Borehole capture gamma ray spectrometry in very dry rock

Using a simulated borehole geometry, prompt gamma-ray spectra have been recorded in four large salt samples, each having a small but different water concentration. Spectra were taken in both dry and water-filled boreholes using a 252Cf neutron source. Analysis of the data shows a very high ratio of fast neutron fluence rate to thermal neutron fluence rate which decreases sharply with increasing concentrations of water. The high ratio is primarily the result of strong thermal neutron absorption by chlorine. The high neutron fluence rate ratios found in dry salt suggest the possibility of significant contributions to the spectra by non-thermal neutron capture gamma-rays. Examination of the counts in the chlorine capture gamma-ray peaks showed no evidence of significant effects on the capture intensities by non-thermal neutron absorption even in dry boreholes and in the driest salt samples measured. This conclusion is supported by the lack of line broadening in the capture gamma-ray spectra. Rock with a macroscopic cross section as high as that of salt is rare; thus the depression of the thermal neutron fluence rate found in dry salt is an extreme case. There need be little concern that capture gamma-ray spectra taken in a borehole in common rocks will be altered by resonance or fast neutron absorption. The thermal neutron fluence rate in relatively dry salt at a source-to-detector distance of 39 cm increases with small increments of formation water. Published data for common rocks indicate the opposite is true for higher concentrations of water. The apparent discrepancy is due to the movement of the cross-over point toward greater distances from the source as the water concentration becomes very low. (author). 26 refs, 6 figs, 4 tabs