Artificial neural network-based method for discriminating Compton scattering events in high-purity germaniumγ-ray spectrometer

To detect radioactive substances with low activity levels,an anticoincidence detector and a high-purity germanium(HPGe)detector are typically used simultaneously to suppress Compton scattering background,thereby resulting in an extremely low detection limit and improving the measurement accuracy.However,the complex and expensive hardware required does not facilitate the application or promotion of this method.Thus,a method is proposed in this study to discriminate the digital waveform of pulse signals output using an HPGe detector,whereby Compton scattering background is suppressed and a low minimum detectable activity(MDA)is achieved without using an expensive and complex anticoincidence detector and device.The electric-field-strength and energy-deposition distributions of the detector are simulated to determine the relationship between pulse shape and energy-deposition location,as well as the characteristics of energy-deposition distributions for fulland partial-energy deposition events.This relationship is used to develop a pulse-shape-discrimination algorithm based on an artificial neural network for pulse-feature identification.To accurately determine the relationship between the deposited energy of gamma(γ)rays in the detector and the deposition location,we extract four shape parameters from the pulse signals output by the detector.Machine learning is used to input the four shape parameters into the detector.Subsequently,the pulse signals are identified and classified to discriminate between partial-and full-energy deposition events.Some partial-energy deposition events are removed to suppress Compton scattering.The proposed method effectively decreases the MDA of an HPGeγ-energy dispersive spectrometer.Test results show that the Compton suppression factors for energy spectra obtained from measurements on ^(152)Eu,^(137)Cs,and ^(60)Co radioactive sources are 1.13(344 keV),1.11(662 keV),and 1.08(1332 keV),respectively,and that the corresponding MDAs are 1.4%,5.3%,and 21.6%lower,respectively.

Optical Methods in Orientation of High-Purity Germanium Crystal

Two optical methods, namely crystal facet reflection and etching pits reflection, were used to orient and high-purity germanium crystals. The X-ray diffraction patterns of three slices that were cut from the oriented and crystals were measured by X-ray diffraction. The experimental errors of crystal facet reflection method and etching pits reflection method are in the range of 0.05° - 0.12°. The crystal facet reflection method and etching pits reflection method are extremely simple and cheap and their accuracies are acceptable for characterizing high purity detector-grade germanium crystals.

Identification of anomalous fast bulk events in a p-type point-contact germanium detector

The ultralow detection threshold,ultralow intrinsic background,and excellent energy resolution of ptype point-contact germanium detectors are important for rare-event searches,in particular for the detection of direct dark matter interactions,coherent elastic neutrino-nucleus scattering,and neutrinoless double beta decay.Anomalous bulk events with an extremely fast rise time are observed in the CDEX-1B detector.We report a method of extracting fast bulk events from bulk events using a pulse shape simulation and reconstructed source experiment signature.Calibration data and the distribution of X-rays generated by intrinsic radioactivity verified that the fast bulk experienced a single hit near the passivation layer.The performance of this germanium detector indicates that it is capable of single-hit bulk spatial resolution and thus provides a background removal technique.

Evaluation of the passivation effect and the first-principles calculation on surface termination of germanium detector

The point-contact high-purity germanium detector(HPGe)has the advantages of low background,low energy threshold,and high energy resolution and can be applied in the detection of rare-event physics.However,the performance of HPGe must be further improved to achieve superior energy resolution,low noise,and long-term reliability.In this study,we combine computational simulations and experimental comparisons to deeply understand the passivation mechanism of Ge.The surface passivation effect is calculated and inferred from the band structure and density of interface states,and further con-firmed by the minority carrier lifetime.The first-principles method based on the density functional theory was adopted to systematically study the lattice structure,band structure,and density of state(DOS)of four different systems:Ge–H,Ge–Ge-NH 2,Ge-OH,and Ge-SiO_(x).The electronic char-acteristics of the Ge(100)unit cell with different passi-vation groups and Si/O atomic ratios were compared.This shows that H,N,and O atoms can effectively reduce the surface DOS of the Ge atoms.The passivation effect of the SiO_(x) group varied with increasing O atoms and Si/O atomic ratios.Experimentally,SiO and SiO_(2) passivation films were fabricated by electron beam evaporation on a Ge substrate,and the valence state of Si and resistivity was measured to characterize the film.The minority carrier lifetime of Ge-SiO_(2) is 21.3 ls,which is approximately quadruple that of Ge-SiO.The passivation effect and mechanism are discussed in terms of hopping conduction and surface defect density.This study builds a relationship between the passivation effect and different termination groups,and provides technical support for the potential passivation layer,which can be applied in Ge detectors with ultralow energy thresholds and especially in HPGe for rare-event physics detection experiments in future.

