The assessment of gamma dose-rate by gamma-ray field spectrometer

A three-channel γ-ray spectrometer, with a 2.5 in NaI probe, was used for separate determination of γ dose coming from U, Th or K soil contents. A new method is proposed for calibration, based on (a) laboratory absorption measurement with point radioactive sources and (b) measurements in holes bored in homogeneous reference lava flows with well determined U, Th and K contents. These calibrations revealed the importance of inhomogeneous media on the γ dose-rate evaluation. Applications are made to cases of practical interest, especially for the determination of the minimum hole depth for probe location that is required for acceptable accuracy. (author)