Design and implementation of the monochromator shielding for the cold neutron spectrometers XINGZHI and BOYA

An innovative monochromator shielding is designed and implemented for the cold neutron spectrometers XINGZHI and BOYA operated by Renmin University of China at China Advanced Research Reactor.Via Monte Carlo simulations and careful mechanical designs,a shielding configuration has been successfully developed to satisfy safety requirements of below 3μSv/h dose rate at its exterior,meanwhile fulfilling space,floor load and nonmagnetic requirements.Composite materials are utilized to form the sandwich-type shielding walls:the inner layer of boron carbide rubber,the middle layer of steel-encased lead and the outer layer of borated polyethylene.Special-shaped liftable shielding blocks are incorporated to facilitate a continuous adjustment of the neutron energy while preventing radiation leakage.Our work has demonstrated that by utilizing composite shielding materials,along with the sandwich structure and liftable shielding blocks,a compact and lightweight shielding solution can be achieved.This enables the realization of advanced neutron scattering instruments that provide expanded space of measurement,larger energy and momentum coverage,and higher flux on the sample.This shielding represents the first of its kind in neutron scattering instruments in China.Following its successful operation,it has been subsequently employed by other neutron instruments across the country.

Correction and verification of HL-2A Tokamak Bonner sphere spectrometer in monoenergetic neutron fields from 100 keV to 5 MeV

A real-time Bonner sphere spectrometer(BSS)has been developed for spectral neutron measurements with the HL-2A Tokamak.To correct and verify the accuracy of the neutron spectrum from the BSS,the BSS system was calibrated using monoenergetic neutron beams in the energy range of 100 keV–5 MeV.The response function of the BSS was corrected based on the calibration results,and the corrected BSS system was verified by unfolding monoenergetic neutron spectra.Fusion neutron spectra on the HL-2A have been obtained from the calibrated BSS system for the first time.

Development of the real-time double-ring fusion neutron time-of-flight spectrometer system at HL-2M

A real-time double-ring neutron time-of-flight(TOFII) spectrometer system has been proposed to achieve plasma diagnosis on HL-2M tokamak with a relatively high count rate and sufficient energy resolution.The TOFII system is in its development stage,and this work describes its characteristics in terms of design principle,system structure,electronic system design,preliminary tests,and neutron transport simulation.The preliminary test results illustrate that the TOFII system can demonstrate the realtime dynamic spectrum every 10 ms.The results also show that based on the support vector machine method,the n-γ discrimination algorithm achieves the discrimination accuracy of 99.1%with a figure of merit of 1.30,and the intrinsic timing resolution of the system is within 0.3%.The simulated flight time spectrums from 1 to 5 MeV are obtained through the Monte Carlo tool Geant4,which also provide the reasonable results.The TOFII system will then be calibrated on mono-energetic neutron sources for further verification.

Unfolding neutron spectra from water-pumping-injection multilayered concentric sphere neutron spectrometer using self-adaptive differential evolution algorithm

A self-adaptive differential evolution neutron spectrum unfolding algorithm(SDENUA)is established in this study to unfold the neutron spectra obtained from a water-pumping-injection multilayered concentric sphere neutron spectrometer(WMNS).Specifically,the neutron fluence bounds are estimated to accelerate the algorithm convergence,and the minimum error between the optimal solution and input neutron counts with relative uncertainties is limited to 10^(-6)to avoid unnecessary calculations.Furthermore,the crossover probability and scaling factor are self-adaptively controlled.FLUKA Monte Carlo is used to simulate the readings of the WMNS under(1)a spectrum of Cf-252 and(2)its spectrum after being moderated,(3)a spectrum used for boron neutron capture therapy,and(4)a reactor spectrum.Subsequently,the measured neutron counts are unfolded using the SDENUA.The uncertainties of the measured neutron count and the response matrix are considered in the SDENUA,which does not require complex parameter tuning or an a priori default spectrum.The results indicate that the solutions of the SDENUA agree better with the IAEA spectra than those of MAXED and GRAVEL in UMG 3.1,and the errors of the final results calculated using the SDENUA are less than 12%.The established SDENUA can be used to unfold spectra from the WMNS.

The Design and Optimization of a Neutron Time-of-Flight Spectrometer with Double Scintillators for Neutron Diagnostics on EAST

Neutron energy spectrometry diagnosis plays an important role in magnetic con- finement fusion. A new neutron time-of-flight （TOF） spectrometer with double scintillators is designed and optimized for the EAST toknmak. A set of optimM parameters is obtained by Monte Carlo simulation, based on the GEANT4 and ROOT codes. The electronic setup of the measurement system is designed. The count rate capability is increased by introducing a flash ADC. The designed spectrometer with high resolution and efficiency is capable of being applied to fusion neutron diagnostics. Applications in mixed-energy and continuous energy neutron fields can also be considered.

