Nuclear instruments & methods in physics research section a-accelerators spectrometers detectors and associated equipment vol:339 issue:1-2 pages:322-328
A low-level high-purity Ge detection system was used to investigate samples of volcanic rock from different locations. A lead shield surrounded the 100 cm(3) germanium crystal and reduced the background radioactivity by a factor of 100 within the energy interval 70 to 3000 keV; an integrated count rate of (0.498+/-0.001) s(-1) was achieved over this inverval, which was mainly attributable to the influence of cosmic rays. Although sample volumes up to 2 litres can be measured, the volcanic samples were held in cylindrical containers and varied in volume from 16 to 20 cm(3). A powdered solid source was prepared of similar chemical composition to the samples, and spiked with suitable radionuclides to calibrate the detector system. The resulting efficiency calibration was compared with a Monte Carlo simulation of the detector efficiency for extended sources. Corrections were made for the given geometry, self-absorption and absorption in calculating the full energy-peak efficiency (epsilon(gamma)). The concentrations of various radionuclides originating from the natural decay chains in the different samples were compared tb a well known reference sample. A wide variation of the concentrations of the measured radionuclides and a breaking of secular equilibrium was observed.