医院中子照射器PGNAA系统初步设计及测量

完成了医院中子照射器(IHNI)瞬发γ中子活化分析(PGNAA)系统的初步设计,并按照设计完成系统搭建。该系统探测器屏蔽部分和中子束屏蔽部分内层均为天然LiF粉末,外层为铅屏蔽层。使用该系统进行的硼浓度测量实验结果表明,该系统可测量10B浓度为10 ppm(1 ppm=10-6),体积为2 mL的样品,10B元素测量灵敏度为0.822 cps(s-1)/ppm。同时发现天然LiF粉末会在待测能区产生较高本底,影响测量精度。     

医院中子照射器性能与特性

介绍了医院中子照射器及其相关系统的设计情况,描述了医院中子照射器的性能和特性。为验证医院中子照射器的固有安全特性,进行了4.2 mk反应性释放实验。实验结果表明,反应堆功率快速上升,在229 s时,功率达到85.7 kW峰值,随后,由于燃料元件多普勒效应和慢化剂的负温度效应会非能动地把功率限制在允许的安全水平之下。满功率运行时,各个工艺房间γ剂量率小于1μSv/h,且中子剂量率为本底水平,具有低辐射特点。     

WIMS/CITATION在IHNI-1堆芯物理计算中的应用

建立了利用WIMS+CITATION计算医院中子照射器Ⅰ型堆堆芯中子学参数的模型,计算了堆芯的功率分布、顶铍反应性价值、控制棒价值、温度系数、堆芯燃耗等中子学参数,计算结果与文献数据一致,表明文章所建立的计算模型可用于医院中子照射器I型堆堆芯的物理计算。     

低浓化医院中子照射器（IHNI-1）堆芯的物理方案设计

采用蒙特卡罗程序MCNP／4B模拟计算了功率为30kW的低浓化医院中子照射器的堆芯物理参数,设计了合理的堆芯布置方案、235U富集度、控制棒价值、后备反应性和停堆深度,得到固有安全性较高、寿期达10年且无需换料、采用低浓化UO2燃料的医院中子照射器的堆芯物理设计方案,为后续反应堆工程设计以及硼中子俘获治疗肿瘤用中子束的设计提供理论依据。     

医院中子照射器试运行

医院中子照射器及其辅助系统于2010年年初完成热调试试验,随后投入试运行至今,共开堆99次(天),累积释放能量1 832.42 kW.h(折合积分中子注量为2.198 9×1017cm-2)。试运行期间实验数据显示,医院中子照射器及其辅助系统满足规定的运行限值和条件,未发生任何事故,具有很好的安全性与可靠性,达到了设计要求。     

核径迹器作辐射测量的新进展

核径迹探测器已应用于中子、α粒子以及重型带电粒子的测量。当前，某些最重要的应用就包括中子剂量测定中的固态径迹探测器，室内和矿山的氡气剂量测定中的固态径迹探测器，空间带电粒子测量以及生物医学用加速器射束，本文将回顾近年来径迹探测器在中子，氡气测量领域的发展。     

核径迹探测器作辐射测量的新进展

核径迹探测器已应用于中子、a粒子以及重型带电粒子的测量。当前，某些最重要的应用就包括中子剂量测定中的固态径迹探测器、室内和矿山的氡气剂量测定中的固态径迹探测器、空间带电粒子测量以及生物医学用加速器射束。本文将回顾近年来径迹探测器在中子、氡气测量领域的发展。     

医院中子照射器I型堆超热中子束流孔道的优化设计

采用蒙特卡罗程序(Monte Carlo neutron and photo transport code,MCNP)对医院中子照射器Ⅰ型堆(IHNI-1)超热中子束流孔道的慢化层、反射层进行了优化设计。首先对FLUENTAL、Al等材料组成的6种慢化体方案进行了分析比较,给出了孔道出口处超热中子通量密度较大的两种设计方案;基于此两种慢化体设计方案,在保持束流孔道外框尺寸不变情况下,对慢化体周围的反射层进行了分析比较,给出了反射层的推荐方案;基于慢化体和反射层优化方案,最后给出了超热中子束流孔道出口处束流参数的空间分布。     

电离辐射计量在核电站的应用

针对核电领域使用的反应堆控制、辐射防护、环境监测类电离辐射计量仪表应用情况进行了叙述,尤其对气态排出物标准源的研制、全身计数器测量系统校准方法的实现和弱贯穿辐射的防护监测,提出了核电领域6.13 MeV高能γ射线剂量监测、高量程γ射线剂量仪表校准、反应堆内外中子通量监测及人员通道中子剂量分布测量的量值溯源需求.     