Extended Bonner sphere spectrometer for dynamic neutron spectrum on HL-2A

The extended Bonner sphere spectrometer （EBSS） at the HL-2A tokamak for the neutron spectrum is described. This device was developed on the basis of previous Bonner sphere spectrometry （BSS）, aiming to obtain a more accurate neutron spectrum in the HL-2A tokamak hall. The previous BSS contained eight Bonner spheres （BS）. This EBSS contains 13 3He-filled detectors embedded in polyethylene spheres （PS）, pre-amplifiers, and a parallel processing data acquisition system （DAQ）. A response matrix is simulated in Geant4 taking the effect of the environment into account.

Neutron time-of-flight spectrometer based on HIRFL for studies of spallation reactions related to ADS project

A Neutron Time-of-Flight(NTOF) spectrometer, based at the Heavy Ion Research Facility in Lanzhou(HIRFL) was developed for studies of neutron production of proton induced spallation reactions related to the ADS project. After the presentation of comparisons between calculated spallation neutron production doubledifferential cross sections and the available experimental data, a detailed description of the NTOF spectrometer is given. Test beam results show that the spectrometer works well and data analysis procedures are established.The comparisons of the test beam neutron spectra with those of GEANT4 simulations are presented.

Simulation of Gamma-Ray and Neutron Spectrometers for Microsatellite Missions

Microsatellites have recently opened windows of frequent and low cost missions for planetary exploration. The performance of gamma-ray and neutron spectrometers on future microsatellite missions is simulated to assess the possibility of observation of hydrogen and major elements, given their concentration on the observation target. The measured elemental abundance will provide important geological constraints, and some of them may serve as space resources. Four different types of target bodies with various hydrogen concentrations in the range of 0 - 20,000 ppm are assumed as target compositions;Earth’s core, C-type, S-type and Martian meteorites. Gamma-ray and neutron emission rates show unique footprints that are related to the different elemental compositions. The starting point is the solid angle subtended between observation target and spectrometers that allow estimating the gamma-ray and neutron count rates emitted by the celestial bodies. In this work, three types of gamma-ray detectors;high-purity germanium (HPGe), CeBr3 and LaBr3(Ce), a neutron spectrometer combining a lithium glass scintillator with a boron loaded plastic scintillator and a dual mode spectrometer Cs2LiYCl6(Ce) (CLYC) are simulated, focusing on their observation backgrounds as a model case for microsatellite based measurements. The background count level of both gamma-ray (except for the LaBr3 detector) and neutron count rates was negligible under these particular conditions. The gamma-ray detectors were compared by the figure of merit, which was determined by their efficiency and energy resolution. It was found that each detector has unique advantages. The HPGe detector has the highest figure of merit due to its excellent energy resolution, whereas the CLYC detector is low in weight and power consumption due to its dual sensitivity to gamma-ray and neutron. The CeBr3 detector is an intermediate choice. The neutron count rates are calculated separately in three energy ranges, i.e. , thermal (<0.5 eV), epithermal (0.5 eV - 500 keV), and fast (>500 keV), as a function of the hydrogen concentration in the 0 - 20,000 ppm range. The thermal and epithermal neutron count rates are found to decrease with hydrogen concentration, while the fast neutron count rate increases with the target average atomic mass. The optimal detector should be decided by the mission restraints on mass, power consumption, and heat thermal design.

Calculation of Response Function for Bonner Sphere Spectrometer Based on Geant4

In order to make further studies on fusion neutron diagnosis on HL-2A /HL-2M,we have developed and succeeded in the calculation of the Response Function for a Bonner sphere spectrometer,which consists of eight polyethylene spheres with ^3He proportional counters inside.The response function of the Bonner spectrometer to neutrons is of fundamental importance for its neutron spectrum unfolding procedure and is directly related to the quality of the unfolded spectrum.In this paper,we calculated the response function to neutrons from 10^-9 MeV to100 MeV by Geant4.In order to test the accuracy of the Geant4 simulation,we apply it to measure an ^241Am-Be neutron source,and the measured neutron counts of the spectrometer and simulated counts are found to be highly consistent,with a relative error up to 9.3%.This has proven the calculation of the neutron response of the Bonner sphere spectrometer by Geant4 to be quite accurate.

