Effect of 14 MeV Neutrons on Space-Charge-Limited Current of Electrons in High Purity Silicon

Alloyed and symmetrical n+?n+ devices made of nominally 75k?cm ?-type Si are analyzed before and after irradiation with 14MeV neutrons at room temperature and doses of 1.2×1011, 5.5×1011 and 4.0×1012n/cm2. Immediately after the application of a large turn-on voltage step at t ? O, the flow of electrons through these devices is by pure, trap free, space-charge-limited current (sclc). From an analysis of this sclc, it is established that the drift velocity-field relationship of electrons in Si is affected by the radiation only at low temperatures in the low field range. For t>0, the current after irradiation decays below its initial trap free sclc value, thus revealing the presence of traps. Two categories are identified: fast traps with energy levels at about ?Etf ? 0.13eV below the conduction band and slow traps with energy levels at roughly ?Ets?005eV. Cross sections are also obtained. Introduction rates are about equal for both (llcm-l for fast traps and 8cm-1 for slow traps). These results demonstrate the sensitivity of sclc as a tool to detect traps and changes in the drift velocityfield relationship caused by radiation. Some implications of these results are discussed and additional experiments are suggested.     

利用瞬发γ射线法测量209Bi的中子非弹性散射截面

在铅铋快堆、空间堆等先进反应堆中,铋作为冷却剂和慢化剂材料被大量使用,其中子核反应截面,尤其是中子非弹性散射截面的准确性对这些核装置的安全性和经济性等具有重要的影响。基于中国原子能科学研究院HI 13串列加速器瞬发γ射线实验平台,通过瞬发γ射线法测量了209Bi在90、105和120 MeV 3个能点的中子非弹性散射截面。在相对于中子束30°、70°、110°和150°方向放置4个Clover探测器测量中子与样品相互作用产生的γ射线。实验采用相对测量,通过测量中子与48Ti发生非弹性散射发射的9835 keV γ射线的产生截面来确定209Bi的截面。209Bi金属样品的尺寸为50 mm×4 mm,参考样品为1块50 mm×1 mm的天然钛金属样品。将实验测量结果与已发表的实验数据、ENDF/B Ⅷ.0、JEFF 33、JENDL 40、ROSFOND 2010和CENDL 31等评价库数据以及Talys 195程序默认参数的计算结果进行对比,发现趋势一致,90、105 MeV能点的测量结果与Talys 195程序的计算结果符合得更好,120 MeV能点的测量结果与ROSFOND 2010评价库数据符合得更好。     

海底原位PGNAA探测含水率对中子分布影响研究

运用MC法模拟14 MeV快中子进入沉积物后的物理过程,得到了不同深度下沉积物中子能谱分布,分析了0~1 eV中子在沉积物中横、纵向分布规律以及含水率与Cl~-浓度对中子分布的影响,探讨了模拟条件下0~1 eV中子扩散的最大深度范围。结果表明:沉积物含水率对中子能谱分布和0~1 eV中子横、纵向分布均有显著影响,进行中子活化的最佳深度为2 cm处;0~1 eV中子扩散的最大深度范围为20~40 cm;海水中Cl~-浓度对中子吸收有影响但影响不显著。     

Single Event Upsets in RAMs Induced by Protons at 4.2 GeV and Protons and Neutrons below 100 MeV

Several different types of random-access-memories (RAMs) have been tested for soft upset susceptibility under a variety of different particle bombardments including thermal neutrons, GeV protons, and protons and neutrons below 100 MeV and with few exceptions found to suffer single event upsets. Devices tested included 4K, 16K and 64K dynamic RAMs and 4K NMOS and 256×4 CMOS static RAMs. Mean upset fluences varied from 106 particles/cm2-upset for 64K dynamic RAMs up to no upsets observed for the 256×4 CMOS RAM. No thermal neutron induced upsets are believed to have occurred. GeV protons, simulating primary cosmic rays, caused upsets at levels of 107 particles/cm2-upset.     

