Monitoring of Natural Soil Radioactivity with Portable Gamma-Ray Spectrometers

Two portable NaI(Tl) spectrometers with four energy windows were used for the recording of gamma-ray counts over soil and rock of differing natural radioactivity. The exposure rates at the field sites were simultaneously measured with a high-pressure argon ionization chamber. Background measurements at sea were carried out in order to estimate the non-terrestrial contributions to the instrument readings. Counts recorded in the three high-energy windows of the spectrometers were converted into radiometrically equivalent concentrations of thorium, uranium, and potassium in the ground. Large concrete calibration pads with known radioelement concentrations were used to determine the stripping ratios and the sensitivity constants required for this conversion. The calibration pads also made it possible to determine the response in the total-count window produced by an exposure rate of 1 μRh-1. The theoretical exposure rates deducible from the experimental radioelement concentrations at the field sites were in good agreement both with the ionization-chamber readings (corrected for cosmic-ray background) and with the exposure rates measured by total gamma-ray counting. From this and other results it is concluded that portable spectrometers of the type used here are suited for in situ assays of natural soil radioactivity