402 V. M. Zaporozhetz and M. M. Filippov 



14 MeV. The high efficiency of the reaction H^((i, n) He* and the high 

 energy of the resultant neutrons, as well as the small values of the accelerat- 

 ing potential necessary for it, make this reaction the most useful among 

 all the reactions known to be used to generate neutrons for radioactive log- 

 ging. 



For the sake of comparison let us consider the reaction lP{d, n) He* 

 produced by bombarding a deuterium target. This reaction previously has 

 been very widely used to generate neutrons, but the quantity of neutrons 

 formed in this reaction is much less than in the previous reaction. The 

 neutron energy is also much less. For instance, when E^ = 0.2 MeV the 

 value of £"„, which depends on the angle of motion of the neutrons, varies 

 within the limits of 2-3 MeV. 



The neutron energy associated with the reaction }i^{d,n) H*, in general, 

 depends on the angle of emission of the neutrons out of the target, but this 

 dependence weakens as the energy E^ of the accelerated ions of deuterium,^ 

 or in other words the accelerating potential, decreases. With the present day 

 techniques of electrification of bore-hole instruments it is difficult to obtain 

 potentials higher than 150-200 kW. The corresponding deutron energy will 

 produce a neutron energy of radiation within the limits of 13-16MeV<^). 



If the deutron energy is < 0.5 MeV the cross -section of the reaction 

 H*(cf, 7i)He* is determined by the following formuJa^^); 



58exp[-1.72£d~°-^] 



a = 



Ed[l + 5705 {Ed-0.096)^' 



where £'j£ is expressed in mega-electron-volts and a in barns. Table 1 shows 

 the dependence of the cross -section of this reaction on the energy of the 

 deutrons. The reaction has a sharply developed maximum o" = 4 bams 

 when E^ ^ 0.26 MeV. Since in the target a neutron is relatively rapidly 

 slowed down, it would be sensible to accelerate the deutrons up to energies 

 higher than those which would promote the maximum efficiency of the re- 

 action. This, however, is precluded by difficulties of insulating the high -voltage 

 electrical mechanism in the well apparatus. This circumstance hmits the 

 highest possible potential to the maximum of 250 kW. 



Let us consider the requirements to be satisfied in the construction of 

 a well neutron generator. 



The well generator has limited dimensions. At any rate its diameter must 

 not exceed 15 cm. The planning of a generator within these limits is a diffi- 

 cult technical problem. In the first place it is difficult to ensure dependable 

 insulation of high -voltage links; and a high voltage is necessary for the work- 

 ing of the acceleration tube. Even a greater difficulty appears owing to the 



