Fast coincidence-summing correction procedures for gamma spectrometric measurements in close geometries

Abstract

The spectrometric analysis of a sample measured in "close geometry" can be affected by errors due to coincidence-summing effects that occur when two or more cascade photons are emitted within the resolution time of the spectrometric system. The probability that these effects occur depends primarily on the nuclide decay scheme, the sample-detector distance, and the intrinsic efficiency of the detector. The values of correction factors are obtained by using well known expressions which terms depend on decay data, Full-Energy-Peak Efficiency (FEPE) and Total Efficiency (TE) values. The experimental determination of FEPE and TE calibration curves performed by using single-γ emitter sources for each measurement geometry is a long and tedious task. Equally time-consuming is the use of a Monte Carlo (MC) simulation both for the validation of the detector model and the number of analyses to be carried out. In this work some faster and simpler procedures to evaluate true coincidence-summing correction factors are tested and the results are compared with the ones obtained through experimental FEPE and TE calibration curves as well as a MC simulation. For point sources a TE behaviour approximation (linear or a constant), the use of Total-to-Peak ratios and a Virtual Point Detector (VPD) approach are considered. For volume sources, a VPD efficiency transfer method by integration of elementary efficiencies over the whole source volume is used. The different approaches give very close results and differences are of the order of a few per cent.