414 



V. M. Zaporozhetz and E, M. Filippov 



The most promising outlook for the GNL method exists in the possibihty 

 of discovering oil-bearing strata from a higher content of deuterium in oil 

 (1.5-3 times exceeding that in water (^)). Since the threshold energy of photo- 

 spHtting of deuterium is not high (2.23 MeV) an accelerator is used which 

 has a relatively low energy of gamma quanta. The building of such an instru- 

 ment will not involve unsurpassable difficulties. In particular, in such a case, 

 no confusion will arise as a result of the formation of photo -neutrons in the 

 material from which the instrument is constructed or the material of the 

 bore-hole casing tubes for which the threshold energy exceeds 6 MeV. 



Table 4. Neutron output per gamma quantum 



The measurement of the density of photo -neutrons can be made on the 

 background formed by the natural neutron radiation of the rocks. However, 

 the intensity of the latter is very small and it will not preclude the detection 

 of photo -neutrons. 



To confirm let us make the following estimate. In petroleum there i& 

 13 per cent of hydrogen; in which the average content of deuterium will be 

 4 X 10~^ per cent (^). If we accept the porosity of sandstones to be 30 per cent 

 we wiU see that deuterium forms 4 xlO~^ x0.13 xO.3 = 1.6 xlO~^ per 

 cent of the mass of the oil-bearing stratum. Since the production of neutrons 

 per one gamma quantum is of the order of 7.4 X 10~* (with a thick target) 

 for deuterium, the probability of obtaining photo -neutrons in an oil bearing 

 layer will be 7.4 x 10"* x 1.6 x 10~^ x 10"^ = 1.2 x 10~^ per gamma quantum. 



Since the investigation depth in the bore -hole rocks is of the order of 

 10 cm the probability of the photo -neutrons, which are formed in the stratum 

 and have an energy of about 2 MeV, to reach the recorder will conform to 

 the following rule. 



P- 



4nr^ 



where : X is the average free path of the fast neutrons in the substance under 

 investigation. 



