Estimation of true coincidence corrections for voluminous sources

To analyze a voluminous radioactive source with the highest possible sensitivity, it is necessary to use both a high efficiency detector and an optimal measurement geometry. The optimal geometry implies positioning the source as close to the detector as possible. It also implies selection of the shape of the source in order to reach the highest efficiency possible (e.g., Marinelli beaker). Under such conditions of measurements, true coincidences may cause systematic errors that can reach levels of more than ten percent for some radionuclides. A method for estimation of the effect of these coincidences was developed. It is based on direct computation of the effect by means of integration of a function which involves the experimentally obtained detection efficiency for the place around the detector. It was found that for the tested detector with a relative efficiency of 15%, the so-called intrinsic peak-to-total calibration may be used in the course of such an integration: It has been shown that theP/T-ratio for the given energy in the working space around the detector may be considered a constant value. Some results from a peak-to-total calibration study in the presence of scattering material are also given.The main part of the material has been presented at the Third International Meeting Nuclear Methods for Protection of the Environment, May 1995, Dubna, Russia.