Design of Lead Spectrometer for Spent Fuel Fissile Assay

The design of a lead slowing down spectrometer is being developed at KAERI （Korea Atomic Energy Research Institute） for an analysis of isotopic fissile content in spent and recycled fuel. The lead spectrometer has an advantage to assay fissile content directly from spent fuel. Accurate fissile material data must be fed back to the spent fuel design and management. For reutilization of PWR spent fuel, an isotopic fissile content analysis is required to obtain safety and economics in the fuel cycle. The LSDS （lead slowing down spectrometer） uses the different fission signatures of each fissile with respect to the interrogation neutron energy. Several simulations were done and parameter was obtained for the system working. Energy between 1 keV to 0.1 eV is very sensitive to isotopic fissile material fission with good energy resolution. An external source neutron is necessary to induce enough fissile fission. A neutron production methodology is being developed for cost effectiveness, easy maintenance and high neutron yield rate. The threshold fission detector screens the prompt fast fission neutrons from an intense radiation background. The detection of a fission signature has a direct relationship with the isotopic fissile content.

Studies on unfolding energy spectra of neutrons using maximumlikelihood expectation–maximization method

Energy spectra of neutrons are important for identification of unknown neutron sources and for determination of the equivalent dose. Although standard energy spectra of neutrons are available in some situations, e.g., for some radiotherapy treatment machines, they are unknown in other cases, e.g., for photoneutrons created in radiotherapy rooms and neutrons generated in nuclear reactors. In situations where neutron energy spectra need to be determined, unfolding the required neutron energy spectra using the Bonner sphere spectrometer (BSS) and nested neutron spectrometer (NNS) has been found promising. However, without any prior knowledge on the spectra, the unfolding process has remained a tedious task. In this work, a standalone numerical tool named ‘‘NRUunfold’’ was developed which could satisfactorily unfold neutron spectra for BSS or NNS, or any other systems using similar detection methodology. A generic and versatile algorithm based on maximum-likelihood expectation– maximization method was developed and benchmarked against the widely used STAY’SL algorithm which was based on the least squares method. The present method could output decent results in the absence of precisely calculated initial guess, although it was also remarked that employment of exceptionally bizarre initial spectra could lead to some unreasonable output spectra. The neutron count rates computed using the manufacturer’s response functions were used for sensitivity studies. The present NRUunfold code could be useful for neutron energy spectrum unfolding for BSS or NNS applications in the absence of a precisely calculated initial guess.

The study of a neutron spectrum unfolding method based on particle swarm optimization combined with maximum likelihood expectation maximization

The neutron spectrum unfolding by Bonner sphere spectrometer(BSS) is considered a complex multidimensional model,which requires complex mathematical methods to solve the first kind of Fredholm integral equation. In order to solve the problem of the maximum likelihood expectation maximization(MLEM) algorithm which is easy to suffer the pitfalls of local optima and the particle swarm optimization(PSO) algorithm which is easy to get unreasonable flight direction and step length of particles, which leads to the invalid iteration and affect efficiency and accuracy, an improved PSO-MLEM algorithm, combined of PSO and MLEM algorithm, is proposed for neutron spectrum unfolding. The dynamic acceleration factor is used to balance the ability of global and local search, and improves the convergence speed and accuracy of the algorithm. Firstly, the Monte Carlo method was used to simulated the BSS to obtain the response function and count rates of BSS. In the simulation of count rate, four reference spectra from the IAEA Technical Report Series No. 403 were used as input parameters of the Monte Carlo method. The PSO-MLEM algorithm was used to unfold the neutron spectrum of the simulated data and was verified by the difference of the unfolded spectrum to the reference spectrum. Finally, the 252Cf neutron source was measured by BSS, and the PSO-MLEM algorithm was used to unfold the experimental neutron spectrum.Compared with maximum entropy deconvolution(MAXED), PSO and MLEM algorithm, the PSO-MLEM algorithm has fewer parameters and automatically adjusts the dynamic acceleration factor to solve the problem of local optima. The convergence speed of the PSO-MLEM algorithm is 1.4 times and 3.1 times that of the MLEM and PSO algorithms. Compared with PSO, MLEM and MAXED, the correlation coefficients of PSO-MLEM algorithm are increased by 33.1%, 33.5% and 1.9%, and the relative mean errors are decreased by 98.2%, 97.8% and 67.4%.

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 （[Oi]） 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 [Oi] at temperatures between 500 and 700℃. It turns out that the large fluctuation of [Oi] after short-time annealing from 500 to 700℃ results from the transformation of fast neutron irradiation defects.