MJTR围桶外中子注量测量及辐照能力评估

MJTR运行期间,在堆芯围桶外壁布置了多组活化探测器,对热中子和快中子探测器注量率及其分布进行测量,评估了MJTR围桶外辐照能力。试验结果表明MJTR围桶外紧贴围桶中平面高度位置2200m.s-1热中子注量率为3.46E＋10 n.cm-2.s-1,E≥1.0MeV快中子注量率为2.21E＋09 n.cm-2.s-1。轴向热中子与快中子注量率近似服从余弦分布,而径向热中子和快中子注量率近似服从指数衰减分布。在MJTR围桶外径向0-1500 mm距离内快中子注量率分布在109-100n.cm-2.s-1,MJTR围桶外能够进行辐照要求在此范围之内的材料考验。     

Determination of the Neutron Flux of the NG-12I Neutron Generator

We describe a method of determining the neutron flux of an NG-12I neutron generator, using Al and F activation detectors. The numerical computation data used were obtained by the Monte Carlo method. __________ Translated from Izmeritel'naya Tekhnika, No. 8, pp. 62–65 August, 2005.     

Fast neutron-induced damage in INTEGRAL n-type HPGe detectors

Several INTEGRAL n-type HPGe detectors have been irradiated by fast neutrons and their degradation studied through the analysis of line shapes. The availability of three different fast neutron beams (5, 16 and 6–70 MeV) allowed a quantitative analysis of the importance of the neutron energy on the amount of damage. A comparison is made with the degradation induced by high-energy proton irradiations. Transient effects on the measured resolution are reported after high voltage cut-off on degraded detectors.     

Large-scale gadolinium-doped water Cherenkov detector for nonproliferation

Fission events from Special Nuclear Material (SNM), such as highly enriched uranium or plutonium, can produce simultaneous emission of multiple neutrons and high-energy gamma-rays. The observation of time correlations between any of these particles is a significant indicator of the presence of fissionable material. Cosmogenic processes can also mimic these types of correlated signals. However, if the background is sufficiently low and fully characterized, significant changes in the correlated event rate in the presence of a target of interest constitutes a robust signature of the presence of SNM. Since fission emissions are isotropic, adequate sensitivity to these multiplicities requires a high efficiency detector with a large solid angle with respect to the target. Water Cherenkov detectors are a cost-effective choice when large solid angle coverage is required. In order to characterize the neutron detection performance of large-scale water Cherenkov detectors, we have designed and built a 3.5 kL water Cherenkov-based gamma-ray and neutron detector, and modeled the detector response in Geant4 [1]. We report the position-dependent neutron detection efficiency and energy response of the detector, as well as the basic characteristics of the simulation.     

分群结构对IHNI-1堆动态参数计算结果的影响研究

基于WIMS和CITATION程序的计算结果,编制了动态参数计算程序CKPWC(calculating kinetic parameters based on WIMS and CITATION),对医院中子照射器Ⅰ型堆(in-hospital neutron irradiator mark 1 reactor,IHNI-1)的动态参数计算进行了研究.首先使用WIMS计算出均匀化栅元截面以及69群通量,再使用CITATION进行四群扩散计算,最后编制动态参数计算程序,计算了IHNI-1动态参数(缓发中子有效份βeff和中子代时间Λ).经过比较研究发现,分群结构对动态参数的计算结果有很大的影响.给出了计算IHNI-1缓发中子份额和中子代时间的最佳四群分群结构.使用文章中的最佳分群结构思想对西安脉冲堆动态参数进行了验证计算,计算结果与设计值符合一致,说明给出的IHNI-1动态参数计算结果具有一定的可信性.     

