High pressure 4He proportional counters as borehole detectors of MeV neutrons

In a conventional borehole logging tool with an isotopic neutron source, the neutron emission energy is typically less than 5 MeV and the slowing down process is dominated by scattering from hydrogen in the rock matrix pores. For a 14 MeV DT source, however, transport from the starting energy to approximately 1 MeV is governed significantly by scattering (elastic and, particularly, inelastic) from nuclei such as C, O, Si, etc., contained in the rock matrix. From 1 MeV to thermal, transport is dominated by hydrogen. Both energy regions are of interest: the high energy region provides information about the rock parameters and the low energy region provides information about the pore volume fraction. Measurement of low energy transport is well established. To measure high energy transport, i.e., the 14→1 MeV slowing down length, detectors capable of measuring 1 MeV flux are required at two different source-to-detector spacings. In this paper, we examine the feasibility of using very high pressure ⁴He counters (up to 40 ATM) as detectors of borehole moderated 14 MeV neutrons, and present a very simple technique for neutron/gamma discrimination. These detectors are candidates because of a scattering resonance near 1 MeV and low scattering and absorption cross sections at lower energies