Development of large-area composite stilbene scintillator for fast neutron detection

Fast neutron applications have gained popularity with the growth of fast neutron production facilities. Covering a larger area and/or wider angle can be one of the advantages of a fast neutron detector. In the present study, a large-area composite stilbene scintillator with the dimensions of 200 mm (D) × 20 mm (H) was fabricated to examine its scintillation properties and to evaluate its applicability to fast neutron detection. The detector response of small- and large-area composite stilbene scintillators for neutrons and gamma rays was measured and compared with that of commercial and small single-crystal stilbene scintillators. To this end, the response of each scintillator was measured for radioisotopes as well as mono-energetic neutrons generated by a Tandem accelerator. The neutron–gamma separation performance of the large-area composite stilbene scintillator was evaluated in terms of figure-of-merit (FoM) using the digital pulse shape discrimination method. The composite stilbene scintillator showed good energy linearity, as determined from its recoil proton spectra, with reasonable n–γ separation capability. The results indicated that the composite stilbene scintillator could be applied to the field of fast neutron detection, especially when a large area and/or a wide angle is to be covered and could be a good alternative to liquid scintillators.     

The Slow Component of Scintillation in Stilbene

Experimental study of the response of stilbene to protons (0,8 to 1,8 MeV) indicates a non-linearity in the intensity of the fast components vs energy and a linear relationship for the slow component. It was found for stilbene and anthracene a decreasing intensity of the slow component with increasing particle mass (electrons, protons, alphas). The temperature dependence of both components has been measured between -130°C and 20°C. A relative increase of the same magnitude, about 20 % has been observed for the fast and the slow emission at low temperature. This paper presents the experimental devices used in these experiments and a discussion of the results in relation to models proposed for the slow emission process (mainly triplet-triplet interaction).     

BC501液体闪烁体对n-γ及能量的分辨与其尺寸的关系

BC501液体闪烁体广泛应用于探测快中子，但测量伴随着很高的γ本底。为寻找具有较好的n-γ及能量分辨的BC501闪烁体，利用脉冲上升时间法，对几种不同尺寸的BC501闪烁体进行n-γ及能量分辨测量。在下阈0.75和1MeV下，分别测量了Am-Be中子源的n-γ分辨谱以及相同条件下的γ上升时间-幅度谱。测量了d-T中子源14MeV的反冲质子脉冲高度分布。对不同尺寸BC501闪烁体的n-γ及能量分辨进行了比较。实验表明，综合考虑n-γ和能量分辨，闪烁体的体积不应太大，长度应在保证效率的条件下适中选择。      

氘化闪烁体的光响应

在迴旋加速器上利用D(d,n)~3He反应和~9Be(d,n)~(10)B反应产生的中子,采用双参数方法测量NE-230和NE-232氘化闪烁体对氘以及国产的ST-451闪烁体对质子的光响应曲线,所测的实验值用二次多项式拟合。对其它性能也作了相对比较。     

Development of a Double Layered Scintillator for Separating and Detecting Low Energy Protons and Electrons

A scintillator system consisting of a thin (5,000 ? - 15,000 ?) CsI(Tl) layer evaporated onto a plastic scintillator (NE-102) has been developed for the purpose of distinguishing low energy protons from electrons and measuring the energy of each species. Evaporations in a high vacuum (10-8 Torr) produced layers of CsI(Tl) that scintillate with an efficiency comparable to optimally doped bulk material, If the CsI(Tl) layer thickness is 15,000 ?, it stops protons with energies below 170 keV and electrons with energies below 18 keV. Thus, protons with energies between about 25 and 250 keV can be distinguished from electrons with energies above 18 keV by examining the shape of the light pulse generated in the dual scintillator. Results obtained with protons and electron beams will be presented.     

