An improved method for determining radionuclide depth distributions using in situ gamma-ray spectrometry

In principle, in situ gamma-ray spectrometry determines the quantities of radionuclides in some medium with a portable detector. The main limitation of in situ gamma-ray spectrometry lies in determining the depth distribution of radionuclides. This limitation is addressed by developing an improved in situ method for determining the depth distributions of gamma-ray emitting radionuclides in large area sources.;This dissertation implements a unique collimator design with conventional radiation detection equipment. Cylindrically symmetric collimators were fabricated to allow only those gamma-rays emitted from a selected range of polar angles (measured off the detector axis) to be detected. Positioned with its axis normal to surface of the media, each collimator enables the detection of gamma-rays emitted from a different range of polar angles and preferential depths. This dissertation also involves sectioning the measured media into several independent depth layers. For this work, a uniform radionuclide distribution was assigned to each depth layer.;From the in situ measurements of a particular source medium, the photopeak count rates acquired with several different collimators were described by a system of equations that incorporate the activity concentrations in the independent depth layers with the counting efficiencies of each depth layer. Solution of the system of equations yielded the activity concentration of radionuclides in each depth layer.;Previous in situ methods require a priori knowledge of the depth distribution shape. However, the absolute method presented in this dissertation determines the depth distribution as a histogram and does not rely on such assumptions. Other advantages over previous in situ methods are that this method only requires a single gamma-ray emission, provides more detailed depth information, and offers a superior ability for characterizing complex depth distributions. Applications of this method for radionuclide contamination in soil and activated concrete in decommissioned nuclear reactors are investigated. Collimated spectrometer measurements of buried area sources demonstrated the capability of the method to yield accurate depth information for both experimental and Monte Carlo calibrations. Based on the results of actual measurements, this method increases the potential of in situ gamma-ray spectrometry as an independent characterization tool in situations with unknown radionuclide depth distributions.