Development of a panorama coded-aperture gamma camera for radiation detection

For radiation detection, the imaging system should have a large field of view (FOV) and high detection efficiency because it has to be used in a radiation environment where the quantity and direction of radioactive sources are unknown. A panorama coded-aperture gamma camera optimized for use in complex nuclear environment has been developed and evaluated with an angular resolution of 3.5°. Typical gamma cameras have the limited field of view ranging from 10° to 60° in both horizontal and vertical direction. The system presented in this paper extends the field of view to 360° in the horizontal direction and 60° in the vertical direction. The partial encoding of coded aperture imaging is mitigated by convolving the data of diverse adjacent modules with a partial transmission function. The experimental feasibility of measuring multiple sources in the 360° horizontal field of view was demonstrated with a panoramic image. The results showed that the system could clearly identify the direction of multiple radiation sources in an unknown extended radiation environment. The system can help to simplify the clean up and decommissioning of nuclear sites.     

Experimental and simulated validation of the energy dependence of saturation thickness of multiple scattered gamma rays

Saturation thickness for multiple scattering gamma rays from multiple sources has been measured experimentally and simulated using the Monte Carlo N-Particle (MCNP) Code. Experimental measurements were performed using a collimated beam of gamma-rays from ⁵⁷Co, ²⁰³Hg, ¹³³Ba, ²²Na, ¹³⁷Cs, ⁶⁵Zn and ⁶⁰Co sources. The gamma rays were directed at rectangular aluminium targets of varying thickness. A NaI (Tl) scintillation detector placed at a backscattering angle of 180° was used to detect the scattered photons. The measured and calculated saturation thickness increases with increasing energy of incident gamma-rays. Experimental and simulated values are compared and are in good agreement.     

Capabilities of the Gamma-400 Gamma-ray Telescope for Observation of Electrons and Positrons in the TeV Energy Range

The space-based GAMMA-400 gamma-ray telescope will measure the fluxes of gamma rays in the energy range from ∼20 MeV to several TeV and cosmic-ray electrons and positrons in the energy range from several GeV to several TeV to investigate the origin of gamma-ray sources, sources and propagation of the Galactic cosmic rays and signatures of dark matter. The instrument consists of an anticoincidence system, a converter-tracker (thickness one radiation length, 1 X₀), a time-of-flight system, an imaging calorimeter (2 X₀) with tracker, a top shower scintillator detector, an electromagnetic calorimeter from CsI(Tl) crystals (16 X₀) with four lateral scintillation detectors and a bottom shower scintillator detector. In this paper, the capability of the GAMMA-400 gamma-ray telescope for electron and positron measurements is analyzed. The bulk of cosmic rays are protons, whereas the contribution of the leptonic component to the total flux is ∼10⁻³ at high energy. The special methods for Monte Carlo simulations are proposed to distinguish electrons and positrons from proton background in the GAMMA-400 gamma-ray telescope. The contribution to the proton rejection from each detector system of the instrument is studied separately. The use of the combined information from all detectors allows us to reach a proton rejection of up to ∼1 × 10⁴.

     

Calibration of a germanium well-detector using 60Co: The effects of the angular correlation of the two gamma rays emitted in cascade,quantified with Monte Carlo simulations

Radiation sources of ⁶⁰Co are commonly measured by means of HPGe gamma spectrometers, either as unknown sources or as calibration sources. However, the two gamma rays that are emitted by this nuclide, at 1.17 MeV and 1.33 MeV respectively, follow each other so rapidly that in the cases where both photons interact with the crystal, the electronics will record a single additive pulse. This is a cascade or coincidence summing effect. Such effect induces in the gamma spectra a “sum-peak”, appearing at 2.5 MeV on the energy axis, generated by the pair of photons which have both been entirely absorbed by the detector. Also, the second photon is correlated for direction to the first one, i.e., it will not be emitted at random, with an isotropic probability. Then the question arises of to what extent the two effects, cascade and correlation, might affect the count rates for the three peaks. Various answers have already been published, but without practical solutions. In this context, the present work was devoted to further explore the question by means of Monte Carlo simulation, in the case of a welltype detector. As a result, inside the well the sum peak only is not affected and it allows accurate evaluation of the detector efficiency at 1.25 MeV. Outside the well, near the detector, none of the three peaks can be directly used for efficiency evaluation, unless the relevant corrections can be evaluated. At a distance from the detector, the two single peaks can be used, but with the drawback of a low efficiency.     

