Preliminary shielding analysis in support of the CSNS target station shutter neutron beam stop design

The construction of China Spallation Neutron Source (CSNS) has been initiated in Dongguan, Guangdong, China. Thus a detailed radiation transport analysis of the shutter neutron beam stop is of vital importance. The analyses are performed using the coupled Monte Carlo and multi-dimensional discrete ordinates method. The target of calculations is to optimize the neutron beamline shielding design to guarantee personal safety and minimize cost. Successful elimination of the primary ray effects via the two-dimensional uncollided flux and the first collision source methodology is also illustrated. Two-dimensional dose distribution is calculated. The dose at the end of the neutron beam line is less than 2.5 μSv/h. The models have ensured that the doses received by the hall staff members are below the standard limit required.     

2GW功率下CFETR中子屏蔽的初步中子学研究

在中国聚变工程实验堆(CFETR)真空室最新设计尺寸下,利用蒙特卡洛中子输运程序(MCNP)建立一维中子学模型,在2GW 的聚变功率下进行了计算。分析了中子反射材料ZrH₂ 对中子的屏蔽效果,发现200mm 的反射层可以屏蔽94.3%的中子通量和94.9%的中子核热。研究CFETR 在运行10 个满功率年(FPY)和20FPY 后,对应不同中子壁载荷的最小屏蔽包层厚度。结果显示,装置运行10FPY 后中子壁载荷在1.0MW·m⁻²、1.5MW·m⁻²、 2.5MW·m⁻² 时所对应的最小屏蔽包层厚度分别为44cm、53cm、65cm;而在装置运行20FPY 后,则需要在径向方向更厚的屏蔽包层才能满足中子屏蔽要求。屏蔽包层的尺寸优化将为目前阶段的CFETR 先进包层设计提供参考。     

The effect of thermal reactor neutron irradiation on semi-insulating GaN

Semi-insulating Gallium nitride was irradiated by fast and thermal neutrons with fluences from 10¹⁴ to 10¹⁶ n/cm². Depth-resolved cathodoluminescence spectroscopy was used to determine defects changes before and after irradiation. The results revealed two kinds of defects affected near-band emission recombination from two opposite directions. One was attributed to irradiation-induced N vacancies that contribute to near-band emission recombination. Another was attributed to irradiation-induced deep level defects that contribute to sub-band gap recombination and thus decrease the near-band emission recombination.     

Inelastic electron irradiation damage in hexagonal boron nitride

We present a study of the inelastic effects caused by electron irradiation in monolayer hexagonal boron nitride (h-BN). The data was obtained through in situ experiments performed inside a low-voltage aberration-corrected transmission electron microscope (TEM). By using various specialized sample holders, we study defect formation and evolution with sub-nanometer resolution over a wide range of temperatures, between −196 and 1200 °C, highlighting significant differences in the geometry of the structures that form. The data is then quantified, allowing insight into the defect formation mechanism, which is discussed in comparison with the potential candidate damage processes. We show that the defect shapes are determined by an interplay between electron damage, which we assign to charging, and thermal effects. We additionally show that this damage can be avoided altogether by overlapping the samples with a monolayer of graphene, confirming this for h-BN and providing a way to overcome the well-known fragility of h-BN under the electron beam.     

The effects of fast neutron irradiation on oxygen in Czochralski silicon

The effects of fast neutron irradiation on oxygen atoms in Czochralski silicon (CZ-Si) are investigated systemically by using Fourier transform infrared (FTIR) spectrometer and positron annihilation technique (PAT). Through isochronal annealing, it is found that the trend of variation in interstitial oxygen concentration ([O_i]) in fast neutrons irradiated CZ-Si fluctuates largely with temperature increasing, especially between 500 and 700℃. After the CZ-Si is annealed at 600℃, the V₄ appearing as three-dimensional vacancy clusters causes the formation of the molecule-like oxygen clusters, and more importantly these dimers with small binding energies (0.1--1.0eV) can diffuse into the Si lattices more easily than single oxygen atoms, thereby leading to the strong oxygen agglomerations. When the CZ-Si is annealed at temperature increasing up to 700℃, three-dimensional vacancy clusters disappear and the oxygen agglomerations decompose into single oxygen atoms (O) at interstitial sites. Results from FTIR spectrometer and PAT provide an insight into the nature of the [O_i] at temperatures between 500 and 700℃. It turns out that the large fluctuation of [O_i] after short-time annealing from 500 to 700℃ results from the transformation of fast neutron irradiation defects.     