Transient and Permanent Effects of Neutron Bombardment on a Commercially Available N-Buried-Channel CCD

A study of the effects of neutron bombardment on a commercially available n-buried-channel CCD (the Fairchild CCD 321A) has been performed. Both transient and permanent changes in dark current density and charge transfer inefficiency were examined. Considerable shortterm annealing of dark current was observed at room temperature for times up to ~104 sec following pulsed neutron irradiation, but negligible annealing of the transfer inefficiency was noted over the period from 1.5 sec to 1000 hours. This transient annealing difference suggests that transfer inefficiency increases are dominated by isolated, as opposed to clustered, defects under the present experimental conditions. Transfer inefficiency data obtained over the range 208-303° K yield trap energy levels with respect to the conduction band of 0.54 eV before irradiation and 0.33 eV after irradiation. The trap introduction rate was determined to be ~2.6 n-1 cm-1. In studies at cryogenic temperatures, a dependence of transfer inefficiency on clock frequency to the one-half power was noted. This dependence is attributed to a trap-limited diffusion process, with trapping occurring at the As donor level. Neutron irradiation at 77° K caused the transfer inefficiency to decrease, and a qualitative model for such behavior is given.     

慢化球探测器组合法测试中子屏蔽性能应用

随着核技术在各领域的应用推广,中子屏蔽材料得到越来越广泛的应用,而目前国内外还没有测试中子屏蔽性能的统一标准。为了探索一种简单可行、能够在较宽中子能量范围内测试材料屏蔽性能的方法,本文对3He正比计数管、计数管外包镉及计数管外包不同直径的聚乙烯(Polyethylene,PE)慢化球共12个模型进行了MCNP(Monte Carlo N Particle Transport Code)模拟计算,得到一种慢化球探测器组合测试方法,使测试能够在1×10-5-1.25 Me V能量范围内有较一致的响应。利用这种方法测试了2 cm和4 cm厚PE对252Cf中子的透射率,与多球谱仪解谱法得到的结果在±1.0%内相吻合,对几种材料的测试结果也符合不同类型材料对中子的屏蔽规律,证实了这种简易组合测试方法的可行性。     

Response of MNOS Capacitors to Ionizing Radiation at 80°k

MNOS capacitors with oxide thicknesses 85?-600? and silicon nitride thicknesses 200-2000? have been irradiated with 2 MeV electrons at 80°K. Measured flatband shifts are found to depend on both polarity and magnitude of the applied field, oxide thickness, nitride thickness, and variations in device processing. For negative gate bias and effective applied fields 1-2?106 V/Cm, ?VFB is independent of device processing and magnitude of the applied field. For these bias conditions, it is shown that flatband shifts in all MNOS samples may be explained by considering only generation and trapping of holes in the oxide. The holes travel a mean free path of 125± 25? in the oxide before being trapped. For positive gate bias, electrons generated in the oxide are trapped at the oxide-nitride interface and/or in the bulk of the nitride, compensating the effect of the positively charged trapped holes in the oxide, and producing a relatively smaller ?VFB for positive bias. The electron trapping process is considerably processing dependent. For high effective applied fields exceeding ± 2×106 V/cm, a strongly field-dependent mechanism of charge generation in the gate insulator is observed.     

Defect-Impurity Relationships in Electron-Damaged Silicon

Crucible grown p-type silicon crystals with various dopants were irradiated with 1 MeV electrons. The Hall coefficient measurements indicated that the introduction rate of the Ev + 0.3 eV energy level was independent of the chemical acceptor atoms and the dislocation density. This energy level was not found in similar float zone crystals. The evidence supports a defect complex with oxygen, but does not involve atoms of the chemical acceptor. Annealing studies of the Ev + 0.3 eV level indicated a first stage annealed at 400°C and a second stage anneal at 550°C. The second phase of this work involves the electron irradiation of heavily-doped samples of floating zone silicon. It was found that the defect introduction rate increased very slowly with phosphorus concentration. The introduction rate decreased linearly with the remaining car rier concentration during irradiation. In the impurity conduction region, the activation energy for conduction increased with electron irradiation. Production of vacancy-phosphorous pairs appears to be responsible for the changes observed.     