Scientific tasks and present status of the GAMMA-400 project

The GAMMA-400 telescope is designed to investigate discrete high-energy gamma-ray sources in the energy range of 0.1–3000 GeV, to measure the energy spectra of galactic and extragalactic diffuse gammaray emissions, and to study gamma-ray bursts and gamma-ray emissions from an active Sun. The gamma-ray telescope has an angular resolution of ～0.01°, an energy resolution of ～1%, and a proton rejection factor of ～10⁶. Its special assignment is to measure fluxes of gamma rays, electrons, and positrons that could be associated with the annihilation or decay of dark matter particles.     

Optimum condition of efficiency functions for HPGe γ-ray detectors in the 121-1408 keV energy range

The optimum condition of three commonly used functions in the Genie 2000 γ spectra analysis software have been studied in the 121-1408 keV energy range.The three functions are applied for fitting the full-energy peak effciency of the HPGe gammaray detector.A detailed procedure to obtain the optimum condition is described.The HPGe detector is calibrated at 11 cm by three radioactive sources of point form(152Eu,137Cs,60Co) providing 11 energy peaks.After data processing,results shows that the three functions used in the Genie 2000 gamma spectra analysis software fit best at orders 3-5.Lastly the standard radioactive source 133Ba is chosen to validate the results.Differences between the standard activity of 133Ba and the result obtained from the fitting functions are below 1.5%.Therefore the optimum orders of the three functions used in the Genie 2000 γ spectra analysis software are 3-5 with the 11 energy peaks.     

Empfehlungen zum Gebrauch einheitlicher Halbwertzeiten einiger wichtiger Radionuklide

The present work represents the results of angular distribution measurements performed for promt gamma-rays in the range of 1.1 … 1.2. MeV due to the ²³³U(n, f) and ²³⁹Pu(n,f) reactions. The measurements yielded anisotropy values found to be consistent with values previously measured and using the same technique at different gamma energy bands. Such consistency, also found to exist between anisotropy values previously measured for the ²³⁵U(n,f) reaction, is a strong evidence about the energy independence of the anisotropy and indicates that the gamma spectra (at 180° and 90° to the fission direction) are essentially the same. The average values of the fragment angular momentum were calculated according to Strutinsky and Nix-Swiatecki theories. It was found that the values of the average angular momentum calculated for ²³⁴U, ²³⁶U and ²⁴⁰Pu according to Strutinsky,s formula (at different gamma energy bands) are consistent and yield average values which are in good agreement with those obtained from direct measurements.     

Characteristics of the GAMMA-400 gamma-ray telescope for searching for dark matter signatures

The GAMMA-400 gamma-ray telescope currently under development is designed to measure fluxes of gamma rays and electron-positron cosmic-ray components, which could be associated with the annihilation or decay of dark matter particles, and to survey in detail the celestial sphere in order to search for and investigate discrete gamma-ray sources; to measure the energy spectra of Galactic and extragalactic dif- fuse gamma-ray emissions; and to study gamma-ray bursts and the gamma-ray emissions of active Sun. The GAMMA-400 energy range is 100 MeV to 3000 GeV. The gamma-ray telescope has an angular resolution of ～0.01°, an energy resolution of ～1%, and a proton rejection factor of ～10⁶. The GAMMA-400 will be installed on Russia’s Navigator space platform. Observations are planned to commence in 2018.     

