THE USE OF ACCELERATORS OF CHARGED PARTICLES 409 



of increasing the speed of measurement and of making measurements with 

 large sondes. This, as is known from theory and from experimental bore -hole 

 investigations (^» ^"^ involves a diminution in the influence of the bore- 

 hole, leading to an increase in the sensitivity of the GGL method to the 

 density variations of rocks. 



The second circumstance favourable for GGL is the possibility of obtaining 

 a high energy with the aid of gamma ray accelerators. The desirabihty of 

 using high energy sources during GGL is obvious from the followdng 

 considerations. 



The use of cobalt sources of gamma quanta with an energy of 1.25 MeV 

 in GGL allows the investigation of a rock stratum (density 2.4-2.8 g cm^) 

 no thicker than 5-7 cm ^^' ^°) in a bore-hole. Consequently, the presence 

 of a mud crust on the bore-hole .wall and a layer of drilhng fluid between 

 the crust and the body of the apparatus leads to a considerable diminution 

 in the accuracy of determining the density of the rock. 



The increase in the gamma quanta energy increases the depth of investiga- 

 tion* and considerably diminishes the influence of the above mentioned 

 factors on the results of investigations. 



Calculations have shown that if the energy of the gamma quanta source 

 is increased from 1 to 15 MeV the average free path of a gamma quantum 

 in the rocks possessing a density of 2-3 g/cm^ increases four to five-fold 

 and correspondingly the depth of investigation also increases. 



The increase in the source energy of the gamma quanta to over 15 MeV 

 is useless since in conjunction with such an increase the phenomenon of 

 formation of electron-position pairs <^°^ also increases and the penetration 

 depth does not grow much. 



In order to obtain more accurate information on the rocks investigated 

 in bore -holes by the GGL method with a high energy gamma quanta generator 

 it is necessary to register the soft part of the spectrum of the scattered 

 gamma ray (0.05-0.07 MeV) by using, for instance, a differential gamma 

 spectrometer. This helps to increase the depth of investigating the rocks. 

 This is connected with the fact that the soft gamma radiation reaching the 

 gamma quanta recorder arrives from those rock strata which are the most 

 distant from the instrument. 



GAMMA-NEUTRON LOGGING (GNL) 



To study the geological sections of bore-holes and to determine the 

 constitution of certain elements in the rocks gamma neutron logging may 



* In this context implies the power of penetration of the rays. [Editor's footnote] 



