THE USE OF ACCELERATORS OF CHARGED PARTICLES 415 



r is the average distance of the centre of formation of photo -neutrons 

 from the recorder. 

 Accepting according to S. A. Kant OR (Private communication) A = 5 cm 

 and r = 10 cm we obtain P = 1.1 xlO"*. 



The volume of the rock with which the gamma radiation (in the instance 

 of a beam of gamma quanta) will interact, will be no less than 1000 cm*. 

 Consequently, the probability of the photo -neutrons reaching the recorder 

 of the fast neutrons mil be: 



1.2 X 10-8 X 1.1 X 10-* X 103 = 1.3 X 10-9 



The effectiveness of the fast neutron scintillation counters (diameter 

 4 cm; length 10 cm) used in bore-hole investigations is approximately 

 0.1 per cent. Consequently, the probability of registering one neutron will 

 be Pg = 1.3 X 10-12. 



The following values of intensity of natural neutron radiation are known: 

 sedimentary rocks in the presence of the cosmic background —230; extrusive 

 rocks (at depth) — 610 ; pegmatites — 3000 n x cm-^ x day-^. Based on these 

 data it is possible to consider that in oil wells the natural neutron back- 

 ground will not exceed 0.003 n X cm-^ x sec"!, whereas in a rock layer with 

 a cross-section of 100 cm^ the natural neutron background I^^ affecting 

 the readings in GNL the neutron stream should exceed several-fold the 

 natural neutron background. For this it is necessary to have a source possessing 

 the following power 



where: m is the demanded increase of the induced neutron radiation over 



the natural; 



M is the power of the source measured in gamma quanta per second. 

 Substituting the previously quoted values for I^^ and Pg and assuming m = 20, 

 we obtain M = 4.6 x lO^^. 



It is interesting to examine the possibility of using the gamma-neutron 

 log for the following purposes. 



1. Prospecting for beryllium-bearing rocks. Using a method similar to 

 the one used above it is possible to show that to distinguish rocks containing 

 up to 5 per cent of beryllium a source with a power of the order of lO^i 

 gamma quanta per sec is necessary. In this connection it is necessary to 

 notice that V. N. Dakhnov's suggestion about beryllium prospecting using 

 the GNL method with an antimony source of gamma quanta from the point 

 of view of safety of bore -hole working is not easily realizable. 



2. Recognition of sandstone layers from their content of the isotope of 