Proposal of Method to Estimate Criticality Correction for Anisotropic Diffusion in Plate Lattice Fast Assembly

Measurements on the neutron and γ-ray attenuation in multi-legged air filled ducts have been carried out using a Cockroft-Walton type neutron generator. The measured spectra were obtained with an NE-213 liquid scintillator using pulse shape discrimination to resolve neutron and γ-ray pulse height data and using a spectral unfolding code to convert these data to energy spectra. Results are given for two rectangular duct geometries. A series of integral measurements have also been carried out with a long counter for fast neutrons and a radiation survey meter to measure the γ-ray dose. The biological dose arising due to the streaming of neutrons was measured with a rem counter. The results show the relative decrement of the integral counts along the axis of the duct. The spectral measurements reveal the presence of resonant windows of oxygen present in the concrete medium. Also the 14 MeV component is found to be dominant in the first leg of the duct whereas degraded neutrons dominate in the second leg. The results are compared with MORSE calculations.     

Energy Loss Distribution of Recoil Protons in Stilbene Scintillator

Formulas are derived for calculating the energy loss distribution of recoil protons in a cylindrical organic scintillator with a broad beam of fast neutrons in the energy range 1 to 15 MeV perpendicularly incident on its base. The assumptions are: (1) the scintillator is composed solely of hydrogen and carbon; (2) only elastic scattering from hydrogen and carbon is considered in relation to the interaction of fast neutrons with the scintillator; (3) single and double scattering alone are considered; (4) wall effect is neglected for recoil protons produced in the second collisions; (5) the radius and the thickness of the scintillator are greater than the range of recoil protons of the highest energy.

As an example, the calculated results are shown for stilbene crystals of 1″ dia. and 1/8″ to 1″ thick. In scintillators of thicknesses below 1/4″, and for incident fast neutrons in the energy range from 3 to 15 MeV, the contribution of double scattering to the energy loss distribution of recoil protons is below about 10% of that of single scattering, while wall effect is quite significant, in particular, for incident neutrons of high energy.     

数字快成形算法用于慢衰减闪烁体的高计数率能谱读出

慢衰减型闪烁体(NaI(Tl)、CsI(Tl)等)探测器经过前放输出的脉冲信号具有较长的上升时间,造成电荷在收集的过程中出现严重的弹道亏损。在后续脉冲成形中为了降低弹道亏损对能量分辨率的影响,一般采用较宽的成形平顶来增加电荷的收集时间。但在高辐射粒子注量率的场合下,由于慢衰减型闪烁体输出的脉冲堆积严重,采用传统的滤波成形方式获得的脉冲计数率和能量分辨率明显降低。为解决此问题,本文提出了一种以数字反褶积为核心的数字快成形算法。该算法可去掉慢衰减型闪烁体探测系统的衰减电流拖尾,获得一理想的冲激脉冲电流,然后再通过滤波成形为一窄脉冲,并彻底消除弹道亏损的影响。通过对~(137) Csγ源测量,使用传统成形算法的能谱测量系统在成形时间为1.5μs时,其光电峰能量分辨率为6.99%,计数率为68 000s~(-1);而使用数字快成形系统,在相同情况下获得6.37%的能量分辨率,计数率可达102 000s~(-1)。因此数字快成形算法可有效地修复在高辐射粒子注量率下,窄脉冲成形引起的信号变形和拖尾,从而提高了脉冲堆积甄别能力。     

用Fisher线性判别方法甄别Cs_2LiYCl_6:Ce~(3+)晶体的n/γ波形

Cs_2LiYCl_6:Ce~(3+)(CLYC)晶体是一种性能优异的无机闪烁晶体,具有良好的n/γ脉冲形状甄别能力。传统的电荷比较法常用来甄别CLYC晶体的n/γ,通过优化积分时间窗口的宽度来获得最优的甄别结果,计算量大。本文提出利用Fisher线性判别进行CLYC晶体的n/γ波形甄别,以获得高甄别品质因子为准则来构建投影向量。n/γ混合场由Am-Be源产生,用泰科DPO7104示波器记录探测器信号。结果表明,构建投影向量后,Fisher线性判别方法计算量小、可移植性强,且可得到优于电荷比较法的甄别结果。     

