Environmental Gamma Rays Research Articles

Basic consideration of a nuclear power monitoring system using neutron-induced prompt gamma rays

ABSTRACTMonitoring only neutron flux in a nuclear reactor core has an advantage of offering reactor power monitoring accuracy. We started the development of a new nuclear instrumentation based on the measurement of prompt gamma rays emitted from metals placed at the neutron flux monitoring positions. The thermal neutron flux at the position of each placed metal piece can be monitored by measuring the prompt gamma rays as the count rate of each energy. The gamma-ray energy range was limited from 6 to 10 MeV to mitigate the interference of environmental gamma rays. Four metals, Ti, V, Ni, and Cu, were chosen as candidates in consideration of their neutron emission rates and self-absorption. In an experiment with a high-purity germanium semiconductor detector, we considered the identification of individual peak energies in an assumed situation where prompt gamma rays were emitted from the four different metals at the same time. Energy resolutions of the peak with the largest energy gap from the nearest energy peak of the other candidate metals were smaller than the gap. Thus, we confirmed that at least one peak for each candidate metal was able to be separated from the peaks derived from other candidate metals.

Optimization of the HPGe detector passive shields by Monte-Carlo simulations

Abstract The selection of appropriate materials and their thicknesses, which should be used for germanium spectrometer shielding, is an important and complex task with the aim to reduce the contribution of background events to detector spectra. These background events have origin in the radioactive decay of environmental radionuclides, and also are produced by cosmic-ray muon induced secondary particles within shielding materials. In this work, the emission of gamma rays arising from members of U-238 and Th-232 series, as well as from K-40, distributed in the surroundings of the cylindrically-shaped shields was simulated, together with cosmic-ray muons of average energy 2 GeV, interacting with the shields. Different types of materials (Pb, Fe, Cu) including variations of the shield thickness were considered in simulations based on Geant4 software, while internal contamination of shielding material and radon buildup inside the shielding were not taken into account. The simulated spectra for germanium detector were analyzed and mutually compared. The results obtained were useful to provide optimal parameters for the shielding of germanium detectors against the external radiation — cosmic-ray muons and environmental gamma rays.

A methodological approach to a radioactive sample analysis with low-level γ-ray spectrometry

In the present study a methodological approach to the analysis of a radioactive sample with low-level γ-ray spectrometry is reported. The sample provided by the IAEA for the 2002 proficiency test was choosen, because it represents a case study, due to its radionuclides content and thus to the different activity corrections necessary to obtain values in agreement with the reference ones. Efficiency curves were obtained for the counting geometry (a 100 ml pillbox on a 96.3% HPGe detector) and subsequently used to quantify radionuclide activities for the unknown sample with software package Gamma Vision 8.0. Both the calibration source and the unknown sample contained radionuclides giving rise to cascade summing effects, whose correction factors as obtained with EFFTRAN (Efficiency transfer and coincidence summing corrections for environmental gamma-ray spectrometry) code were employed. Obtained radioisotopes activities were compared with those certified and results reported.

Optimization of thickness of GAGG scintillator for detecting an alpha particle emitter in a field of high beta and gamma background

At the site of the Fukushima Daiichi nuclear power plant (FDNPP), there is a large quantity of beta and gamma emitters such as 90Sr and 137Cs. Moreover, radon (Rn) progeny, which are naturally occurring radionuclides, exist and emit alpha and beta particles. To detect plutonium isotopes (238Pu, 239Pu, and 240Pu) in a field of high beta and gamma background, an alpha particle detector with low beta- and gamma-ray sensitivity and good energy resolution for an alpha particle is required to distinguish plutonium isotopes from Rn progeny. Previously, we developed an alpha particle imaging detector by combining a 0.1-mm-thick gadolinium aluminum gallium garnet (GAGG) scintillator with a silicon photomultiplier (SiPM). However, this detector was sensitive to environmental gamma and beta rays. In this study, we optimized the thickness of the GAGG scintillator for alpha particle detection in a field of high beta and gamma background. We prepared three GAGG scintillators with thicknesses of 0.05 mm, 0.07 mm, and 0.1 mm. Each of the GAGG scintillators was coupled optically to the SiPM array, which was used as the photodetector. Alpha, beta, and gamma rays were irradiated onto the developed alpha particle detector, and their spectra were obtained. The energy resolution of the 0.05-mm-thick GAGG for 5.5-MeV alpha particles (∼11.6% full width at half maximum [FWHM]) was the best among the three GAGG scintillators. All GAGG scintillators used in this study were not sensitive to gamma rays with a dose rate of 1 mSv/h. The beta particle count above the lower level discriminator (LLD) decreased because the scintillator was thinner, and the beta count of the 0.05-mm-thick GAGG was only 1/100 that of the 0.1-mm-thick GAGG. Because the alpha particle detector with the 0.05-mm-thick GAGG scintillator had low beta and gamma-ray sensitivity and good energy resolution for alpha particles, it is promising from the viewpoint of detecting plutonium contamination in a field with high beta and gamma background, such as the FDNPP site.

