Measurement of uranium concentrations in situ by gamma-ray spectrometry: An assessment

Portable instruments have often been used to measure radioisotope concentrations in their natural environment. These units offer the advantages of speed and lower cost, but their accuracy has been questioned, particularly in soil and rock that is naturally stratified and heterogeneous. In our study, low concentrations of uranium in rock and soil were determined in situ by a portable γ-ray spectrometer (GSPEC), and compared with the concentrations determined for samples of rock and soil collected and analyzed by neutron-activation delayed-neutron counting (DNC). The study shows that the portable γ-ray spectrometer must be used with caution when uranium concentrations in soils are needed. For both organic and mineral soils, careful attention to soil stratification is important. Because peat soil does not shield γ-rays as effectively as mineral soil, the GSPEC will detect γ emitters that lie deeper in peat soil. Hence the effect of a layer of high concentration influences the result differently in the two types of soil. If substrate concentrations in the top 15 cm of soil are required for calculating the traditional concentration ratio (CR) values used in environmental impact assessments, the GSPEC soil-concentration values may be appropriate for mineral soils, but unreliable for organic soils. Accurate concentrations were not obtained for rock.