The effect of heat treatment on the thermoluminescence of naturally-occurring calcites and their use as a gamma-ray dosimeter

The feasibility of using naturally-occurring calcite for gamma-ray dosimetry was investigated. Anneal treatment above 350°C increased the sensitivity of all radiation-induced TL peaks except the glow peaks above 300°C. On the other hand, annealing in air, at a temperature of 700°C caused a collapse in the TL sensitivity. The increase in TL efficiency was found to depend on the annealing temperature and time. Heating at 600°C for 5 h and quenching in ambient air are the optimum conditions for TL sensitivity enhancement in the calcite materials investigated. These results are explained using the energy scheme of the pre-dose model of Zimmerman (1971) and in terms of the impurity rearrangements in the crystal lattice induced by heating. It was found that the values of the kinetic parameters E, s and b for TL glow peaks remained unchanged for annealed samples. The TL dose–response curves for stable dosimetric peaks of annealed and unannealed calcite samples could be fitted to the same linear mathematical function. This implies that the annealing process probably does not change the nature of the trapping centers except the low temperature TL peaks at 125 and 160°C of flowstone. The TL dosimetric parameters of calcite samples annealed, including glow curves, fading characteristics, dose–responses, dose-rate responses and energy responses, have also been studied in detail. The response to gamma-rays of annealed calcite samples was found to be linear from 0.05 to 10⁴ Gy. The lower limit of observable doses for each calcite sample was about 0.05 Gy. This offers the possibility of applying the investigated materials for gamma-ray dosimetry within this useful range. These dosimeters can be used in various applications, such as, in industries related to chemical technology (polymerization), food processing and in determining the dose received by the patient during medical examination and treatment.     

Dark matter and generation of galactic magnetic fields

We look for possible spectral features and systematic effects in the Fermi LAT publicly available high-energy gamma-ray data by studying photons from the Galactic center, nearby galaxy clusters, nearby brightest galaxies, AGNs, unassociated sources, hydrogen clouds and from the Earth limb. Apart from the already known 130 GeV gamma-ray excesses from the first two sources, we find no statistically significant excesses from any of the cosmological sources nor from any control region. Therefore our main effort goes to the study of gamma rays appearing from the Earth limb. In the energy range of 30 to 200 GeV the Earth limb gamma-ray spectrum follows a power-law with spectral index 2.86±0.05 at 95 % CL, in a good agreement with the PAMELA measurement of the cosmic ray proton spectral index of 2.82–2.85, confirming the physical origin of the limb gamma-rays. In subsets of the Earth limb data at small photon incidence angle spectral features occur, including a feature at 130 GeV. We observe a systematic ～2σ-level difference in the Earth limb spectra with small and large incidence angles. The behavior of those spectral features as well as the background indicates that those may be statistical fluctuations or complicated unknown systematic effects of the Fermi LAT. In the latter case, only the Fermi LAT Collaboration can give the final answer having access to raw data and all details of the reconstruction.     

基于FPGA的数字反符合γ谱仪系统设计

本文设计了基于FPGA的数字反符合γ谱仪系统来降低天然本底和康普顿散射对低活度放射性测量的影响。该系统选用Φ145 mm×95 mm×80 mm的NaI （Tl）环形探测器与Φ75 mm×75 mm的NaI （Tl）主探测器构成反符合探测器,采用FPGA和高速ADC同步采样主探测器脉冲信号和反符合环形探测器输出信号。在FPGA中实现了核脉冲信号采集、缓存、反符合甄别、梯形成形等相关算法。在天然本底测量实验中,数字反符合γ谱仪系统的计数率为191.80 cps,本底抑制系数为2.69;对137Cs放射源的测量实验表明,在反符合探测器端面中心处,反符合测量峰总比为0.41,能量分辨率为6.99%;在反符合探测器侧面中间部位,反符合测量峰总比为0.30,能量分辨率为7.48%。实验结果表明,基于FPGA的数字反符合γ谱仪系统明显降低了天然环境本底和康普顿散射对测量的影响,适用于低活度放射性测量、现场就地放射性测量。     

