Design, construction and characterisation of a portable gamma-ray spectrometer for low-level natural occurring radioactive material ex-situ measurement.

The activity concentrations in naturally occurring radioactive material (NORM) samples are conventionally measured using a gamma-ray spectrometer with a single detector (mostly HPGe or NaI:Tl) enclosed in a lead shield. In this work, a passive water shield to reduce background radiation reaching the detectors was designed using GEANT4-toolkit Monte Carlo simulations and then constructed. This measurement system is portable and cost-effective for ex-situ measurements. IAEA-375 soil and beach sand each placed in Marinelli beakers were measured using two LaBr:Ce detectors in singles and coin-cidence modes. A novel method of background reduction by using photon time-of-flight was employed together with the measurement of the two photons energies. These samples were also measured in singles mode using a NaI:Tl detector inside the constructed water shield and HPGe detector shielded with lead to compare and validate the results of LaBr:Ce detectors measurement. Both the simulated and measured results show that the water shield (500 mm thick) attenuates 2614.5 keV gamma rays by 90 %. The minimum detectable activity (MDA) measured for 24 h using the NaI:Tl detector without shield and inside the water shield are; 546 Bq kg and 146 Bq kg at 1460.8 keV (K), 194 Bq kg and 15 Bq kg at 1764.5 keV (U series), and 131 Bq kg and 15 Bq kg at 911.2 keV (Th series), respectively. The measured activity concentrations of U and Th series and K radionuclides inside IAEA-375 soil agree with certified values to within measurement uncertainties. The measured activity of La in the LaBr:Ce detector crystal is 263.8 ± 26.8 Bq kg. The internal activity of the LaBr:Ce detector increases the MDA at 1460.8 keV and 2614.5 keV, which limits the measurement of primordial radionuclides with low activity concentration in singles mode. The activity concentrations of U and Th series radionuclides in beach sand were measured using NaI:Tl and LaBr:Ce detectors. The results are consistent with those from HPGe measurement to within uncertainties.