Use of Lanthanum Bromide Detectors to Augment Site Surveys for Depleted Uranium

Abstract

A variety of systems have now been described for use in surveying sites for anthropogenic radiological contamination. Virtually all of these include use of sodium iodide detectors and register detection data with global positioning satellite data. This paper demonstrates how lanthanum bromide detectors can be used to augment existing field surveys in a manner to reduce uncertainty in areas of low count rates and to discriminate between depleted uranium and naturally occurring uranium. The survey system described is equipped with large (20 × 20 × 100 cm) sodium iodide (Alpha Spectra) and 7.5 × 7.5 cm lanthanum bromide (Saint Gobain) detectors. Additional radiological detection equipment (DigiBase) was obtained from ORTEC with survey data collected in the List Mode. Data collected in surveys were then used to generate digital maps using GeoSoft’s Oasis Montaj. Software has been developed to automatically identify areas of increased count rates using user-defined thresholds. This software can collect the count data for the masked area and generate a composite spectrum that can be compared to a reference spectrum believed to represent an uncontaminated area. Ratios of counts attributed to protactinium-234m (Pa-234m) are compared to counts attributed to Bismuth-214 (Bi-214) for both the composite filed survey spectrum and the reference spectrum. Soil samples have been collected from selected sites over a range of soil and geology types for the purpose of collecting data comparing high purity germanium (HPGe) detector and lanthanum bromide (LaBr) detector spectra. These samples have come from areas believed to be devoid of depleted uranium contamination and from areas expected to have higher concentrations of naturally occurring uranium. A library of HPGe and LaBr spectra have been collected comparing: (1) background soil samples with, (2) spectra from the same samples that have been doped with half the remediation threshold activity of depleted uranium, and (3) the remediation threshold activity of depleted uranium. Ratios of the Pa-234m:Bi-214 for both HPGe and LaBr detectors are provided in this paper. This process can be repeated for any site of interest. Background soil samples can be obtained prior to surveying and an equivalent library of spectral ratios generated. Field data from three different sites will be used to show how LaBr detectors can be effectively used as an infield HPGe surrogate for rapid discrimination between DU contamination and areas of high naturally occurring uranium. Areas of maps of questionable contamination are selected and composite LaBr spectra are generated along with Pa-234m:Bi-214 ratio. This is compared to library data to determine the approximate activity of DU present. All areas suspected of DU contamination can also be selected and excluded from the remainder of map data. A composite spectrum from the areas believed to be uncontaminated can be generated and spectral ratios compared library data for clearance purposes.