173Lu和174gLu衰变数据的测量

175Lu（n;2n）174m,gLu（n;2n）173Lu反应在高能中子通量监测中具有重要作用,其产物的放射性活度通常用HPGe探测器测量,这就需要174m,gLu和173Lu的半衰期和γ射线发射率等衰变数据必须准确无误。在主要的评价核数据库中,只有ENDF/B 7.1和JEFF 3.1.1数据库给出了173Lu和174Lu的半衰期和γ射线的发射几率等衰变数据,其中173Lu的射线发射几率ENDF/B 7.1库比JEFF 3.1.1库总体偏高,其他评价数据两个数据库非常一致。在中国原子能研究院的串列加速器上用20 MeV质子辐照金属Yb靶生产了含有173Lu,174m,gLu的放射性溶液,用激光共振电离质谱（LRIMS）和热表面电离质谱（TIMS）两种同位素稀释法测定了该溶液中173Lu和174m,gLu核素浓度,然后制备测量源在HPGe探测器上进行了近7年的跟踪测量,发现173Lu的半衰期为1.45 a,比目前评价的数据1.37 a高6.1%,636.1 keV射线发射几率的偏差最大,比ENDF/B 7.1的评价数据偏高7%。174gLu半衰期的测量结果3.37 a175Lu（n;2n）174m,gLu（n;2n）173Lu反应在高能中子通量监测中具有重要作用,其产物的放射性活度通常用HPGe探测器测量,这就需要174m,gLu和173Lu的半衰期和γ射线发射率等衰变数据必须准确无误。在主要的评价核数据库中,只有ENDF/B 7.1和JEFF 3.1.1数据库给出了173Lu和174Lu的半衰期和射线的发射几率等衰变数据,其中173Lu的射线发射几率ENDF/B 7.1库比JEFF 3.1.1库总体偏高,其他评价数据两个数据库非常一致。在中国原子能研究院的串列加速器上用20 MeV质子辐照金属Yb靶生产了含有173Lu,174m,gLu的放射性溶液,用激光共振电离质谱（LRIMS）和热表面电离质谱（TIMS）两种同位素稀释法测定了该溶液中173Lu和174m,gLu核素浓度,然后制备测量源在HPGe探测器上进行了近7年的跟踪测量,发现173Lu的半衰期为1.45 a,比目前评价的数据1.37 a高6.1%,636.1 keV γ射线发射几率的偏差最大,比ENDF/B 7.1的评价数据偏高7%。174gLu半衰期的测量结果3.37 a,比评价数据3.31 a高约1.8%,174gLu 76.5 keV和1 241.8 keV γ射线的发射几率比ENDF/B 7.1的评价数据分别低1.87%和12.8%。175Lu(n,2n)174m,gLu(n,2n)173Lu are important neutron reactions to monitor the high energy neutron fluxes and the residual nuclei, 174m,gLu and 173Lu, are usually measured by a HPGe spectrometer conveniently so that the decay data such as half lives and gamma-ray emission probabilities of 174m,gLu and 173Lu must be accurate. There are evaluated decay data only in ENDF/B 7.1 and JEFF 3.1.1 among the major evaluated nuclear data libraries, where the most data are almost same besides the gamma-ray emission probabilities of 173Lu are higher in ENDF/B 7.1 than that in JEFF 3.1.1. Yb metal as a target was irradiated by 20 MeV proton beams on a tandem accelerator in CIAE to produce 174m,gLu and 173Lu, and 176Lu isotopic dilution methods based upon a laser resonance ionization mass spectrometer (LRIMS) and a thermal surface ionization mass spectrometer (TIMS) were employed to determine the numbers of nuclides of 174Lu and 173Lu in a solution containing the irradiated target. Several radioactive sources made from the solution had been measured by a HPGe detector during the past 7 years and the peak intensities of the characteristic gamma-rays from 174gLu and 173Lu were analyzed to determine the half lives and gamma-ray emission probabilities. According to the measurements, the half life of 173Lu is 1.45 a, which is about 6.1% longer than the evaluated 1.37 a, and the gamma-ray emission probabilities of 173Lu is also different from the evaluated data, especially the emission probability of 636.1 keV gamma-ray of 173Lu is 7% higher than the value in ENDF/B 7.1. The half life of 174gLu is 3.37 a, about 1.8% longer than the evaluated 3.31 a, and the emission probabilities of 76.5 keV and 1 241.8 keV gamma-rays of 174Lu are 1.87% and 12.8% lower than the evaluated data in ENDF/B 7.1, respectively.     