An improved underground cosmic-ray detector made of RPCs

In order to suppress muon-induced background, veto detector is very important, even if the particle physics experiments are deployed underground. Owing to its high efficiency, low cost and low sensitivity to environmental gamma-rays, resistive plate chamber (RPC) is a good candidate for large-area underground cosmic-ray detectors. Based on the experiences and lessons from the RPC veto detector in Daya Bay neutrino experiment, an improved design has been proposed considering the extremely low muon flux, which results in underestimation of RPC layer detection efficiency in self-calibration. Additionally, allowing for the studies of 9Li/8He and fast neutrons, the tracking ability has been considered in design. Finally, an improved design of an RPC underground cosmic-ray detector has been achieved.

Environmental Low Energy Gamma Rays Measurements in Brazilian Tropics Region During 2016

Environmental Low Energy Gamma Rays Measurements in Brazilian Tropics Region During 2016

Simulation of background reduction and Compton suppression in a low-background HPGe spectrometer at a surface laboratory

High-purity germanium (HPGe) detectors are well suited to analyse the radioactivity of samples. In order to reduce the environmental background for an ultra-low background HPGe spectrometer, low-activity lead and oxygen free copper are installed outside the probe to shield from gamma radiation, with an outer plastic scintillator to veto cosmic rays, and an anti-Compton detector to improve the peak-to-Compton ratio. Using Geant4 tools and taking into account a detailed description of the detector, we optimize the sizes of these detectors to reach the design requirements. A set of experimental data from an existing HPGe spectrometer was used to compare with the simulation. For the future low-background HPGe detector simulation, considering different thicknesses of BGO crystals and anti-coincidence efficiency, the simulation results show that the optimal BGO thickness is 5.5 cm, and the peak-to-Compton ratio of 40K is raised to 1000 when the anti-coincidence efficiency is 0.85. In the background simulation, 15 cm oxygen-free copper plus 10 cm lead can reduce the environmental gamma rays to 0.0024 cps/100 cm3 Ge (50 keV-2.8 MeV), which is about 10−5 of the environmental background.

A performance study of an electron-tracking Compton camera with a compact system for environmental gamma-ray observation

An electron-tracking Compton camera (ETCC) is a detector that can determine the arrival direction and energy of incident sub-MeV/MeV gamma-ray events on an event-by-event basis. It is a hybrid detector consisting of a gaseous time projection chamber (TPC), that is the Compton-scattering target and the tracker of recoil electrons, and a position-sensitive scintillation camera that absorbs of the scattered gamma rays, to measure gamma rays in the environment from contaminated soil. To measure of environmental gamma rays from soil contaminated with radioactive cesium (Cs), we developed a portable battery-powered ETCC system with a compact readout circuit and data-acquisition system for the SMILE-II experiment [1,2]. We checked the gamma-ray imaging ability and ETCC performance in the laboratory by using several gamma-ray point sources. The performance test indicates that the field of view (FoV) of the detector is about 1 sr and that the detection efficiency and angular resolution for 662 keV gamma rays from the center of the FoV is (9.31 ± 0.95) × 10−5 and 5.9° ± 0.6°, respectively. Furthermore, the ETCC can detect 0.15 μSv/h from a 137Cs gamma-ray source with a significance of 5σ in 13 min in the laboratory. In this paper, we report the specifications of the ETCC and the results of the performance tests. Furthermore, we discuss its potential use for environmental gamma-ray measurements.

Discrimination between natural and other gamma ray sources from environmental gamma ray dose rate monitoring data.

