Testing of different true coincidence correction approaches for gamma-ray spectrometry of voluminous sources

Abstract

To achieve the highest possible sensitivity of analysis for environmental samples it is common practice to use both a high efficiency detector and a close measurement geometry with a large sample size (e.g. Marinelli beaker). Under such conditions, the typical efficiency calibration procedure results in a biased activity value for many nuclides due to the tree coincidence summing effect. While there are a few methods to correct for this effect with special calibration standards, such calibrations can be both time consuming and expensive. Due to these calibration difficulties, the tree coincidence summing effect is often simply ignored. Recently, it has been demonstrated that the coincidence summing correction can be performed mathematically even for voluminous sources. This new method consists of an integration of the coincidence correction factor over the sample volume while taking into account its chemical composition and the container. In this paper, we will discuss the latest approaches for establishing the peak efficiency and peak-to-total efficiency curves, which are required for this method. These approaches have been tested for HPGe detectors of two different relative efficiencies.

Mapping of true coincidence effect value for voluminous sources measured with HPGE

“…The 60% detector was fully characterized using the procedure described in the previous paper [1]. The TCE for numerous points around the detector was calculated by combining the characteristics of the detector with the P/T-efficiency curve from a previous series of experiments [1].…”

“…The contribution of the scattered radiation may reach 30 % and more in some cases. Some authors have suggested to use the experimental P/T-ratios for small subsamples during the integration of the coincidence effect and have demonstrated improvement of the results [1]. We had previously assumed that the intrinsic P/T-calibration is suitable only for small detectors, whereas the scattering must be taken into account for large detectors [2,4].…”

“…Since the TCE is a geometry effect, placing samples close to the detector results in an increasing need to make the correction. For example, a 60% HPGe detector may exhibit TCE in the range of tens of percents [1].…”

“…The method developed for the computation of this correction factor is based on: (i) a full peak efficiency map generated in advance [1]; (ii) an intrinsic P/T-calibration calculated from previous measurements. It has been demonstrated that such an approach works quite well for HPGe detectors with the relative efficiency up to 60 % [1].…”

“…Since the TCE is a geometry effect, placing samples close to the detector results in an increasing need to make the correction. For example, a 60% HPGe detector may exhibit TCE in the range of tens of percents [1].To compute the overall TCE correction factor for voluminous samples it is necessary to integrate a volumetric TCE function over the sample volume [2]. The method developed for the computation of this correction factor is based on: (i) a full peak efficiency map generated in advance [1]; (ii) an intrinsic P/T-calibration calculated from previous measurements.…”