Response of Ge photon detectors to beta-rays and possible reduction of photopeak areas due to beta–gamma coincidence summing effects

https://doi.org/10.1016/j.apradiso.2013.11.068Get rights and content

Highlights

  • The possible effects arising from β–γ and X–CE coincidence summing are shown.

  • A spectrum of CEs was measured without absorbers, positioned close to detector window.

  • Results suggest that β-absorbers are required in γ-spectrometry to eliminate coincidences with electrons.

Abstract

In the use of photon detectors having sensitivity to β-rays or internal conversion electrons (CEs), β–γ and/or X–CE coincidence summing may arise. In particular, an n-type germanium detector has a thin dead layer and window, causing considerably higher sensitivity to β-rays and/or CEs especially when sources are located at a small source-to-detector distance. In the present research, reduction of the photopeak area due to β–γ or X–CE coincidence summing has been studied for three germanium detectors.

Introduction

In precise quantitative analyses of photon spectrometry with a germanium detector, corrections for the reduction of peak-areas due to coincidence summing effects are sometimes required if a radioactive decay involves cascade transitions, especially for close source-to-detector distance and for a larger detector. So far, corrections for γ–γ or X–γ coincidence summing have been extensively studied, and some inter-comparisons have been carried out to verify the correction methods (Lépy et al., 2010). However, coincidence summing effects may also occur between photon and β-particles provided that the detector has a considerably higher sensitivity to β-particles. In such a case, the resultant reduction of photopeak areas due to simultaneous detection of β-rays or internal conversion electrons (CEs) should be further considered.

Recently large n-type germanium detectors are being commonly used. Such detector types have thin dead layers and windows made of beryllium or carbon composite material, and may be highly sensitive to β-rays. For instance, in the case of thin samples such as aerosol filters, nearly the same situation may arise in X-ray spectrometry, in which X-ray peaks may be reduced because of the simultaneous detection of X-rays with CEs. In this study, possible effects on photon spectrometry due to the simultaneous detection of β-rays or CEs with photons were examined using several types of germanium detectors for various nuclides.

Section snippets

Reduction of photopeak area due to sum effects

The effects of γ–γ coincidence summing appear for nuclides emitting several γ-rays simultaneously. Especially at small source-to-detector distance, significant coincidence summing can occur. For nuclides with a simple β–γ decay scheme, in which one β-transition feeds one γ-ray transition, the gamma-ray efficiency, εγ, reduces to εγ in the following relation:εγεγ=(1Tβ),where Tβ is the total efficiency of the γ-detector for β-rays.

Nearly the same situation may arise in X-ray spectrometry, in

Experiments

For the experiments, aliquots of 106Ru–106Rh, 134Cs, 137Cs, and 152Eu were deposited on 0.8 mg/cm2 Mylar films, and one drop of diluted colloidal silica solution (LUDOX SM-30, dilution factor; 8000) was added for each source to reduce the self-absorption.

After drying, each source was sandwiched with another film. These sources were measured with a photon spectrometer at several source-to-detector distances, with and without absorbers made of polymethylmethacrylate (PMMA). The experiments were

Concluding remarks

Reduction of photopeak areas due to β–γ or X–CE coincidence summing has been illustrated in this study. The results revealed that absorbers of low-Z material with an appropriate thickness are required if accurate photon spectrometry is to be performed with n-type or planar germanium detectors, in order to reduce the effects of β–γ and X–CEs coincidence summing. Even for a conventional p-type detector, sensitivity to high-energy β-rays was demonstrated, so possible problems arising from β–γ or

Acknowledgments

The authors would like to express their sincere thanks to Mr. Unno of NMIJ for many experimental advises and also to our colleagues of JRIA for their encouragements and supports.

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Cited by (1)

  • Calculation of coincidence summing in gamma-ray spectrometry with the EGS5 code

    2015, Applied Radiation and Isotopes
    Citation Excerpt :

    However, coincidence summing occurs also for β-rays and γ-rays, for conversion electrons (CEs) and γ-rays, as well as for Auger electrons and γ-rays. In particular, the importance of β–γ coincidences has been pointed out by Ishizu et al. (Ishizu et al., 2014). We implemented decay schemes for 22Na, 24Na, 60Co, 88Y, 110mAg, and 134Cs in the EGS5 code (Hirayama et al., 2005), which handles not only γ-rays and X-rays, but also β-rays and CEs as source particles.

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