Theoretical assessment of collimated submerged detector measurements for improving the accuracy of in-situ gamma spectrometry

In-situ gamma spectrometry is a potentially powerful technique for assessing radioactive contamination in the ground but accurate quantification relies on knowledge of the radioactivity variation with depth. Previous methods to rapidly derive such information have been proposed but are limited to using simple models of the activity-depth profile. This paper describes a technique to obtain this information from measurements made with a collimated spectrometer submerged in the ground at a series of discrete positions. The type of collimation was first considered and then the method was optimised using computer modelling for the number and position of measurement depths and the acquisition time. The method was compared with the best of the previously-proposed methods (the Lead Plate Method) using computer modelling with real activity-depth data and taking into account random and systematic uncertainties and was found to be significantly better, with estimated activity concentrations within a factor of 1.2 on average compared with 1.6 for the Lead Plate Method.