In this study, a method to discriminate between natural and other γ-ray sources from environmental γ-ray dose rate monitoring data was developed, and it was successfully applied to actual monitoring data around nuclear facilities. The environmental dose rate is generally monitored by NaI(Tl) detector systems in the low dose rate range. The background dose rate varies mainly as a result of the deposition of (222)Rn progeny in precipitation and shielding of the ground by snow cover. Increments in the environmental dose rate due to radionuclides released from nuclear facilities must be separated from these background variations. The method in the present study corrects for the dose rate variations from natural sources by multiple regression analysis based on the γ-ray counting rates of single-channel analysers opened in the energy ranges of γ-rays emitted by (214)Bi and (208)Tl. Assuming a normal distribution of the results and using the one-sided type I error of 0.01 while ignoring the type II error, the detection limit of the γ-ray dose rate from artificial sources was 0.77 nGy h(-1).

Remote Monitoring of Environment by Android Smartphone through Secure Sockets Layer

A multichannel data acquisition device connected to a linux ubuntu computer was used to measure environmental gamma rays intensity and energy spectrum. LabVIEW program stored the data in memory mapped MySQL table during acquisition, java server program sent data to android client through secure sockets layer (SSL). Generation of keystore files and implementation of SSL communication were discussed.

III 環境γ線の特徴と被ばく線量との関係

環境中における適切な外部被ばく線量評価に必要な基本情報を提供する。まず，環境中に分布する典型的な線源から放出されるγ線の基本的な性質について説明するとともに，この性質を考慮して行われた被ばくシミュレーションにより得られた，広い年齢層に対する線量換算係数をまとめて紹介する。さらに，様々な要因による被ばく線量の変動の様子，また，空間線量率の測定値と被ばく線量の関係についても議論する。

Environmental gamma-ray dose rate measurement by using ultra-high sensitive LiF:Mg,Cu,Si TLD

Environmental gamma-ray dose rates were measured by using ultra-high sensitive LiF:Mg,Cu,Si thermoluminescence dosimeters (TLDs) during short-term burial. The resultant gamma-ray dose rates were compared with those measured by using optically stimulated luminescence (OSL) of Al2O3:C and assessed by using HPGe spectrometer. Regardless of the short-term burial and the change of environmental conditions as well as the mismatch of the effective atomic number (Zeff) with the soil (mainly SiO2), good agreement was observed between the doses measured by TLDs and OSL dosimeters or the assessed doses. LiF:Mg,Cu,Si TLD has been found to be an effective alternative for the measurement of environmental gamma-ray annual dose rates for use in optical dating.

Calculation of true coincidence summing corrections for extended sources with EFFTRAN

A deterministic code was developed for the calculation of true coincidence summing correction factors and has been incorporated into the EFFTRAN tool. The approach is aimed at the analysis of extended samples measured on p-type HPGe detectors in environmental gamma-ray spectrometry and was verified against the results of a state-of-the-art full Monte Carlo code. The two sets of results matched on average within 1%. Our code requires no measurements in addition to a standard full-energy-peak calibration, has a very short run time and takes into account the spatial variation of the efficiency across the sample volume. The EFFTRAN code is free software, available from the authors on request.

An accurate method for the determination of 226Ra activity concentrations in soil

A quantitative method to determine the activity concentration of 226Ra in soil samples was established using high performance environmental gamma-ray spectrometry. In this method, a semi-empirical calibration procedure was developed for full energy peak efficiency calculation utilizing the elemental composition of the soil sample. Aatami software was used to deconvolute the 235U and 226Ra doublet at 185.7 keV and 186.2 keV, respectively, and to fit the baseline of the soil gamma-spectrum for the determination of 226Ra activity. The results indicated that the Aatami doublet deconvolution procedure provides a rapid and accurate analysis of a complicated spectrum in comparison with other cumbersome spectral interference correction methods. The study also compared the results with those obtained by radon progeny (214Pb, or 214Bi) measurements and found that the deconvolution method provided a more accurate 226Ra activity as it is independent of the error caused by radon diffusion. This error can be quite large since the amount of escaped radon gas through the sample container walls and sealing cannot be accurately quantified.