Detector Development for the abBA Experiment

We have developed a new type of field-expansion spectrometer to measure the neutron beta decay correlations (a, b, B, and A). A precision measurement of these correlations places stringent requirements on charged particle detectors. The design employs large area segmented silicon detectors to detect both protons and electrons in coincidence. Other requirements include good energy resolution (< 5 keV), a thin dead layer to allow observation of 30-keV protons, fast timing resolution (~1 ns) to reconstruct electron-backscattering events, and nearly unity efficiency. We report results of testing commercially available surface-barrier silicon detectors for energy resolution and timing performance, and measurement of the dead-layer thickness of ion-implanted silicon detectors with a 3.2 MeV alpha source.     

Characterisation of CVD diamond detectors used for fast neutron flux monitoring

Natural diamond detectors (NDD) have been successfully used for fast neutron spectrometry on various fusion installations in plasma diagnostics. These detectors can work at high temperature, are radiation hard and exhibit a high energy resolution. However, the use of NDD is limited by the availability of IIa type diamonds exhibiting high electronic properties. With the recent advance in the growth of high quality chemically vapour deposited (CVD) diamond at LETI, CVD diamond appears to be a very promising material for plasma diagnostics. We present here for the first time results of the use of CVD diamond detectors for fast neutron flux monitoring on a neutron generator. The characteristics of CVD diamond detectors are compared with that of high quality NDD made by TRINITI. Pulse height spectra have been measured with CVD detectors and NDD under both 5.5 MeV alpha particles and 14.1 MeV neutrons. The quality of CVD diamond enables the recording of structured spectra allowing the distinction between the different neutron reactions on carbon. The efficiency of CVD diamond monitors and their actual limitations are analysed and discussed.     

A multiwire chamber system for measurements of neutron spectra

A new type of detector for measuring neutron flux and energy over a wide range of angles and energies is being developed. Measurements of neutron elastic and inelastic scattering as well as neutron energy continua are possible. Time-of-flight is not used for measuring outgoing neutron energy, and so for continuum measurements this system has some distinct advantages over conventional neutron detectors. Neutron energy measurement is carried out by measuring the energy and angle of the recoil proton produced by the neutron in a CH₂ converter. Spectra from ⁷Li(p, n)⁷Be at 62 MeV and ⁴⁰Ca(n, n′χ) at 65 MeV are presented.     

Neutron imaging at Peking University

Neutron imaging techniques were investigated at Peking University based on a 4.5 MV Van de Graaff accelerator. The thermal neutron radiography, fast neutron radiography and fast neutron resonance radiography were tested. The low neutron flux limits the image quality. A new radio frequency quadrupole (RFQ) accelerator based neutron source with a yield of 1012 n/s is being set up.     

Hexagonal boron nitride epitaxial layers as neutron detector materials

Micro-strip metal-semiconductor-metal detectors for thermal neutron sensing were fabricated from hexagonal boron nitride (hBN) epilayers synthesized by metal organic chemical vapor deposition. Experimental measurements indicated that the thermal neutron absorption coefficient and length of natural hBN epilayers are about 0.00361 μm⁻¹ and 277 μm, respectively. A continuous irradiation with a thermal neutron beam generated an appreciable current response in hBN detectors, corresponding to an effective conversion efficiency approaching ∼80% for absorbed neutrons. Our results indicate that hBN semiconductors would enable the development of essentially ideal solid-state thermal neutron detectors in which both neutron capture and carrier collection are accomplished in the same hBN semiconductor. These solid-state detectors have the potential to replace ³He gas detectors, which faces the very serious issue of ³He gas shortage.     

包装物对爆炸物中子检测的影响

文章概述了目前主要的爆炸物中子检测方法,介绍了用热中子俘获γ射线法（TNA法）检测爆炸物的原理,用模拟实验装置对模拟爆炸物进行了检测,得到了被测物的俘获γ射线能谱。实验装置采用d-D中子管产生2.5MeV的快中子,再经慢化之后得到热中子,并采用多个NaI探测器提高检测效率。着重对TNA法中不同外包装物材料以及多层复合外包装物材料对探测结果的影响进行了探索研究和结果分析,发现铜外壳对γ射线的屏蔽较大程度地降低了检出率,而例如聚四氟乙烯等塑料外壳也可能造成类似的影响。