Characterization of Concealed Mercury Using a Compact Cadmium Zinc Telluride Spectrometer and Prompt Gamma-ray Neutron Activation Analysis

Contaminated process equipment and pipes pose dangers to workers engaged with decontamination and deactivation activities. Non-intrusive methods are preferred because contamination can be detected and characterized without opening equipment or pipes, so the probability of release is decreased. One specific case is mercury contamination in legacy buildings at the Y-12 site in Tennessee. H3D, Inc. has investigated the use of a cadmium zinc telluride imaging spectrometer and neutron generator to detect mercury within steel pipes via neutron activation analysis. The method takes advantage of the thermal capture cross-section of mercury, which is about two orders of magnitude higher than most building materials. The imaging spectrometer, H1600, is compact (< 12 lbs) and provides good energy resolution (≈1.1% FWHM at 662 keV) at room temperature. Previous work by H3D has demonstrated the method with a single, large sample of mercury with moderated californium, and then with a neutron generator. In this work, we investigate non-intrusive characterization of mercury, with a focus on detection of mercury inside steel pipes, because that is most critical to waste segregation and safety. Mercury is assumed to be in its metallic form, which represents the most extreme self-shielding scenario. Detection is further complicated by 352-keV and 366-keV radiative capture signatures from iron-56, which can interfere with the 368-keV radiative capture signature from mercury-199. We show measured linearity between the area of exposed mercury and its net count rate. We then model the attenuation of neutrons and gammas to predict the detection sensitivity as a function of the mercury and the intervening steel thickness.