Accelerated oxygen precipitation in fast neutron irradiated Czochralski silicon

Annealing effect of the oxygen precipitation and the induced defects have been investigated on the fast neutron irradiated Czochralski silicon （CZ-Si） by infrared absorption spectrum and the optical microscopy. It is found that the fast neutron irradiation greatly accelerates the oxygen precipitation that leads to a sharp decrease of the interstitial oxygen with the annealing time. At room temperature （RT）, the 1107cm^-1 infrared absorption band of interstitial oxygen becomes weak and broadens to low energy side. At low temperature, the infrared absorption peaks appear at 1078cm^-1, 1096cm^-1, and 1182cm^-1, related to different shapes of the oxygen precipitates. The bulk microdefects, including stacking faults, dislocations and dislocation loops, were observed by the optical microscopy. New or large stacking faults grow up when the silicon self-interstitial atoms are created and aggregate with oxygen precipitation.     

Cluster dynamics simulation of point defect clusters in neutron irradiated pure iron

The effect of the size dependence of the production rate of point defect clusters is taken into account in the cluster dynamics (CD) modeling of the simultaneous formation of vacancy clusters and self-interstitial atom clusters in neutron-irradiated pure iron. The calibration of material parameters has been carried out. The correspondence between small angle neutron scattering, transmission electron microscopy and positron annihilation spectroscopy and CD data is studied.     

中子辐照直拉硅中的本征吸除效应

对经中子辐照的直拉硅中的本征吸除效应进行了研究.结果表明:经中子辐照后,直拉硅片经一步短时退火就可以在硅片表面形成完整的清洁区.清洁区宽度受辐照剂量和退火温度所控制,清洁区一旦形成,就不随退火时间变化.大量的缺陷在中子辐照时产生,并同硅中氧相互作用,加速了硅片体内氧的沉淀,是快速形成本征吸除效果的主要因素,从而把热历史的影响降到次要地位 关键词： 本征吸除 中子辐照 直拉硅     

Radiation damage in triglycine sulphate due to fast neutron irradiation

Abstract

The change in electrical properties of TGS crystals due to induced defects created by fast neutron irradiation of two different energies (2 and 14 MeV) and different integrated neutron fluxes have been studied in the vicinity of phase transition. It is observed that the electrical conductivity increases with increase of neutron fluence up to 1.7 × 10¹⁰ n · cm^?2 and the values of the relative change of electrical conductivity in case of 2 MeV are higher than that of 14 MeV neutrons at the same neutron fluence (φ)     

用多群蒙特卡罗方法计算快中子核临界K_(eff)和通量密度

用多群蒙特卡罗方法对快中子核裂变系统进行了临界计算。有效增殖因子κｅｆｆ的计算值与实验结果符合。计算所得中子通量密度的空间分布在球形裂变系统中随半径增大单调下降。中子通量密度的能量分布在由高浓缩铀组成的活性区内里单一能量极大值,其对应能量对于裸球核裂变系统和具有反射层裂变系统分别为０．３５ＭｅＶ和０．２５ＭｅＶ,而在由天然铀组成的反射层中在０．１ＭｅＶ附近出现能量双峰。由通量密度所得中子能谱在无反射层球形裂变系统中随半径增加变硬,在有反射层球形裂变系统中随半径增加变软。     

