In-Situ gamma-ray spectrometry: the influence of topography on the accuracy of activity determination

Determination of gamma-ray activity in soil by in-situ spectrometry with portable high purity germanium detectors is highly dependent on the nature of the site and the source distribution in the soil. Disturbances of the model distribution of radionuclides related to ground irregularities, incidence angle of the particles transporting activity at the time of contamination and plant cover affect the accuracy of the interpretation of the measurements. The effects of these disturbances on the different distributions currently used in spectrometry are analysed in this paper. It appears that in the initial stage of deposition, as the contamination is mainly on the surface, the contribution of these disturbances is not negligible. Later, however, the transfer of the radionuclides in the soil switches off significantly the contribution of distant regions from the detector. Its effective field is therefore reduced, and, thus, the effects of inhomogeneities related to disturbing factors vanish with time. As a result, irregularities of the order of a few centimetres are sufficient to decrease the measured fluence by a factor of two for a surface activity, and a penetration of the nuclides of about 1 cm in soil reduces the radius of the detector effective field from 100m to 30m.