Investigation of cosmic-ray induced background of Germanium gamma spectrometer using GEANT4 simulation

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Highlights

  • Contribution of various components of cosmic rays on an HPGe gamma spectrometer were studied by Geant4.

  • The simulated total spectrum shows good agreement with the measurement background spectrum from 3 MeV up to 50 MeV.

  • Muonic component dominates with 86% of background.

  • Muons secondary particle shower contributes mostly to the spectrum in the energy region of below 3 MeV.

Abstract

In this article, a GEANT4 Monte Carlo simulation toolkit was used to study the response of the cosmic-ray induced background on a High-Purity Germanium (HPGe) gamma spectrometer in the wide energy range, up to 100 MeV. The natural radiation background measurements of the spectrometer were carried out in the energy region from 0.04 to 50 MeV. The simulated cosmic-ray induced background of the Ge detector was evaluated in comparison with the measured data. The contribution of various cosmic-ray components including muons, neutrons, protons, electrons, positrons and photons was investigated. We also analyzed secondary particle showers induced by the muonic component.

Introduction

Germanium (Ge) gamma spectrometers have been useful tools for analyzing radionuclides in environmental and food samples due to high efficiency and low background. The sensitivity of a Ge spectrometer is influenced by its detection efficiency, energy resolution and the natural radiation background sources at the measurement site. The background spectrum measured by a Germanium detector results from environmental gamma radiation, 222Rn and its gamma-ray-emitting daughters in the shield, cosmic rays and an intrinsic contamination of Ge detector and shield materials (Heusser, 1986, Heusser, 1993, Heusser, 1994, Vojtyla, 1996). To reduce the environmental gamma radiation, the Ge detector is mounted inside a passive shielding made of low-activity lead, iron or copper that is able to suppress most of the radiation from outside. To reduce a contribution from 222Rn and its daughters, nitrogen gas has been used to flush the shield. Cosmic rays component can be suppressed in underground laboratories or in ground laboratories if anticoincidence system is used Heusser, 1993, Thomas et al., 2013. More recently Cagniant et al. (2015) used a cosmic veto to design a new versatile ultralow background photon spectrometer installed in a ground laboratory level.

To understand the effect of cosmic rays to the Ge gamma spectrometers, there have been some works experimentally to study the cosmic-ray induced background to the Ge detector (Haines et al., 2011, Solc et al., 2014, Bikit et al., 2014). Cosmic rays can contribute to the background spectrum because of their penetrating power and large number of physical processes leading to background induction. An effective way to understand a contribution of cosmic rays to the Ge detector background is to use a Monte Carlo simulation of the detector background (Vojtyla, 1995, Vojtyla, 1996, Joković et al., 2009, Breier and Povinec, 2010, Solc et al., 2014). However, these studies were mostly compared without measured data (Vojtyla, 1995, Vojtyla, 1996, Joković et al., 2009, Breier and Povinec, 2010) or with the Ge-detector background measured bellow 25 MeV (Solc et al., 2014). The aim of this work was to study the response of cosmic-ray induced background in a High-Purity Germanium (HPGe) gamma spectrometer using GEANT4 simulation toolkit. The background measurements were carried out at ground laboratory level up to 50 MeV and compared with simulation data. We investigate contributions of cosmic-ray components, including muons, neutrons, protons, electrons, positrons and photons on the background spectrum. We also analyzed secondary particle showers induced by the muonic component. The simulations and analysis of the deposited energy in the Ge-detector by cosmic rays were carried out in the wide energy range, up to 100 MeV.

Section snippets

Experimental set-up

The HPGe detector (Canberra model GC2018) (Canberra, 2013) is of coaxial p-type (SEGe) with a relative efficiency of 20%, an active volume of 104 cm3 and a resolution of 1.8 keV at the 1332.5 keV gamma rays of 60Co. The Ge detector was mounted inside a cylindrical shield (Canberra model 747E) (Canberra, 2013) with 10 cm thick low-background lead. Energy range of measurements was setup from 0.04 to 50 MeV. The background counting rates measured during 5 days were 100 counts/min and 75 counts/min at

Comparison of experimental and simulated spectra

Fig. 2 shows measured and simulated deposited energy spectra of the Ge detector of radiation background (blue curve) and cosmic-ray induced background (black cure), respectively. In the high-energy region (above 3 MeV), there is a good agreement between the measured and simulated data. A peak in the region of 30–42 MeV, resulted from direct hits of comic-ray particles with the Ge material. In the low-energy region (below 3 MeV), the measured spectrum is higher than the simulated spectra because it

Summary

The cosmic-ray induced background of the HPGe gamma spectrometer was investigated using the GEANT4 Monte Carlo simulation and measured background data. The simulated deposited energy spectrum of the Ge detector showed a good agreement with the measured background spectrum in the energy region from 3 to 50 MeV. The so-called muon peak at about 36 MeV resulted from the direct hits of comic-ray muons with the Ge detector. In the low energy-region (below 3 MeV), the simulated data showed cosmic-ray

Acknowledgement

This work was supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under Grant numbers 103.04–2015.103.

References (18)

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