NE213闪烁体的n-γ分辨

NE213闪烁体广泛使用于探测快中子,但却伴随很高的g本底。本文使用快信号门与总信号门的两门积分方法,用快信号与总信号进行二维作图来分辨粒子。用252Cf 中子源和d+D核反应产生的单能中子研究了 f5"2"、f5"5"和f8"2"三种尺寸的NE213闪烁体的n-g分辨性能,通过选择合适的门宽,获得了极好的n-g分辨效果。对f5"2"的NE213,最佳快信号门宽为30ns。 摘要 NE213闪烁体广泛使用于探测快中子,但却伴随很高的g本底。本文使用快信号门与总信号门的两门积分方法,用快信号与总信号进行二维作图来分辨粒子。用252Cf 中子源和d+D核反应产生的单能中子研究了 f5"2"、f5"5"和f8"2"三种尺寸的NE213闪烁体的n-g分辨性能,通过选择合适的门宽,获得了极好的n-g分辨效果。对f5"2"的NE213,最佳快信号门宽为30ns。     

EJ339A型液体闪烁探测系统光输出响应函数的理论模拟与实验研究

对液体闪烁探测器EJ339A中子与伽马辐射测量问题,采用理论模拟与实验分析方法,结合自主设计搭建的基于Lab VIEW的数字化信号处理系统,分别完成22Na、133Ba、137Cs和60Co等4种不同能量的γ源等效电子能量测量与刻度。在此基础上,利用中子飞行时间测量原理,在不同时间窗下将锎(252Cf)源近似分化成若干个单能中子源,获得1.9 7.8 Me V范围内中子在探测器中的光输出响应函数。结果表明,理论模拟与实验测量值在低能段比较吻合,而高能段存在约7.3%的误差。     

Characterization of the new scintillator Cs_2LiYCl_6:Ce~(3+)

The first domestic inorganic scintillator, Cs_2LiYCl_6:Ce~(3+)(CLYC), was grown at Beijing Glass Research Institute using the vertical Bridgman method. In this work, we evaluated the performance of this new CLYC crystal in terms of its gamma-ray energy resolution and pulse shape discrimination(PSD) capability between neutrons and gamma rays. The decay times associated with different scintillation mechanisms were obtained by fitting decay functions to the neutron and gamma-ray waveform structures. We found an energy resolution of * 4.5% for 662-ke V gamma rays and efficient neutron/gamma PSD with a figure of merit of * 2.6. Under gamma-ray excitation, there is an ultrafast scintillation mechanism in CLYC with a decay time of approximately 2 ns, whereas there is no evidence of ultrafast decay under thermal neutron excitation. This work contributes to the promotion of domestic development of CLYC.     

基于6LiF/ZnS(Ag)和闪烁光纤的宽能谱中子探测技术研究

为了解决传统中子探测器在狭窄空间、强电磁干扰、远距离传输等复杂环境下探测中子时存在的不足,本研究将6LiF/ZnS(Ag)混合材料和闪烁光纤相结合,设计了一种可用于宽能谱中子测量的新型闪烁体光纤中子探测器。基于蒙特卡罗粒子输运计算程序FLUKA对该新型光纤中子探测器的中子探测性能进行了模拟研究,完成了闪烁体光纤探头的优化设计。结果表明,当入射中子的能量在0.01~10 eV和0.5~10 MeV范围时,该新型中子探测器具有较高的中子探测效率,可用于热中子-快中子宽能谱范围中子的探测;通过对比脉冲幅度的差异,该新型中子探测器能够实现n-γ信号的甄别。      

Stilbene and NE 213 Characteristics Related to Their Use for Neutron Spectroscopy

The pulse discrimination technique permits scintillators to be used for neutron spectroscopy, in the presence of gamma rays, to investigate a wide variety of problems. The influence of shape and size of stilbene and NE 213 scintillators on their gamma-ray and neutron efficiencies and on the shape correction has been investigated. The neutron efficiency and the shape correction factor enter into the spectrum conversion function which is needed to reduce a recoil-proton energy distribution to a neutron spectrum. Gamma-ray and neutron efficiencies from 0.1 to 24 MeV have been graphed as a function of length. Graphs of the shape correction factor and the spectrum conversion function, up to 24-MeV neutron energy, have been made for various dimensions and similar volumes. These graphs and the summary tables can be used to select the size and shape of a detector to minimize the spread of the correction factor over a given range of neutron energy and to increase the neutron efficiency consistent with conditions of a given experiment or application.     