Environmental gamma radiation measurement in District Swat, Pakistan

External exposure to environmental gamma ray sources is an important component of exposure to the public. A survey was carried out to determine activity concentration levels and associated doses from (226)Ra, (232)Th, (40)K and (137)Cs by means of high-resolution gamma ray spectrometry in the Swat district, famous for tourism. The mean concentrations for (226)Ra, (232)Th and (40)K were found to be 50.4 +/- 0.7, 34.8 +/- 0.7 and 434.5 +/- 7.4 Bq kg(-1), respectively, in soil samples, which are slightly more than the world average values. However, (137)Cs was only found in the soil sample of Barikot with an activity concentration of 34 +/- 1.2 Bq kg(-1). Only (40)K was determined in vegetation samples with an average activity of 172.2 +/- 1.7 Bq kg(-1), whereas in water samples, all radionuclides were found below lower limits of detection. The radium equivalent activity in all soil samples is lower than the limit set in the Organisation for Economic Cooperation and Development report (370 Bq kg(-1)). The value of the external exposure dose has been determined from the content of these radionuclides in soil. The average terrestrial gamma air absorbed dose rate was observed to be 62.4 nGy h(-1), which yields an annual effective dose of 0.08 mSv. The average value of the annual effective dose lies close to the global range of outdoor radiation exposure given in United Nations Scientific Committee on the Effects of Atomic Radiation. However, the main component of the radiation dose to the population residing in the study area arises from cosmic ray due to high altitude.

Development of Wide-energy Range X/γ-ray Survey-meter

The X-ray radiation produced from various X-ray generating machines has energies between 8 keV to 300 keV and for monitoring environmental gamma rays, the energy range to be measured is from 40 keV up to 1500 keV. We developed a wide-energy range X/γ-ray survey-meter , which can measure ambient dose equivalent H*(10), covering the energy range of 8 keV to 1500 keV by dividing the two energy ranges, from 8 to 300keV for X-ray mode and from 40 to 1500keV for γ-ray mode, both energy responses are within 25% from 10keV up to 300keV and from 50keV up to 1500keV using a 12.7mm diameter by 12.7mm thick NaI(Tl) scintillator.

An underground cosmic-ray detector made of RPC

Owing to its high efficiency, low cost and low sensitivity to environmental gamma-rays, resistive plate chamber (RPC) is a good candidate for large area underground cosmic-ray detectors. We report in this paper such a design for the Daya Bay reactor antineutrino experiment based on calculations and simulations for the efficiency, dead space control, noise and gamma-ray backgrounds. Experimental tests are performed, and good agreements with calculations and simulations are obtained, showing that the design is appropriate.

First tests of the active shield for a gamma ray spectrometer

The operational problems of the gamma ray spectrometer shielded passively with 12 cm of lead and actively by five 0.5 m × 0.5 m × 0.05 m plastic veto shields are described. The active shielding effect from environmental gamma ray, cosmic muons, and neutrons was investigated. Wide range of scintillator pulses, corresponding to the energy range of 150 keV–75 MeV, was used for anticoincidence gating. With the optimal set up, the integral background of 0.31 c/s was achieved for the energy region of 50–3000 keV. The detector mass-related background was 0.345 c/kg s. The 511 keV annihilation line was reduced by factor of 7 by the anticoincidence gate. It is shown that the plastic shields increase the neutron capture gamma line intensities due to neutron termalization.

A method for calculation of true coincidence summing correction factors for extended sources

In gamma-ray spectrometry, true coincidence summing correction factors for an extended sample can be calculated from full-energy-peak and total efficiencies as if the sample were a point source, if the so-called linear-to-square- (LS) curve, introduced by Blaauw and Gelsema, is known and properly applied. A method is described for obtaining the efficiencies and the corresponding LS-curve for an arbitrary cylindrical sample from calibration measurements in a reference geometry. The approach is aimed at the analysis of samples measured on p-type HPGe detectors in environmental gamma-ray spectrometry and was successfully verified against experimental data.

Calculation of total efficiencies of extended samples for HPGe detectors

Calculation of coincidence summing correction factors in gamma-ray spectrometry can be carried out by making use of full-energy-peak and total efficiencies for a given sample geometry and detector setup. In the case of extended samples, the contribution of gamma-rays that scatter in the sample itself to the total efficiency needs to be taken into account. A method is described for calculating this contribution based on calibration measurements with point sources positioned on the detector housing. The approach is aimed at environmental gamma-ray spectrometry analysis of cylindrical samples on p-type HPGe detectors and is verified against full Monte Carlo calculations of total efficiencies with the GEANT code.