Close-geometry efficiency calibration in gamma-ray spectrometry using radio-nuclides with a two-step cascade decay

When efficiency calibration is performed in gamma-ray spectrometry with point sources in close geometry, radio-nuclides emitting photons of a single energy are usually utilized in order to avoid problems arising from true coincidence summing. Radio-nuclides emitting gamma-rays in a simple two-step cascade are therefore not considered suitable for such measurements. It is, namely, not possible to determine the full-energy-peak and total efficiencies for the gamma-rays such radio-nuclides emit from the system of equations which determine the number of counts registered in individual peaks in their spectra. A method was developed to overcome this difficulty by making use of additional constraints, based upon sound physical grounds, which can be imposed on these equations to render the combined system solvable. The accuracy of the method was successfully tested with point sources of 60Co, 46Sc and 94Nb. The method provides six additional energies in the range between 700 and 1400 keV for which full-energy-peak and total efficiencies can be determined, which is important in view of the fact that only seven single-energy emitters are generally available for close-geometry calibration. We applied the method to several distances of the point source from the detector and studied the influence of angular correlations on the determination of the efficiencies. The effect is significant for total efficiencies and larger distances of the source from the detector, which has not been noted before.