Measurements of thorium and uranium in environmental samples using gamma-gamma coincidence spectrometry.
We built a new coincidence spectrometer, consisting of two NaI(Tl) detectors facing each other inside a low-background passive shield, to measure thorium and uranium daughter activities in environmental samples. The spectrometer was simulated using Geant4 to correct sample efficiencies for self-absorption effects. The coincidence efficiencies for gamma-ray cascades from 208Tl and 214Bi decay are small (<1%), but at the same time the coincidence room background is lower by three orders of magnitude compared to the singles count rate. We investigated the most favorable coincidence gating schemes, detection efficiencies, background count rates, and minimum detectable activities. The device was employed to estimate thorium and uranium daughter activities in three different samples: Brazil nuts, potting mix, and magazine paper. Our uranium and thorium activities for Brazil nuts agree with some, but not all, previous measurements. Neither thorium nor uranium activities have previously been reported for commercial potting mix. Our measured values are in the range of activities reported for various soils in the United States. For magazine paper, our measured activities are lower than previously determined results.
The figure above shows the response of the coincidence spectrometer to a sample containing 232Th. For more in formation, see Tillett et al., NIM A 871, 66 (2017) and Tillett et al., App. Rad. Isotop. 141, 24 (2018). Andrew Tillett is a former UNC undergraduate student.