13N超高纯锗单晶的制备与性能研究

13N超高纯锗单晶是制作超高纯锗探测器的核心材料。本文通过还原法获得还原锗锭,再由水平区熔法提纯获得12N高纯锗多晶,最后由直拉法生长得到13N超高纯锗单晶。通过低温霍尔测试、位错密度检测、深能级瞬态谱(DLTS)测试对13N超高纯锗单晶性能进行分析。低温霍尔测试结果显示,晶体头部截面平均迁移率为4.515×10^(4)cm^(2)·V^(-1)·s^(-1),载流子浓度为1.176×10^(10)cm^(-3),导电类型为p型,位错密度为2256 cm^(-2);尾部截面平均迁移率为4.620×10^(4)cm^(2)·V^(-1)·s^(-1),载流子浓度为1.007×10^(10)cm^(-3),导电类型为p型,位错密度为2589 cm^(-2)。晶体深能级杂质浓度为1.843×10^(9)cm^(-3)。以上结果表明该晶体是13N超高纯锗单晶。

Study on cosmogenic radioactive production in germanium as a background for future rare event search experiments

Rare event search experiments are one of the most important topics in the field of fundamental physics,and high-purity germanium(HPGe)detectors with an ultralow radioactive background are frequently used for such experiments.However,cosmogenic activation contaminates germanium crystals during transport and storage.In this study,we investigated the movable shielding containers of HPGe crystals using Geant4 and CRY Monte Carlo simulations.The production rates of 68Ge,65Zn,60Co,55Fe,and 3H were obtained individually for different types of cosmic rays.The validity of the simulation was confirmed through a comparison with the available experimental data.Based on this simulation,we found that the interactions induced by neutrons contribute to approximately 90%of the production rate of cosmogenic activation.In addition,by adding an optimized shielding structure,the production rates of cosmogenic radionuclides are reduced by about one order of magnitude.Our results show that it is feasible to use a shielding container to reduce the cosmogenic radioactivity produced during the transport and storage of high-purity germanium on the ground.

High‑accuracy measurement of Compton scattering in germanium for dark matter searches

Compton scattering with bound electrons contributes to a significant atomic effect in low-momentum transfer,yielding background structures in direct light dark matter searches as well as low-energy rare event experiments.We report the measurement of Compton scattering in low-momentum transfer by implementing a 10-g germanium detector bombarded by a^(137)Cs source with a radioactivity of 8.7 mCi and a scatter photon captured by a cylindrical NaI(Tl)detector.A fully relativistic impulse approximation combined with multi-configuration Dirac–Fock wavefunctions was evaluated,and the scattering function of Geant4 software was replaced by our calculation results.Our measurements show that the Livermore model with the modified scattering function in Geant4 is in good agreement with the experimental data.It is also revealed that atomic many-body effects significantly influence Compton scattering for low-momentum transfer(sub-keV energy transfer).

Development of Low Dark Current SiGe Near-Infrared PIN Photodetectors on 300 mm Silicon Wafers

SiGe offers a low-cost alternative to conventional infrared sensor material systems such as InGaAs, InSb, and HgCdTe for developing near-infrared (NIR) photodetector devices that do not require cooling and can operate with relatively low dark current. As a result of the significant difference in thermal expansion coefficients between germanium (Ge) and silicon (Si), tensile strain incorporated into SiGe detector devices through specialized growth processes can extend their NIR wavelength range of operation. We have utilized high throughput, large-area complementary metal-oxide semiconductor (CMOS) technology to fabricate Ge based p-i-n (PIN) detector devices on 300 mm Si wafers. The two-step device fabrication process, designed to effectively reduce the density of defects and dislocations arising during deposition that form recombination centers which can result in higher dark current, involves low temperature epitaxial deposition of Ge to form a thin p+ seed layer, followed by higher temperature deposition of a thicker Ge intrinsic layer. Phosphorus was then ion-implanted to create devices with n+ regions of various doping concentrations. Secondary ion mass spectroscopy (SIMS) has been utilized to determine the doping profiles and material compositions of the layers. In addition, electrical characterization of the I-V photoresponse of different devices from the same wafer with various n+ region doping concentrations has demonstrated low dark current levels (down to below 1 nA at -1 V bias) and comparatively high photocurrent at reverse biases, with optimal response for doping concentration of 5 × 1019 cm-3.

