Uranium, radium and thorium in soils with high-resolution gamma spectroscopy, MCNP-generated efficiencies, and VRF non-linear full-spectrum nuclide shape fitting

¹ Radiation Safety Engineering, Inc, 3245 North Washington Street, Chandler, AZ, USA
² White Rock Science, PO Box 4729, Los Alamos, NM, USA
³ Snakedance Scientific, LLC, 13509 Quaking Aspen Place NE, Albuquerque, NM, USA

^* Corresponding author: rlmetzger@radsafe.com

Published online: 25 September 2017

Abstract

A new method for analysis of uranium and radium in soils by gamma spectroscopy has been developed using VRF (“Visual RobFit”) which, unlike traditional peak-search techniques, fits full-spectrum nuclide shapes with non-linear least-squares minimization of the chi-squared statistic. Gamma efficiency curves were developed for a 500 mL Marinelli beaker geometry as a function of soil density using MCNP. Collected spectra were then analyzed using the MCNP-generated efficiency curves and VRF to deconvolute the 90 keV peak complex of uranium and obtain ²³⁸U and ²³⁵U activities. ²²⁶Ra activity was determined either from the radon daughters if the equilibrium status is known, or directly from the deconvoluted 186 keV line. ²²⁸Ra values were determined from the ²²⁸Ac daughter activity. The method was validated by analysis of radium, thorium and uranium soil standards and by inter-comparison with other methods for radium in soils. The method allows for a rapid determination of whether a sample has been impacted by a man-made activity by comparison of the uranium and radium concentrations to those that would be expected from a natural equilibrium state.