Search for UHE cosmic sources with a large-area underground muon detector

Summary The newly discovered gamma-ray emission from Cyg-X 3 with energies up to 2·10¹⁶eV gives the first evidence that particle acceleration in galactic gamma-ray sources can reach energies of the order of 10⁴ TeV. We discuss the detectability of this type of sources by an underground muon tracking detector with area of the order of ∼1000 m² and the physical information which could be obtained in this way. Paper presented at the 2° Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984.     

A New Way of Looking at Nuclear Resonant Scattering

A new model for nuclear-resonant scattering of gamma radiation from resonant matter has been developed and is summarized here. This “coherent-path” model has lead to closed-form, finite-sum solutions for radiation scattered in the forward direction. The solution provides a unified microscopic picture of nuclear-resonant scattering processes. The resonant absorber or scatterer is modeled as a one-dimensional chain of “effective” nuclei or “effective” planes. The solution is interpreted as showing that the resonant radiation undergoes sequential scattering from one absorber “nucleus” or “plane” to another before reaching the detector. For recoil-free processes the various “paths” to the detector contribute coherently. The solution for this case gives calculated results that are indistinguishable from those using the classical optical model approach, although the forms of the solutions are completely different. The coherent-path model shows that the “speed-up” and “dynamical beating” effects are primarily a consequence of the fact that the single “effective” nuclear scattering processes are 180° out of phase with the incident radiation while the double nuclear scattering processes are in phase with the incident radiation. All multiple scattering paths are, and must be, included. The model can also treat the incoherent processes, i.e., processes involving gamma emission with recoil or conversion-electron emission. The source of the resonant gamma radiation can be from a radioactive source or from synchrotron radiation: both cases are treated. The model is used to explain and understand the results when each of the following experimental procedures is applied: time-differential Mössbauer spectroscopy, time-differential synchrotron radiation spectroscopy, enhanced-resolution resonant-detector Mössbauer spectroscopy, and the “gamma echo”.     

Experimental calculations of number,energy and dose albedos for various materials using 662 keV gamma rays

ABSTRACT

Number, energy and dose albedos are measured at a scattering angle of 180° for a broad beam of 662 keV gamma rays obtained from a radioactive source of ¹³⁷Cs (having strength in µCi; 1 Ci?=?3.7?×?10¹⁰ disintegrations per second). The gamma beam is incident on semi-infinite thick targets of variable atomic numbers. The scattering media is divided into three sets, which are pure elements, alloys and composite materials. Experiments are carried out using a 3^″?×?3^″ NaI(Tl) scintillation detector. To obtain precision in data, the response unfolding of a scintillation detector is used, which converts the observed pulse-height distribution to a true photon spectrum over the energy range of 2.5 to 640 keV. The detector response unfolding results in the true intensity of back-scattered gamma flux by shifting the events resulting from partial absorption of photons to the full energy peak of the spectrum. In the present study, albedo factors are studied as a function of target thickness and their atomic number. The experimentally calculated numbers of back-scattered gamma photon are in good agreement with theoretically generated numbers of multiple back-scattered counts by using a Monte Carlo simulation code. The experimental data on energy and intensity of 662 keV gamma photons are used to evaluate the number, energy and dose albedos for different materials under investigation.     

GdBr3:Ce in glass matrix as nuclear spectroscopy detector

An alumina-silica glass cylinder with cerium-activated gadolinium bromide, 2.5 cm in diameter and 3 cm long was cast to form a scintillation detector for spectral analysis of nuclear radiation, especially gamma rays. The detector, operated with a conventional photomultiplier tube and associated power supply, amplifier, and multichannel analyzer, shows for the first time for this material system a set of spectra that include well-defined full-energy gamma-ray peaks from 59.5 keV to 2500 keV. The full-energy peaks were characterized in terms of pulse height linearity, resolution, and counting efficiency. Energy spectral resolution also was demonstrated for alpha particles, beta particles, and ²³⁹Pu/Be neutrons. Gamma-ray detection efficiency was comparable to a conventional NaI(Tl) detector of similar volume but peak resolution at 662 keV in this new detector type was currently only 27%, compared to about 7% for the NaI(Tl) detector. Improved light output and thus resolution is being sought by optimizing glass compositions and processing.     