Ge-based Medium Wave Infrared MCT 1280×1024 Focal Plane Detector

Medium-wave HgCdTe thin films grown on germanium-based substrates by molecular beam epitaxy were treated by large area n-on-p injection junction and flip-flop mixing process.The chips interconnected with low-noise and multimodal options readout circuit composed a 1280×1024 Medium-wave Infrared Focal Plane Cooling Detector whose pixel spacing was 15 microns.Its main photoelectric properties are average NETD equivalent to 18.5 mK,non-uniformity equivalent to 7.5%,operability equivalent to 98.97%.The paper also studies the substrate-removal technique on Germanium-based chip,which improves the stability and reliability of detector.

基于低本底高纯锗γ谱仪测量茶叶和烟草中^(210)Pb和^(40)K的比活度

茶叶和烟草中存在^(210)Pb和^(40)K等放射性核素,其放射性含量一直备受关注。本研究选择基于低本底高纯锗γ谱仪的γ能谱方法,并结合Geant4无源效率刻度技术,对典型的茶叶和烟草样本进行了^(210)Pb和^(40)K比活度测量。其中,低本底高纯锗γ谱仪由Canberra BEGe 5030高纯锗探测器和15 cm厚低本底钢+2 cm厚高纯无氧铜屏蔽室组成,在30~3 000 keV能量范围内,全谱计数率为1.98 s-1。基于以上实验设备,选择了4种茶叶和5种产地的香烟作为测量对象,测量结果显示,茶叶中^(210)Pb的比活度为8.15~49.67 Bq/kg,^(40)K的比活度为296.58~351.69 Bq/kg;烟草中^(210)Pb的比活度为21.74~31.16 Bq/kg,^(40)K的比活度为470.15~522.88 Bq/kg。根据样品中^(210)Pb的平均比活度估算,饮茶导致的^(210)Pb有效剂量应小于35.15μSv/a,吸烟导致的^(210)Pb有效剂量约为11.00μSv/a,饮茶和吸烟造成的相应核素的内照射剂量对人体健康的影响均较小。

基于应变锗的金属—半导体—金属光电探测器

通过对锗(Ge)半导体材料进行拉伸应变和n型掺杂,可以提高其电子迁移率和发光性能。将薄膜卷曲技术与微电子加工技术相结合,制备出悬浮的Ge微米带结构,实现了单轴和双轴2种不同的应变状态,并且可以通过调整光刻图案来控制Ge的应变状态和大小。利用这一方法,制备出了高性能双轴应变Ge对称型肖特基接触金属-半导体-金属(MSM)光电探测器。经测试,在1 V偏压和入射功率为1 000 nW的863 nm激光下,该探测器具有低至nA量级的暗电流和高达713的电流开/关比。该性能的实现主要是依赖于双轴应变和掺杂对Ge能带的修饰。研究结果展示了应变Ge在光电探测领域的优势,也证明了其在Si兼容光学通信设备中的应用潜力。

Radiometric and Geophysical Investigations on Exposure Levels and Excess Cancer Risk in Kargi-Kenya

Radiation is considered one of the possible causes of cancer disease with natural background sources including cosmic, terrestrial and internal radiation. A number of cancer disease cases have been reported in Kargi with their causes not properly documented. The present work characterized the radioactivity in soil and water, to find out possible causes of radiation in KARGI-KENYA by studying magnetic intensities, anomalous zones with depth to magnetic sources and delineating subsurface structures. A total of 117 soil and 14 water samples were collected from the entire area and analysed for radionuclides due to 40K, 232Th and 226Ra. Measurement methods of proton magnetometer and gamma spectrometry employing a high purity germanium (HPGe) detector were employed basically to evaluate the magnetic survey and radiological hazard of radioactivities respectively. A total of 51 magnetic field measurements were taken on the eastern part of Kargi, a place suspected to have more concentration of radionuclides. The results showed that there could have been a fractionation during weathering period or metasomatic activity of the radioelements involvement. This study also reveals that the mining activities in the nearby study area could have affected the geologic formation causing more fracturing in rocks and pronounced subsurface structures as a result of mining that could have served as passage for leachates from pollutants as well as the level of radiation in the study area.

高纯锗多晶材料区熔速度优化的数值模拟

为提高探测器级高纯锗多晶材料的制备效率,开展对锗材料多次区熔过程的参数优化的数值模拟.利用分凝原理对高纯锗多晶材料制备的区熔过程进行数值模拟,针对杂质分凝系数小于1的情况,比较了不同区熔速度下,单次和多次区熔的提纯效果.结果表明,虽然速度越慢单次区熔效果越好,但对多次区熔的累计效果要采用相对快速多次的方法,以实现相同提纯效果下总时间最短,即多次累计的区熔效率最高.给出了区熔速度的优化方法,以指导实验提高区熔效率.