Neutron Imaging by Boric Acid

In this paper a new type of passive neutron detector based on the already existing one, CR39, is described. Its operation was verified by three different neutron sources: an Americium-Beryllium (Am241-Be) source;a TRIGA type nuclear reactor;and a fast neutron reactor called TAPIRO. The obtained results, reported here, positively confirm its operation and the accountability of the new developed detecting technique.

基于半经验法多球中子谱仪校准

基于半经验法扣除中子散射本底,提出了一种多球中子谱仪校准的新方法。首先用热中子参考辐射装置校准多球中子谱仪各中子球内He-3正比计数器的探测效率。用多球中子谱仪各中子球单独测量参考^(241)Am-Be中子源,结合半经验法扣除散射,给出了各中子球的注量响应。结合两者实验结果,获得^(241)Am-Be源能谱并给出谱仪注量响应。结果表明,校准的多球中子谱仪具备快中子区测量能力。

多场耦合原位拉伸中子衍射实验装置综述

极端环境服役材料的研发一直是航空航天等国家战略发展的“瓶颈”,不同环境因素会影响材料的性能。中子由于其强穿透性、轻元素敏感等特点,可与同步辐射技术互为补充,运用原位装置还原材料在真实工况条件下的受力变形过程,并利用中子探针原位观察材料在服役条件下晶格应变、织构、相变和残余应力的演化。多个国家的中子谱仪均配备了不同的原位拉伸装置,实现在不同的加载环境下对样品进行原位应力加载,对样品材料的微观结构进行测试分析,能够解决材料工程领域的重要科学机理问题,进而推动材料的发展应用。介绍了国内外不同中子源谱仪原位拉伸装置的情况,重点阐述了应用于中子衍射技术的多场耦合原位拉伸装置的设计原理与结构特点,凸显了工程材料研究的发展方向。

面向地外行星探测的脉冲中子元素分析仪

以模拟月球的地层为研究对象,设计了一种小型化的高产额中子管,开展了集成化中子-伽玛谱仪研究,建立了元素分析谱库和解谱方法,通过搭建已知元素含量的模拟井和地面测试验证平台,实现了对模拟月球次表层常量元素、稀有元素等元素的定量分析。中子发生器的产额在107~108个/s之间(分别对应脉冲和直流模式),探测时间在分钟量级,可达到5%的精度。该研究工作实现了对行星探测任务的中子-伽玛谱仪轻小型化关键技术的重要突破,提高了探测速度,为在未来深空探测任务中的载荷应用工程化奠定基础。

中子织构谱仪原位加载装置

原位加载过程中织构演化研究对于深入理解材料在不同变形工艺及服役条件下变形方式和性能演化机制具有重要意义。本文基于中国先进研究堆(CARR)中子织构谱仪,设计并研制了一台专用原位加载装置。该装置的最大加载力为10 kN,加载速率在0.01~3 mm/min范围内连续可调,可绕中子织构谱仪欧拉环χ角在0°~90°旋转、沿装置拉伸轴进行0°~360°自转,以实现多晶材料在原位拉伸和挤压过程中微观结构和体织构实时演变测量。利用该装置对热挤压Mg-1.2Al-0.3Ca-0.47Mn(MMK1)合金原位拉伸过程中织构演化的研究发现,随着拉伸位移的增加,(0002)基面织构强度不断增加,表明该装置可实现多晶材料原位加载过程中织构演化测量。CARR中子织构谱仪原位加载装置的设计和研制,为了解多晶材料微观结构与力学性能关系,尤其是在外部载荷条件下材料结构演化的规律和机理研究提供了重要的技术支撑。

月表元素组成的粒子探测现状与展望

月表元素组成是国际上月球探测和研究的热点。伽马射线谱仪和中子谱仪等粒子探测器广泛用于月球遥感探测,提供了月表天然放射性元素(U、Th、K)、主要元素(Fe、Ca、Si、Al、Mg等)和水冰(H元素)的分布情况。这些数据极大地促进了人类对月球起源与演化历史的研究,同时也推动了近年来世界各国开发利用月球资源以及建设月球科研站的步伐。在国际月球探测新热潮背景下,本文从粒子物理和原子核物理的角度阐述了携带有元素特征信息的伽马粒子和中子的产生机制,介绍了伽马谱仪和中子谱仪的探测原理和探测器响应,并综述了近30年来粒子探测器在月表元素组成上所取得的主要研究成果以及相关数据分析方法。这些内容可为我国未来嫦娥七号和其他深空探测任务的相关研究提供参考。