闪烁探测器信号波形识别的人工神经网络实现

根据闪烁探测器中激发荧光的快、慢成分的差异,定义了代表信号波形的三个特征物理量:即信号幅度、信号在上升时间段的积分以及中频段的频谱积分.研究了人工神经网络对不同波形的识别方法.用计算机模拟了实际情况下的信号波形,用神经网络对这些信号波形进行识别,得到了较好的识别结果并进行了讨论.     

A pulse shape discrimination of CsI(T1) crystal with 6He beam

The performance test of a CsI(T1) crystal (70×27×23 mm~3) was performed by applying the pulse shape discrimination technique for identification oflight charged pafflcles , The crystal is coupled to a photomultiplier tube during an experiment with ~6He beam. The pulse waveform is fully recorded by employing a high precision digital oscilloscope. The fast and slow gates are used for the pulse shape discrimination and the best values for the gate widths were determined to be 0.5 μs and 1.67 μs, respectively. The ~6He, ~4He and ~3He are successfully discnminated with this technique.     

基于BP神经网络的n/γ甄别方法研究

常用的有机闪烁体探测器对中子和γ射线均敏感,所以消除或减弱γ射线在中子探测技术中的影响是必要的。考虑到BP神经网络能实现分类器的功能,因此本文结合脉冲形状甄别技术与BP神经网络,将BP神经网络应用在中子与γ射线混合场的粒子甄别中。通过训练BP神经网络达到记忆、分类测试样本的目的。对BP神经网络应用于n/γ脉冲波形甄别的准确性进行验证后与电荷比较法及频域梯度分析法甄别结果进行了对比。结果表明,BP神经网络甄别法不仅能为混合辐射场提供有效的甄别,而且在甄别时间上较电荷比较法与频域梯度分析法有所提高。     

Detection of Fast Neutrons with Nal(Tl) Crystal

An attempt has been made of using Nal(Tl) crystal as fast neutron detector in conventional Time-of-flight apparatus. Typical pulse hight distributions, in the γ-ray energy range below 1 MeV, produced by the incidence of fast neutrons are presented. The characteristics of this neutron detection system have been studied, using pulsed neutrons from the T(p, n) and D(d, n) reactions. Neutron detection efficiency curves are presented, covering both whole γ-rays and those from a few low excited levels of ¹²⁷I nucleus. The detection bias can be stably set at the first excitation energy (58 keV), which is lower than possible for an organic scintillator.     

基于MPD-4电子学插件的液体闪烁体中子光响应特性实验研究

中子光响应函数是液体闪烁体探测器的重要参数,准确的光响应函数和探测阈值是精确确定中子探测效率的基础。近年来,专门用于液体闪烁体中子探测器的高集成度新型电子学插件（Mesytec公司的MPD-4）广泛应用于中子探测系统中,该电子学插件集成了脉冲高度（PH）、脉冲形状甄别（PSD）和恒比定时（CFD）测量功能,且1个NIM插件即可实现4路探测器的同时测量。对于同一中子探测器,利用这一电子学系统与传统电子学系统对中子的光响应函数进行了实验比对,结果表明,两套系统得到的中子光响应函数具有明显的差异。因此,在使用MPD-4电子学系统进行中子探测的实验中,需对光响应函数进行重新标定。     

A Zero-Crossing Pulse Shape Discriminator for Space Applications

A circuit for discriminating neutrons, gammas and charged particles from each other in a phoswich scintillator is described. The scintillator is intended for use as a fast neutron detector. Test results indicate that the particle discrimination capability, noise rejection, and pulse pile up rejection are good. The circuit is capable of operating with very little degradation over a wide temperature range. Energy information for neutrons and gammas is also simultaneously obtained. The dynamic range is at least 20:1. The circuit has the capability of accepting count rates up to 4 × 105 per second without loss of information.