HIGH-RESOLUTION GAMMA-RAY SPECTROMETRY LOGGING FOR CONTAMINATION ASSESSMENT

A well-logging system containing a high-purity germanium (HPGe) detector provides quantitative assessment of gamma-ray-emitting contaminants and natural radionuclides in the subsurface. The system is in operation at the Hanford Site of the U.S. Department of Energy where radionuclide-laden effluent from the production of nuclear materials in the past has contaminated the soil. The system is calibrated by using standard formations that have known concentrations of K, U and Th. This produces calibration constants for K, U and Th, and a detector inverse efficiency function for the energy range 0.1-3 MeV. The inverse efficiency function, along with a knowledge of the energies and gamma-ray production coefficients for artificial radionuclides, allows the system to be calibrated for these artificial gamma-ray sources, even though they are not contained in the calibration standards. Corrections for a steel borehole casing and the diameter of a water-filled borehole are also derived from measurements with the calibration standards. The borehole logging measurements provide concentrations (activity per gram of earth material) of the following isotopes: K-40, Ra-226 (via Bi-214), U-238 (via Pa-234m), U-235, Th-232 (via Tl-208), Co-60, Sb-125, Cs-137, Eu-152, Eu-154, Np-237 (via Pa-233), Pu-239, Pu-240 and Am-241.