Neutron capture logging calibration and data analysis for environmental contaminant assessment

This paper describes a method to calibrate a neutron capture sonde equipped with a high resolution γ-ray detector, and analyze log data. The method utilizes the 1460.8-keV passive γ-ray of 40K, the 770.3-keV capture γ-ray of 39K, and a capture γ-ray from a target element. An equation containing the spectral line intensities for the two capture γ-rays, nuclear capture data, and the detector efficiency function expresses the concentration of the target element as a multiple of the 39K concentration. The concentration of 39K is easily deduced from the 40K concentration, which is calculated directly from the line intensity for the 1460.8-keV γ-ray in a passive γ-ray spectrum.The calibration automatically adjusts to changes in the neutron transport properties of the logged medium that may result, for example, from variations in the H density and the concentrations of neutron poisons. Fluctuations in the neutron source output are similarly accommodated. The calibration utilizes U.S. Department of Energy (DOE) passive γ-ray calibration standards that contain well established concentrations of K, U, and Th. The passive γ-rays from K, U, and Th (and the U and Th decay progenies) provide data for the detector efficiency function determination.Data for proof-of-principle demonstrations of the method were acquired by logging boreholes penetrating the shallow subsurface at a DOE waste site with a simple, reliable neutron capture logging system. The system had a 252Cf source and a high purity germanium (HPGe) detector. Time gating could not be used to sort signals originating from capture and activation, but the excellent energy resolution permitted capture γ-ray identifications based solely on the γ-ray energies. Cl, H, and other elements were detected and assessed.A conventional calibration and data analysis method was also employed. The method was specific to Cl and was based on measurements in two Cl-impregnated concrete blocks. Cl concentrations inferred with this method were often consistent with the concentrations determined with the new method. When the two methods produced different Cl concentrations, the discrepancies could be explained by variations in formation parameters.