Gamma-ray Full Spectrum Analysis for Environmental Radioactivity by HPGe Detector

Research Paper J. Astron. Space Sci. 31(4), 317-323 (2014) http://dx.doi.org/10.5140/JASS.2014.31.4.317 Gamma-ray Full Spectrum Analysis for Environmental Radioactivity by HPGe Detector Meeyoung Jeong1,2, Kyeong Beom Lee2†, Kyeong Ja Kim3, Min-Kie LEE2, Ju-Bong HAN1 1 Radiation Measurement and Radiation Safety, University of Science and Technology, Daejeon, Korea Korea Research Institute of Standards and Science, Daejeon 305-600, Korea 3 Korea Institute of Geoscience & Mineral Resources, Daejeon, Korea 2 Odyssey, one of the NASA’s Mars exploration program and SELENE (Kaguya), a Japanese lunar orbiting spacecraft have a payload of Gamma-Ray Spectrometer (GRS) for analyzing radioactive chemical elements of the atmosphere and the surface. In these days, gamma-ray spectroscopy with a High-Purity Germanium (HPGe) detector has been widely used for the activity measurements of natural radionuclides contained in the soil of the Earth. The energy spectra obtained by the HPGe detectors have been generally analyzed by means of the Window Analysis (WA) method. In this method, activity concentrations are determined by using the net counts of energy window around individual peaks. Meanwhile, an alternative method, the so-called Full Spectrum Analysis (FSA) method uses count numbers not only from full-absorption peaks but from the contributions of Compton scattering due to gamma-rays. Consequently, while it takes a substantial time to obtain a statistically significant result in the WA method, the FSA method requires a much shorter time to reach the same level of the statistical significance. This study shows the validation results of FSA method. We have compared the concentration of radioactivity of 40K, 232Th and 238U in the soil measured by the WA method and the FSA method, respectively. The gamma-ray spectrum of reference materials (RGU and RGTh, KCl) and soil samples were measured by the 120% HPGe detector with cosmic muon veto detector. According to the comparison result of activity concentrations between the FSA and the WA, we could conclude that FSA method is validated against the WA method. This study implies that the FSA method can be used in a harsh measurement environment, such as the gamma-ray measurement in the Moon, in which the level of statistical significance is usually required in a much shorter data acquisition time than the WA method. Keywords: gamma-ray full spectrum analysis, gamma-ray spectrometer, lunar orbiter payload, anticoincidence background suppression 1. INTRODUCTION The geological matrix in the Earth and the Moon contain the primary natural radionuclides. These are the single nuclide as 40K and the decay series of 232Th and 238U. The decay series of 232Th and 238U emit three different types of radiation: alpha particles, beta particles and gamma-rays. As gamma-rays have more extreme radiation penetration than alpha and beta particles, gammas are used in the radiation measurement. The activity concentrations of 232Th and 238 U are same as those of their progenies due to radioactive equilibrium (secular equilibrium). Typically, gamma-ray spectroscopic system utilizes Window Analysis (WA) method in order to measure soil based radioactivity. Hendriks, PHGM et al. (2001) utilized Full Spectrum Analysis (FSA) method in the development of high-efficiency BGO scintillation detector system for a comparison with conventional WA system which uses NaI detector. Also, Maphoto KP (2004) compared two analysis methods using an HPGe detector, and utilized FORTRAN program for FSA. The two methods showed a difference of 1% ~ 61 % depending on the samples. In this study, a FORTRAN program was developed for the technology development of FSA to replace the WA method This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http:// creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Received Nov 6, 2014 Revised Dec 3, 2014 Accepted Dec 4, 2014 †Corresponding Author E-mail: , ORCID: 0000-0002-5202-2004 Tel: +82-42-868-5379, Fax: +82-42-868-5671 Copyright © The Korean Space Science Society 317 http://janss.kr plSSN: 2093-5587 elSSN: 2093-1409 J. Astron. Space Sci. 31(4), 317-323 (2014) Table 1. The data of used materials. and a low-level background gamma-ray spectrometer developed by Korea Research Institute of Standards and Science (KRISS) was used. This gamma-ray spectrometer is equipped with a passive shield to reduce background signals and an active shield to eliminate cosmic rays. It is operated by anti-coincidence mode with HPGe detectors. The purpose of this study is to ensure the validity of FSA method by evaluating uncertainties between the measurement and analysis results of the WA and the FSA. WA method requires long measurement time and only takes into account of the interested peaks in measured spectra, thus, the accuracy is low. Moreover, it has a disadvantage of slow analyzing speed. In this study, a validation is performed for the Full Spectrum Analysis (FSA) method which complements the WA method to account for all the energy peaks from full spectrum in addition to the contributions of the Compton scattering, thus enabling measurement of statistical significance in a short period of time. Mesh Mass (kg) Density (g/cm3) Activity concentration (Bq/kg) RGU(238U) RGTh(232Th) KCl(40K) soil 240 240 200 200 0.1076 0.1089 0.0825 0.0855 1.196 1.210 0.916 0.950 4940 3250 16602 2.2 Methods Gamma-ray spectroscopic system detects gammas emitted from sample materials using a High Purity Germanium (HPGe) detector (Khandaker 2011). As radioactive isotopes emit gammas of intrinsic energy, if we measure the energy of gammas, we can tell apart the kind of isotope and its activity. Gamma-ray spectroscopic system is used to identify the isotopes and activities by measuring gammas from a sample material. Most of HPGe detectors use high-density and high atomic numbered lead as a shield to reduce background noise due to radiation and radioactive dirt existing around the detectors. HPGe gamma-ray spectrometers are used widely for the measurement of environment radiations (Murray et al. 1987). 2. MATERIALS AND METHODS 2.1 Materials The reference materials, IAEA-RGU-1 and IAEA-RGTh-1, were prepared by the Canada Centre for Mineral and Energy Technology on behalf of the International Atomic Energy Agency (IAEA), while the 40K reference material (KCl of 99.5% purity) was produced in Japan (Wieser 2006). The reference materials were sampled in a 90 ml can and were kept for radioactive equilibrium between radium and radon daughter nuclides. For the background data acquisition, a 90 ml can was filled with pure water. Table 1 lists the information of the reference sources from the samples, such as mass and activity concentration. The detailed sample preparation process is summar (...truncated)