Effect of triangle vacancy on thermal transport in boron nitride nanoribbons

Recently, triangle vacancy in hexagonal boron nitride is observed experimentally. Using nonequilibrium Green’s function method, we investigate thermal transport properties of boron nitride nanoribbons (BNNRs) with a triangle vacancy. The effect of triangle vacancy on the phonon transmission of zigzag-edged BNNRs (Z-BNNRs) is different from that of armchair-edged BNNRs (A-BNNRs). The triangle vacancy induces antiresonant dips in the spectrum of Z-BNNRs. Moreover, the boron-terminated triangle vacancy causes antiresonant zero-transmission dip and the number of the zero-transmission dip increases with the geometrical size of triangle vacancy. For the A-BNNRs with triangle vacancy, except some antiresonant dips, a resonant peak is found in the transmission. The antiresonant and resonant phenomena are explained by analyzing local density of states and local thermal currents. Although the antiresonant dip and the resonant peak are both originated from quasibound states, their distributions of local thermal currents are distinct, which leads to the transport discrepancy. In addition, the thermal conductance of BNNRs decreases linearly with increasing the vacancy size.     

快中子辐照直拉硅中受主和施主的研究

通过傅里叶红外光谱、正电子湮没寿命谱和Hall技术研究了高剂量快中子辐照直拉硅的辐照 缺陷、电阻率、载流子迁移率、载流子浓度随退火温度的变化.经快中子辐照，直拉硅样品 的导电类型由n型转变为p型.在450和600℃热处理出现两种受主中心，分别由V2_{2O22，V22O，VO22，V-O-V及V44型缺 陷引起，这些缺陷态的出现使得样品中空穴浓度迅速增加；大于650℃热处理这些受主态 缺陷迅速消失， 关键词： 快中子辐照 空位型缺陷 受主 施主}     

Measurement of dose enhancement by neutron capture of boron in Cf interstitial brachytherapy

The effect of boron concentration in water on the gamma, fast and slow neutrons and alpha particles components at the central, forward and backward surfaces inside tumor phantom of 4.2 cm diameter and 4.4cm height, during brachytherapy by neutrons from ²⁵²Cf were investigated. The source was at the centre of a cubic shaped water phantom of 30 cm side. The study was carried for different concentrations of boron from H₃BO₃, Li₂B₄O₇ and H₃¹⁰BO₃. The effect of source to tumor distance on the different components of radiation was also measured. The results indicated that the use of ¹⁰B compounds enhances the damage and is recommended for successful boron neutron capture therapy (BNCT).     

A small angle neutron scattering (SANS) experiment using very cold neutrons (VCN)

This paper describes the results of SANS measurements of small samples using the very cold neutron (VCN) beam of the PF2 instrument at the Institut Laue Langevin (ILL), France. In addition to a classical SANS pinhole collimation, the experiment used a polarizing supermirror as a monochromator and a magnetic sextupole lens to focus the neutron beam in order to gain intensity and avoid any material in the neutron beam besides the sample.     

An optical study of the D-D neutron irradiation-induced defects in Co-and Cu-doped ZnO wafers

Room-temperature photoluminescence and optical transmittance spectroscopy of Co-doped(1×1014,5×1016,and 1×1017cm-2) and Cu-doped(5×1016cm-2) ZnO wafers irradiated by D-D neutrons(fluence of 2.9×1010 cm-2) have been investigated.After irradiation,the Co or Cu metal and oxide clusters in doped ZnO wafers are dissolved,and the wu¨rtzite structure of ZnO substrate for each sample remains unchanged and keeps in high c-axis preferential orientation.The degree of irradiation-induced crystal disorder reflected from the absorption band tail parameter(E0) is far greater for doped ZnO than the undoped one.Under the same doping concentration,the Cu-doped ZnO wafer has much higher irradiation-induced disorder than the Co-doped one.Photoluminescence measurements indicate that the introduction rate of both the zinc vacancy and the zinc interstitial is much higher for the doped ZnO wafer with a high doping level than the undoped one.In addition,both crystal lattice distortion and defect complexes are suggested to be formed in doped ZnO wafers.Consequently,the Co-or Cu-doped ZnO wafer(especially with a high doping level) exhibits very low radiation hardness compared with the undoped one,and the Cu-doped ZnO wafer is much less radiation-hard than the Co-doped one.     