高纯锗探测器探测效率的MCNP模拟

利用同轴型高纯锗(HPGe)探测器测量152Eu和133Ba在15cm处的探测效率,调节探测器死层厚度和冷指尺寸,利用Monte Carlo方法对同轴型HPGe探测器的全能峰效率进行模拟计算,并将计算效率与实验效率进行比较.结果表明,当HPGe探测器的死层厚度为0.22cm,冷指半径和长度分别为0.301cm和1.00cm时,模拟效率与实验效率相符。

能谱解析法稀土萃取过程在线分析的研究

应用能量色散X荧光分析原理,采用高分辨半导体探测器和微机多道能谱仪直接获取萃取过程中由放射源激发的稀土元素K系X荧光能谱,用能谱解析方法在线分析萃取过程15个稀土元素的含量。该分析系统具有较宽的分析范围和较小的分析误差,能够较好地满足萃取过程在线分析的要求。

测定锗的二极管阵列检测-流动注射分光光度法

研究了在溴化十六烷基三甲铵（ＣＴＭＡＢ） 存在下， 锗与２，３ ，７ － 三羟基－ ９ － 水杨基荧光酮－ ６（ＳＡＦ） 的显色反应， 采用二极管阵列检测器， 建立了检测微量锗的流动注射分光光度法。 将该法应用于食品和中草药中锗的测定， 取得了满意的结果。 锗在０ ～３ ．０ ｍｇ／Ｌ 范围内线性良好， 检出限为０ ．０１２ ｍｇ／Ｌ， 样品加标回收率为８１ ．０ ％ ～９８ ．５ ％ ， 相对标准偏差小于８ ．４ ％ 。 实验表明， 该法灵敏、快速， 准确度高、精密度好， 操作简便， 易于推广使用。

9μmp^＋—GexSi1—x／p—Si异质结内光电红外探测器

用分子束外延方法生长了ｐ＋－ＧｅｘＳｉ１－ｘ／ｐ－Ｓｉ异质结，并用平面工艺制成了内光电红外探测器，器件截止响应波长达９μｍ，在５２Ｋ时，Ｒｖ５００Ｋ＝３．３×１０３Ｖ／Ｗ．

高纯锗能谱仪系统研制

介绍了高纯锗能谱仪系统的研制。项目突破了探测器级高纯锗单晶提纯、高纯锗探测器表面钝化保护、高计数率数字化多道分析器以及无源效率刻度等关键技术,研制出高纯锗能谱仪系统样机。开展了高纯锗能谱仪在核燃料包壳破损在线监测、中高放射性废物桶核素源项无损检测和材料点缺陷特性分析示范应用研究。性能测试表明:85%探测效率的能谱仪的能量分辨率为1.98keV(1.33 MeV伽马射线),峰康比好于61。

γ射线谱中符合相加修正系数的Monte-Carlo模拟

用Monte Carlo方法模拟放射性核素的衰变过程,得到用HPGe探测器应能测到的多道γ能谱理论谱形,同时给出各特征能峰的探测效率和各特征峰的真符合相加修正系数。采用本方法开发的I、140Co、131Cool2000软件,可模拟60Co、88Y、134Cs、133Ba、95Nb、57Co、103Ru、106Ru、110Agm、124Sb、144Ce、65Zn、58La、241Am、99Mo、166Hom、152Eu、125Sb、214Bi、212Bi(α衰变链)和208T1,共计24种核素的衰变过程。对于60Co圆柱形体源发射的两条主要γ射线进行测量和分析结果表明:符合修正系数的模拟计算值与实验值在±1%的偏差范围内相符。同时给出了一种校正同轴探测器死层厚度和冷指尺寸的方法。

铟活化诊断氘氘中子产额不确定度分析

介绍了铟活化诊断氘氘中子产额的测量原理,分析了中子产额测量不确定度的来源及评定方法。中子产额测量不确定度主要由灵敏度标定不确定度、活化射线净计数不确定度、立体角测量不确定度及测量系统的随机误差等构成。评估了灵敏度标定过程中加速器中子与聚变中子能量差异、大厅散射中子本底等因素对灵敏度标定的影响,并评估了宇宙射线本底对活化射线净计数测量的影响。分析了中子产额处于不同量级时起主要作用的不确定度分量,提出了减小灵敏度标定不确定度的方法。以实验数据为基础,对具体的实验数据进行了分析计算。结果表明:利用伴随粒子法在加速器中子源上标定出铟活化测量系统灵敏度的相对标准不确定度为4.3%。中子产额低于1010时,产额测量不确定度大于7%,活化射线净计数误差是产额测量误差的主要来源;产额大于1010时,测量不确定度好于7%,中子产额测量不确定度主要由灵敏度标定不确定度引起。