ON THE APPLICABILITY OF THE EFFECTIVE SOLID ANGLE CONCEPT IN ACTIVITYDETERMINATION OF LARGE CYLINDRICAL SOURCES

Well known activities of two large cylindrical gamma-sources (a mixtur e of radionuclides in solution, and a geological sample) are measured using a semiempirical method for calculating photon detection efficien cies (epsilon(p)) of Ge-detectors via effective solid angles (<(Omega) over bar>), in order to analyse the feasibility and the accuracy of th e method. It is applied here under conditions not favourable to the me thod itself, which are however quite common in practice: (1) HPGe dete ctor supplied only with standard detector specification and performanc e data, and (2) large source just above the detector end-cap. The impa ct of the probable range of uncertainties in knowledge of the detector and source geometrical parameters on the calculated efficiencies is c onsidered. The fact that the crystal of the detector used is not coaxi al with the end-cap, discovered by making X-ray radiographs, is of a s pecial concern with respect to the applicability of the method under s uch conditions. Therefore, the method for the calculations of Omega is extended to cases of parallel but not coincident axes of the cylindri cal sources and detectors. True coincidence effects, which introduce t ypically 5% errors for the counting geometry used, are not dealt with.