Antimatter in the Universe: constraints from gamma-ray astronomy

We review gamma-ray observations that constrain antimatter – both baryonic and leptonic - in the Universe. Antimatter is probed through ordinary matter, with the resulting annihilation gamma-rays providing indirect evidence for its presence. Although it is generally accepted that equal amounts of matter and antimatter have been produced in the Big Bang, gamma-rays have so far failed to detect substantial amounts of baryonic antimatter in the Universe. Conversely, positrons are abundantly observed through their annihilation in the central regions of our Galaxy and, although a wealth of astrophysical sources are plausible, their very origin is still unknown. As both antimatter questions – the source of the Galactic positrons and the baryon asymmetry in the Universe - can be investigated through the low energy gamma-ray channel, the mission concept of a dedicated space telescope is sketched out.     

NaI探测器对23.8 MeVγ射线的效率刻度方法

低能D(d,γ)4He辐射俘获反应截面的研究在聚变领域和天体物理等领域中起到非常重要的作用。由于受到标准γ源能量的制约,在研究D(d,γ)4He反应高能γ射线产额实验过程中不能用标准源进行效率刻度。采用实验测量与计算相结合的方法实现NaI探测器对23.8 MeV γ射线的效率刻度,先对6.13MeVγ射线效率进行实验刻度,然后用蒙卡程序MCNP－4C模拟计算出NaI探测器的效率曲线,再用实验数据与模拟结果进行对比校正,计算出NaI探测器在23.8MeV能量点对γ射线的探测效率。该方法对高能γ射线效率刻度给出了一种参考依据。In the study of D(d,γ)4He radiative capture reactions research, which is very important for fusion and astrophysics, the efficiency calibration of the detector is necessary. Due to the restriction on energy of the standard gamma source, the efficiency calibration of high-energy gamma-rays can not be calibrated by a standard source. In this paper, the method combining the experimental measurements and calculations for the efficiency calibrations of high energy gamma rays is given in the experiment of 6.13MeV gamma rays efficiency calibration using NaI detector, the efficiency curve of which is calculated by MCNP-4C and corrected experimentally. In this case, the detection efficiency of 23.8MeV gamma ray is known. The method provides a reference to high energy gamma ray efficiency calibration.     

用于n,γ混合场的新型脉冲中子探测器研究

新型脉冲中子探测器采用特殊工艺将两个PIN半导体组合而成.利用脉冲γ辐射研究了探测器对γ的响应；利用脉冲中子源研究了探测器对DT中了的响应，并与闪烁探测器进行了比较 .结果表明：脉冲中子探测器对脉冲γ辐射基本不灵敏，对脉冲中子辐射的灵敏程度依赖于中子辐射体，是一种用于n，γ混合脉冲辐射场中子测量的新型探测器. 关键词： 硅半导体 差分补偿 脉冲中子探测器 n γ混合场     

Effect of gamma and UV-B/C radiation on plant cells

The biological effect of gamma-rays is based on the interaction with atoms or molecules in the cell, particularly water, to produce free radicals, which can damage different important compounds of plant cell. The UV-B/C photons have enough energy to destroy chemical bounds, causing a photochemical reaction. The biological effect is due to these processes. This paper is focused on the structural and biochemical changes of the cell wall and plastids after gamma and/or UV-B irradiation. Gamma-rays accelerate the softening of fruits, causing the breakdown of middle lamella in cell wall. They also influence the plastid development and function, such as starch-sugar interconversion. The penetration of UV-B light into the cell is limited, while gamma-rays penetrate through the cells. For this reason, UV-B light has a strong effect on surface or near-to-surface area in plant cells. UV-B radiation influences plastid structure (mostly thylakoid membranes) and photosynthesis. Some kinds of pigments, such as carotenoids, flavonoids save plant cells against UV-B and gamma irradiation. Plant cells are generally ozone sensitive. The detoxifying systems operate at the cellular level. Methods for studying structural changes in plant cells develop in direction to molecular biology, combined with immunoassays and new microscopical techniques. Nowadays, UV-B radiation is undergoing much research, being an environmental factor which causes damage to both humans and plant cells.