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

In-situ gamma spectrometry is a potentially powerful technique for assessin g radioactive contamination in the ground but accurate quantification relie s on knowledge of the radioactivity variation with depth. Previous methods to rapidly derive such information have been proposed but are limited to us ing simple models of the activity-depth profile. This paper describes a tec hnique 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 pre viously-proposed methods (the Lead Plate Method [4]) using computer modelli ng with real activity-depth data and taking into account random and systema tic 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.