Use of generator produced neutrons in coal analysis. Final report

Abstract

A 14 MeV neutron generator is satisfactory for making capture gamma-ray analysis in a borehole in rocks for a limited number of elements. In coal, a thermal-capture gamma-ray spectrum will yield data for a relatively larger number of elements. However, by the time 14 MeV neutrons are reduced to thermal energies and absorbed by the target elements the neutrons are a considerable distance from the neutron source and the detector. Consequently, the activated coal sample is dispersed and a long way from the detector, and results in a reduced sensitivity. If the neutron generator source emitted 2.5 MeV neutrons by a (D,D) reaction, the neutrons would be thermalized close to the detector. In this case, the sensitivity of the neutron activation method would be substantially enhanced. To produce a borehole generator which would produce 2.5 MeV neutrons, we started with 14 MeV generator using a tube-transformer assembly (TTA) as developed by the Sandia Corporation. A similar TTA was constructed using a deuterium rather than a tritium loaded target. The new TTA did not respond adequately with the exiting electronics. In the course of this investigation, a fast-response high-energy neutron detector using the {sup 16}O(n,p){sup 17}N reaction was developed which worked quite well and was a great help in adjusting the electronic parameters of the 14 MeV neutron generator. As the flux rate of the 2.5 MeV neutron generator was inadequate for activation analysis of coal, Cf-252 was substituted in order to obtain comparable spectra using 14 and 2.2 MeV neutrons. Using a specially constructed coal irradiation facility with a central borehole, experiments were made using neutrons of both energies and comparable fluxes. It is clear from the results that lower energy neutrons yield spectra which are more useful for coal analysis.