Zinc (tris) thiourea sulphate (ZTS): A single crystal neutron diffraction study

The crystal structure of ZTS has been determined by neutron diffraction with a finalR-value of 0.026. Using the structural parameters, the contributions from the structural groups to the linear optical susceptibility and linear electro-optic coefficients have been evaluated. Results showed a significant contribution from the hydrogen bonds in the structure.     

Prompt neutron emission spectra and multiplicities in the thermal neutron induced fission of235U

The emission spectra of prompt fission neutrons from mass and kinetic energy selected fission fragments have been measured in²³⁵U(n _th,f). Neutron energies were determined from the measurement of the neutron time of flight using a NE213 scintillation detector. The fragment energies were measured by a pair of surface barrier detectors in one set of measurements and by a back-to-back gridded ionization chamber in the second set of measurements. The data were analysed event by event to deduce neutron energy in the rest frame of the emitting fragment for the determination of neutron emission spectra and multiplicities as a function of the fragment mass and total kinetic energy. The results are compared with statistical model calculations using shell and excitation energy dependent level density formulations to deduce the level density parameters of the neutron rich fragment nuclei over a large range of fragment masses.     

Monotonous and oscillatory recombination kinetics in neutron irradiated crystalline chromates

Abstract

Kinetics of post recoil thermal annealing of chromate salts after ⁵⁰Cr(n, γ)⁵¹Cr nuclear reaction reveals a monotonous and an oscillatory part amounting 1–2% of the total retention. The permanent recombination after subtracting the oscillatory part has been found to consist of two first order processes producing ⁵¹Cr(VI) from ⁵¹Cr(III) during the examined first hour. The oscillatory phenomenon is possibly explained by thermodynamics of irreversible processes.     

大空腔探测系统的Am-Li中子优化屏蔽

为了减少Am-Li中子本底对高浓铀部件质量主动多重性测量的影响,对大空腔探测系统（NPL-NMC）Am-Li中子本底的优化屏蔽进行了模拟研究,提出了一个基于高密度聚乙烯为中子屏蔽体的优化方案。通过对比模拟结果与屏蔽前实验测量结果,发现屏蔽使Am-Li中子本底探测效率明显降低,从原来的15.77%降为屏蔽后的1.94%,大约降低了87.7%;而屏蔽对裂变中子计数的影响却相对较小,只比屏蔽前降低约2.4%。本底中子计数的降低明显提高了系统对铀部件质量测量的灵敏度,在3000 s测量时间内,其质量测量下限从原来的大约6.4 kg下降到屏蔽后的2.6 kg;同时,屏蔽后的NPL-NMC系统在相同测量条件下,铀部件质量测量准确性提高50%以上。     

大空腔探测系统的Am-Li中子优化屏蔽

为了减少Am-Li中子本底对高浓铀部件质量主动多重性测量的影响,对大空腔探测系统(NPLNMC)Am-Li中子本底的优化屏蔽进行了模拟研究,提出了一个基于高密度聚乙烯为中子屏蔽体的优化方案。通过对比模拟结果与屏蔽前实验测量结果,发现屏蔽使Am-Li中子本底探测效率明显降低,从原来的15.77%降为屏蔽后的1.94%,大约降低了87.7%;而屏蔽对裂变中子计数的影响却相对较小,只比屏蔽前降低约2.4%。本底中子计数的降低明显提高了系统对铀部件质量测量的灵敏度,在3000s测量时间内,其质量测量下限从原来的大约6.4kg下降到屏蔽后的2.6kg;同时,屏蔽后的NPL-NMC系统在相同测量条件下,铀部件质量测量准确性提高50%以上。