 beam of gamma quanta of high energy while photo -neutrons, which are 

 knocked out by the gamma quanta from the atomic nuclei, are registered 

 •with the aid of a neutron recorder. 



Table 3 gives a list of rock -forming elements, which participate in photo - 

 nuclear reactions, the threshold values of the gamma quanta energy which 

 may lead to the appearance of photo-neutrons, and also the maximal energy 

 of the gamma quanta which produce the maximum emission of neutrons. 



K 



60 



80 



100 120 140 160 180 200 



A 



Fig. 7. The dependence of the neutron output per one gamma quantum on the 

 elemental atomic weight A for thick targets of these elements. The gamma-quanta energy 

 of the low atomic weight elements (up to 20) is 2.76 MeV and for the other elements 



is 17.6 MeV. 



Table 3 shows that the values for the energy of the gamma quanta — 

 ■which cause the knocking of neutrons out of the elemental nuclei — is 

 least for beryllium (1.67 MeV) and deuterium (2.23 MeV). For all the other 

 elements the value of this energy is no less than 6 MeV. 



Table 4 and Fig. 7 show the emission of neutrons for 1 gamma quantum. 

 Evidently with the increase in the atomic weight of an element the production 

 of neutrons per one gamma quantum at first grows and then becomes constant 

 and approximately equal to 2 x 10~^ neutrons per 1 gamma quantum. The 

 diminution of the gamma quantum energy leads to the diminution of the 

 production of neutrons per 1 gamma quantum. 



Thus, for silver the diminution of the gamma quanta energy from 17.6 

 to 10 MeV leads to a 70 times lower neutron production per 1 gamma 

 quantum <^^ 