强激光与固体靶相互作用所致硬X射线剂量和能谱的实验测量

为了研究强激光与固体靶相互作用产生的电离辐射危害,本文在星光Ⅲ300TW强激光装置上开展了一系列激光打靶实验。实验使用的激光功率密度为5×10~(18)~4×10~(19)W/cm~2,激光脉冲能量为60~153J,靶为直径1mm、厚度1mm的Ta圆柱,本文分别对X射线剂量、X射线能谱和超热电子能谱进行了测量。实验结果表明,测量到的单发最大X射线剂量约为16.8mSv,靠近激光传播方向(0°),距靶50cm处;激光0°方向的X射线剂量随激光功率密度的增加而显著增加,激光90°方向的X射线剂量随激光功率密度的变化相对较小;测量到的X射线能谱可大致用含有两个X射线温度的指数分布函数描述,其中0°方向测量到的X射线温度为0.4~1.15 MeV,90°方向测量到的X射线温度为0.25~0.54 MeV;实测超热电子温度与Wilks定标率符合较好。     

Study of n- and p-Type Silicon Exposed to Highly Energetic Radiation

For an understanding of displacement effects in semiconductors, it is important to establish a correlation of the magnitude of these effects with various types of radiation. A study has been made of the effect of highly energetic radiation on n-type silicon using two techniques. First, with electron-spin resonance studies, the energy dependence of formation of the Si-Bl center at 80° and 300°K by 1.5, 2.5, 5, and 30 MeV electrons and by reactor neutrons has been determined. In addition, the energy dependence of the production of the divacancy by electrons with energies between 2.5 and 30 MeV in n-type silicon have been obtained. Second, using galvanomagnetic techniques, information has been obtained on the irradiation response and subsequent recovery during annealing of the conductivity, Hall coefficient, and Hall mobility of n-and p-type silicon. The samples were bombarded at 80°K with 5 to 30 MeV electrons and isochronally annealed to 400°K. A considerable amount of annealing was observed in all cases, but the recovery was never complete.     

Theoretical and Experimental Determinations of Neutron Energy Deposition in Silicon

Theoretical calculations have been made of the energy deposited in silicon in ionization and elastic interactions by neutrons in the energy range of 60 keV to 15 MeV. In contrast to earlier determinations, care was taken to calculate accurately the effects of atomic recoils. These are of primary importance for permanent effects in silicon at all neutron energies and account for about 30% of the transient ionization at 14 MeV. To test the calculation of the energy deposited by 14 MeV neutrons, experiments were performed with a pulsed d-t generator. The experimental values in rads per unit fluence were (11 ± 2) × 10-10 rad-cm2 for special p-n junctions and (8 ± 2) × 10-10 rad-cm2 for power transistors. Both experimental values agree, withi jerror, with the calculated value of (8 ± 1) × 10-10 rad-cm2. The calculated variation of radiation damage in silicon with neutron energy was compared with existing experimental data. There is general agreement of theory with experiment from 200 keV to 14 MeV.     

