Correction factors to account for minor sample height variations in gamma-ray spectrometry

Abstract

It often happens in gamma-ray spectrometry measurements that there is less sample material than required for the given measuring geometry. One approach to this problem is to stick to the original geometry, though a correction factor is to be introduced to account for the difference in the height of the sample material. This correction factor C h is expressed as the ratio of spectrometer efficiency for the nominal sample height ε( h 0) to that obtained for the actual height ε( h). The author determined the correction factor C h for several radiation energies E, 81.0, 356.0, 661.7 and 1173.2 keV. Two measurement geometries were considered: a Marinelli beaker 710 cm 3 in volume, and a cylindrical sample 31.5 mm in height. The correction factors were obtained experimentally and by Monte Carlo simulation method for h falling within the range h 0±8 mm. C h values obtained by these two methods are consistent. For E⩾356 keV, C h value almost does not depend on energy. C h value varies linearly with d h. For Marinelli beaker for E⩾356 keV, the correction is 0.9% (1.0% for E=81 keV) for each millimeter of sample height change; for a cylindrical geometry the correction is 1.5% (1.7%). Monte Carlo method was further used to compute C h values for several other cylindrical geometries. The lower the nominal height of the cylindrical samples, the more sensitive the method to sample height variations. The knowledge of the value C h enables us to estimate the uncertainty of the measurements, associated with the sample height uncertainty.