Characterization and calibration of a portable HPGe detector for in situ gamma-ray spectrometry using large-sized calibration pads

For in situ gamma-ray spectroscopy, a study was performed for characterization of a portable high-purity germanium (HPGe) detector utilizing four large-sized concrete pads, which were already used for the calibration of airborne gamma-ray detectors. Each of the four concrete pads contains the known concentrations of 238U, 232Th, and 40K, including one so-called 'blank pad', which contains background activity. An efficiency transfer method has been adopted to calibrate the present HPGe detector using small-volume International Atomic Enery Agency (IAEA)-certified RGU-1, RGTh-1, and RGK-1 certified materials. To this end, each of four concrete calibration pads, having dimensions of (8 × 8 × 0.375) m, was marked by meshing into (1 × 1) m grids to enable determining activity in measurement positions. Then, the measurements were carried out in 49 positions on each calibration pad by using a 40% relative efficiency HPGe detector. The experimental efficiency values of this detector were also compared with Monte Carlo simulation results. As a result, the activity concentrations determined in this study showed good agreement with the declared activity values for 40K and 232Th contained in the calibration pads reported by MTA; however, the results for 238U exhibited a discrepancy of ~10% from the declared value. In order to demonstrate the applicability of the proposed calibration methodology in real environmental conditions, additional in situ measurements were carried out on the soil at the Sarayköy site of NÜKEN. The obtained activity concentrations were compared with those obtained from laboratory gamma spectrometric analyses for soil samples taken from the same locations. The study indicates that if large-sized calibration pads are used for the characterization of a portable in situ HPGe detector, thereby enabling accurate in situ gamma-ray spectrometric assessments.