HIGH-RESOLUTION GAMMA-RAY SPECTROMETRY LOGGING FOR CONTAMINATION ASSESSMENT

A well-logging system containing a high-purity germanium (HPGe) detect or provides quantitative assessment of gamma-ray-emitting contaminants and natural radionuclides in the subsurface. The system is in operati on at the Hanford Site of the U.S. Department of Energy where radionuc lide-laden effluent from the production of nuclear materials in the pa st has contaminated the soil. The system is calibrated by using standa rd formations that have known concentrations of K, U and Th. This prod uces calibration constants for K, U and Th, and a detector inverse eff iciency function for the energy range 0.1-3 MeV. The inverse efficienc y function, along with a knowledge of the energies and gamma-ray produ ction 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 ste el borehole casing and the diameter of a water-filled borehole are als o derived from measurements with the calibration standards. The boreho le logging measurements provide concentrations (activity per gram of e arth 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.