BNCT人头体模内剂量分布计算

用修正的Synder人头体模几何模型和ICRU-46中的材料数据，用MCNP-4B程序对0．0253ev、1kev、2keV、10keV、100keV、1MeV单能中子束，0．2、0．5、1、2、5、10MeV单能光子束，以及与当前硼中子俘获治疗(BNCT)临床中使用的超热中子相似的超热中子束，计算了在人头体模中的剂量分布，计算结果与有关文献报道的结果一致，初步校验了我们正在编制的BNCT治疗计划软件。     

High-Energy Radiation Damage in Silicon Transistors

An experimental investigation of electron and gamma ray damage in silicon transistors is presented. At low values of fluence (?e < 1014 electrons/cm2), loss in common-emitter dc current gain of medium frequency n-p-n planar transistors at collector currents of one to 10 milliamperes is attributed to changes in the surface recombination velocity. Displacement-induced recombination centers in the base region cause a reduction in gain when ?e is greater than 1014 electrons/cm2. A technique of saturating the surface damage with low energy electrons (E = 125 kev) so as to permit a separation of surface and bulk damage is demonstrated. The minority-carrier lifetime-damage constant, K?, has been estimated from the separated bulk-damage curve. It agrees with the value determined from electron irradiation of a low frequency (f?b = 1.25 Mc/sec) mesa n-p-n transistor which is shown to suffer degradation in gain only from bulk recombination current losses within the base region. Surface damage from both electrons and gamma rays is annealed at 250°C or by injecting emitter currents of 200 milliamperes which generate a high internal temperature. In contrast to this behavior, electron irradiation of p-n-p transistors caused loss in gain which is attributed to bulk damage. Damage constants, K?, determined from the data show that p-n-p transistors suffer bulk radiation damage about five times greater than n-p-n transistors.     

基于D-T中子源的板状铌样品的评价数据基准检验实验

利用中国原子能科学研究院核数据国家重点实验室的脉冲化氘氚聚变中子源产生的145 MeV单能中子,通过飞行时间法,测量了5、10、15 cm厚度板状铌（Nb）样品在与60°和120°两个方向上的泄漏中子飞行时间谱。利用蒙特卡罗模拟软件MCNP 4C进行了泄漏中子飞行时间谱的模拟计算,分别获得了CENDL 31、ENDF/B Ⅷ0和JENDL 40 3个数据库中Nb评价数据的模拟结果。通过各数据库不同能区的模拟结果与实验结果的比值（C/E）,对3个数据库中93Nb与145 MeV中子作用的角分布和双微分截面等相关评价数据进行了检验,重点分析了CENDL 31库的数据。结果表明,CENDL 31数据库的模拟结果在弹性散射能区、非弹性散射能区以及(n,2n)反应能区与实验结果均存在一定的偏差。而JENDL 40数据库除在120°弹性散射能区有高估现象,其他能区的模拟结果与实验结果均符合较好。ENDF/B Ⅷ0数据库的模拟结果除在60°方向弹性散射峰偏低外,其他能量范围的模拟结果均高于实验。     

Target optimization for a 10-MeV E-beam neutron converter

Accelerator-based target design and optimization is an approach for neutron generation. The target plays an important role for a neutron source on an electron accelerator. For optimizing a neutron source using 10 MeV electron beams of Rhodotron-TT200, Pb, Ta, or W alloys with Be were calculated as photo-neutron converter. The neutron yield, flux and energy were simulated using the MCNPX code. The results indicate that a 10 MeV electron beam is capable of producing high-intensity neutron flux of 1013n·cm–2·s–1 with average energy of 0.8 MeV.     

国产Am-Be中子源能量低于1.5MeV中子所占份额的实验测定

本工作提出了测定Am Be中子源发射的能量低于1.5MeV中子所占份额的1种实用实验方法。用4.438MeVγ射线伴随的飞行时间法测量了中子源的局部中子谱（n₁群中子）。通过已准确测量的中子源发射4.438MeVγ射线与中子强度的比值(R=R_γ/S_n)和n₁群中子谱与测量的能量为1.5MeV以上中子总谱在3.2MeV能量处归一后的面积比值,求得国产Am-Be中子源能量低于1.5MeV中子的所占份额为(19.1±1.9)%。     

Computational-experimental studies of the neutronics of the SM reactor with different variants of the neutron-trap arrangement

