398 V. M. Zaporozhetz and E. M. Filippov 



to a relatively small magnitude. Thus, for instance, the maximum energy 

 of the gamma rays from natural sources does not exceed 3 MeV, whereas 

 with the aid of a betatron with a heavy element target one can obtain gamma 

 quanta with an energ}' of tens of millions of electron-volts. The energy of 

 most neutrons produced by the commonly used Po+Be or Ra+Be sources 

 does not exceed 8 to 9 MeV whereas with the use of a neutron generator it 

 is not difficult to obtain neutrons with an energy of the order of 14 MeV. 



The third feature of the accelerators, which is advantageous in radio- 

 active logging, consists of the possibility of obtaining with their aid, a mono- 

 energetic radiation, or at any rate a radiation with a distinctly limited maxi- 

 mum energy. This provides a basis for the development of new methods 

 of bore-hole investigation. 



However, the most important advantage of the accelerators of charged 

 particles is the possibility of governing their radiation. This allows investiga- 

 tion of bore-holes using a variable intensity of the source and the Avorking- 

 out of a vast variety of intermittent methods of investigation. 



NEUTRON GENERATOR 



In investigating bore -holes the use of the so-called neutron generators- 

 which produce beams of neutrons, is of a paramount interest. The most 

 promising is the use of a neutron generator in neutron-gamma logging (NGL). 



In interactions involving the high energy neutrons and the nuclei of the 

 rock forming atoms the principal role is played by the inelastic collisions 

 which produce gamma radiation. A fast neutron as a result of inelastic collisions 

 loses a considerable part of its energy and thereafter, owing to a large number 

 of elastic collisions, it is slowed down to a thermal state. Afterwards, it gets 

 captured by some atomic nucleus, frecjuently involving the emission of one 

 or several gamma quanta. 



The gamma rays produced by the initial inelastic collision of the neutrons 

 with the nuclei of rock-forming atoms can be considered to come into exist - 

 ance instantaneously. Retardation of the neutrons to the thermal state is- 

 a relatively lengthy process. Its continuity depends on the nature of the 

 decelerator and is inversely proportional to the hydrogen content of the 

 latter. For instance, in lead the average life of the neutrons with an energy 

 of 14 MeV is 500-3000 /^ sec <i) whereas in water it is 2-5 ^sec^^). In 

 consequence, the gamma rays produced by the capture of the slow neutrons 

 are separated from the gamma rays produced by inelastic collisions with 

 atomic nuclei by relatively long intervals of time. Thus, if during NGL the 

 rocks are subjected to short duration (of the order of microseconds) spurts 



