Accounting for the depth distribution of 137Cs in on-line mobile gamma spectrometry through primary and forward-scattered photons.

Stationary and mobile field gamma spectrometry is a useful tool for rapid estimation of environmental radioactivity inventories on and in the ground. A weak point however, is that the depth distribution of the activity in the ground must be known in order to calculate the true activity per unit area or unit mass from an observed photon fluence rate. A promising method for converting incoming spectral data into both true activity content and depth distribution in real time is the peak-to-valley method, which is based on an analysis of the ratio between count rates from primary and forward-scattered photons. In this study the peak-to-valley method was adapted to car-borne mobile gamma spectrometry, where the depth distribution of (137)Cs is fitted to a Lorenz function. Results from field experiments with a large HPGe detector, utilising point sources at different depths, are presented. It was found that the method can be useful for mobile measurements with a measuring time of 5-10 min for activity concentrations of about 100 kBq.m(-2) or higher, resulting in an uncertainty in the estimate of the true activity of about 50%.