The results of experimental studies of the neutronics of the high-flux SM reactor with different arrangements of the neutron trap are presented. The MCU series of high-precision computer programs implementing the Monte Carlo method is used for computations. Experimental data on reactivity effects, the effectiveness of safety and control rods, and the coefficients of nonuniformity of energy release in the core have been obtained in experiments on a critical assembly – a physical model of the SM reactor – and directly in experiments in the reactor. The error is 4.2–10% in determining the reactivity parameters and 5–10% for the relative energy release in the fuel elements. Information on the neutron field formed in the volume of the neutron trap has been obtained for two arrangements of the beryllium and water moderators. The differential and integral energy spectra of the neutrons in the energy interval from 0.5 eV to 20 MeV are obtained for three points inside the trap (external, central series, center). The flux density of thermal, superthemal, and fast neutrons are determined.     

A ctivation measurement of neutron distribution in intermediate energy heavy ion target area

The distribution of neutrons produced by the reaction of 50MeV/u ^12C-ion on a thick Cu target are studied.The neutrons are measured with threshold activation detectors.Al.F,C,Al and In activation samples were used to measure neutrons with energy greater than 7,11,20,50MeV and thermal neutrons,respectively,The fluence rate,energy and angular distributions of neutrons,total neutron yield of 12C-ion and the emission rate in the forward direction of neutrons over 11 and 20MeV were obtained.     

Shielding design and analyses of KIPT neutron source facility

Argonne National Laboratory (ANL) of USA and Kharkov Institute of Physics and Technology (KIPT) of Ukraine have been collaborating on the conceptual design development of a neutron source facility. An electron accelerator drives a sub-critical facility (ADS) is used for generating the neutron source. The facility will be utilized for performing basic and applied nuclear researches, producing medical isotopes, and training young nuclear specialists. Monte Carlo code MCNPX has been utilized as the major design tool for the design, due to its capability to transport electrons, photons, and neutrons at high energies. However the ADS shielding calculations with MCNPX need enormous computational resources and the small neutron yield per electron makes sampling difficulty for the Monte Carlo calculations. The high energy electrons (E > 100 MeV) generate very high energy neutrons and these neutrons dominant the total radiation dose outside the shield. The radiation dose caused by high energy neutrons is ∼3-4 orders of magnitude higher than that of the photons. However, the high energy neutron fraction within the total generated neutrons is very small, which increases the sampling difficulty and the required computational time. To solve these difficulties, the user subroutines of MCNPX are utilized to generate a neutron source file, which record the generated neutrons from the photonuclear reactions caused by electrons. This neutron source file is utilized many times in the following MCNPX calculations for weight windows (importance function) generation and radiation dose calculations. In addition, the neutron source file can be sampled multiple times to improve the statistics of the calculated results. In this way the expensive electron transport calculations can be performed once with good statistics for the different ADS shielding problems. This paper presents the method of generating and utilizing the neutron source file by MCNPX for the ADS shielding calculation and similar accelerator facilities, and the accurate radiation dose analyses outside the shield using modest computational resources.     

基于150 MeV电子束的光中子特性模拟分析

高能中子源是研究高能太空宇宙射线中子对人体和电子仪器辐射损伤的必备装置,基于高能电子加速器的光中子源是目前能够提供较高能量白光中子的方式之一。本工作以清华大学先进加速器实验室的激光电子加速器束流参数为基础,借助Geant4对产生的光中子的能量特性、产额特性、角分布特性、时间特性进行了分析。模拟结果表明,Φ2 cm×2 cm的圆柱体Ta靶时,150 MeV电子束流可产生最高能量约为110 MeV、中子产额约为1.2×10~5n/10~7e-、出射时间在0~100 ns之间呈负指数分布的几乎各向同性的光中子。根据拟合的中子能量-出射时间离散指数函数,估算得到对产生的1~100MeV中子,在飞行距离为5m时中子飞行时间的时间分辨率好于2.23%。本工作为该加速器的光中子产生和实验测量工作提供了参考依据。