INVESTIGATION OF THE NATURAL RADIOACTIVITY OF VOLCANIC ROCK SAMPLES USING A LOW-BACKGROUND GAMMA-RAY SPECTROMETER

A low-level high-purity Ge detection system was used to investigate sa mples of volcanic rock from different locations. A lead shield surroun ded the 100 cm(3) germanium crystal and reduced the background radioac tivity by a factor of 100 within the energy interval 70 to 3000 keV; a n integrated count rate of (0.498+/-0.001) s(-1) was achieved over thi s inverval, which was mainly attributable to the influence of cosmic r ays. Although sample volumes up to 2 litres can be measured, the volca nic samples were held in cylindrical containers and varied in volume f rom 16 to 20 cm(3). A powdered solid source was prepared of similar ch emical composition to the samples, and spiked with suitable radionucli des to calibrate the detector system. The resulting efficiency calibra tion was compared with a Monte Carlo simulation of the detector effici ency for extended sources. Corrections were made for the given geometr y, self-absorption and absorption in calculating the full energy-peak efficiency (epsilon(gamma)). The concentrations of various radionuclid es originating from the natural decay chains in the different samples were compared tb a well known reference sample. A wide variation of th e concentrations of the measured radionuclides and a breaking of secul ar equilibrium was observed.