钍俘获反应率离线伽马测量方法

为了测定聚变-裂变反应堆模型钍包层中的钍俘获率以及钍-铀转化率, 探索了一种新的钍活化离线γ测量法. 利用测量²³²Th俘获反应产物²³³Th衰变链中²³³Pa衰变放出的311.98 keV 特征γ射线, 来反推计算并最终确定²³²Th(n,γ)²³³Th的反应率, 测试实验中不确定度约6% (²³³Th/²³²Th量级为10^-17情况下). 详细介绍了此方法的背景和原理方法, 并进行简单的校验实验, 证明其能够较好地得到模拟装置中的俘获率. 与瞬发γ测量法以及质谱分析法进行对比, 本方法更适合用于聚变-裂变反应堆模型钍包层中的钍俘获率以及钍-铀转化率测量, 并有望进一步测量其他相关参数.     

临界装置绝对裂变率测量

临界装置是在低功率下运行的能够维持可控自持链式裂变反应的装置，一般运行的热功率在1kW以下，与动力堆相对应，一般称为零功率装置。在临界装置上进行的各项研究中，一个重要的参数就是裂变反应率(简称裂变率)，它是其它参数测量的基准，如在脉冲式的反应堆中，还是脉冲裂变总数的测量基准，是脉冲堆的一个主要指标。     

重离子裂变反应动力学研究

从实验角度评述了重离子裂变反应动力学研究现状．内容包括动力学裂变宽度、裂变时标、核粘滞性的温度和形变相关性以及从裂变实验上研究核粘滞性的方法等． The present status of the dynamics study for heavy ion induced fission reaction includesing the dynamical fission width,the fission time scale,the temperature and deformation dependence of the nuclear viscosity and the methods of the extracting viscosity from fission experiments induced by heavy ion reactions is reviewed.      

非全熔合反应裂变角分布

采用核固体径迹探测器测量了¹⁶O+¹⁵⁹Tb、¹⁶O+¹⁹⁷)Au和¹⁶O+²⁰⁹Bi反应碎片角分布,扣除中等质量碎片(IMF)的贡献,用过渡态统计模型(TSM)和单个自旋标准理论拟合实验的裂变角分布,提取裂变核的自旋. 讨论了它随质心系入射能量的变化.     

基于裂变γ标识技术的瞬发裂变中子谱测量新方法

瞬发裂变中子谱（prompt fission neutron spectrum,PFNS）是用于核实验诊断过程中十分重要的参数数据,传统的测量主锕系核素（U,Pu）PFNS的技术手段是采用裂变室,利用裂变碎片标识裂变中子,通过中子飞行时间技术获得裂变中子谱.目前出现了一种新的用于PFNS测量的技术,其原理是基于如下的物理事实：在一次裂变过程中,释放中子的同时伴随着释放7–8个γ射线光子,而非弹性散射效应产生的γ射线光子只有1–2个.据此,可以通过裂变γ射线的多重性将裂变中子和其他杂散中子甄选出来,达到测量PFNS的目的.本文建立了基于裂变γ标识技术的PFNS测量实验系统.利用该系统对²⁵²Cf中子源的PFNS进行了实验测量,测量结果与传统的裂变碎片标识法及ENDF/B-VⅡ数据库的标准谱进行了比较,对新方法的裂变标识率以及实验不确定度也一并进行了分析.     

测量电介质的电容率

简述了应用计算机技术改进测量电介质电容率的一种方法。     

BFEL束团长度测量系统的离线实验研究

短和超短束团长度测量技术是发展短波长FEL和高级加速器的关键技术之一. 为测试研制的短和超短束团长度测量系统雏形装置, 利用黑体辐射仪的连续远红外-毫米波辐射替代BFEL的4ps微束团产生的亚毫米波-毫米波CTR, 进行了离 线模拟实验, 获得良好调制的干涉图. 实验表明, 该雏形装置软硬件系统的运行满足初期的设计构想; 干涉仪经细致的光学调节, 可获得较为调制充分的干涉图; LabVIEW编制的测控软件满足控制和采集的基本需要. 提出了进一步改善的措施.     

248 Cm和252Cf自发裂变瞬发中子谱测量

在飞行时间方法测量 2 0 0keV— 1 2MeV能区内2 48Cm和2 5 2 Cf自发裂变瞬发中子谱 ,以国际原子能机构推荐的2 5 2 Cf自发裂变瞬发中子谱为标准 ,可以免去探测器效率刻度的问题并消除系统误差 ,提高测量精度 .实验用一个微型电离室作为裂变碎片探测器 ,两个芪晶体中子探测器测量中子及一套基于微机的距离分别为 3 2 ,50和 1 0 0cm ,经过对数据的处理和分析给出了 2 0 0keV— 1 2MeV的中子能区内的实验数据 .用Maxwell分布对实验数据进行拟合 ,得到的核温度为 (1 4 0 1± 0 0 0 6)MeV .     

脉冲裂变中子总数测量新方法的理论研究

简述了作者提出的脉冲裂变中子测量原理,介绍了一种采用中子灵敏度补偿原理建立的中子测量探测系统,即由辐射衰减器+载6Li塑料闪烁探测器组成的探测系统。采用数值模拟方法对该探测系统的参数进行了优化计算,给出了理论优化计算结果并进行了分析讨论,对这种方法和探测系统的适应性进行了分析,预测了这种方法的应用前景。     

Measurement of fast neutron induced fission cross section of thorium using Lexan plastic track detector

Fission-track registration characteristics of Lexan solid state nuclear track detectors have been used to measure the fast neutron induced fission cross section of²³²Th. The fast neutrons (?14.2MeV) were produced with the help of an AN-400 model Van-de-Graaff accelerator at Banaras Hindu University laboratory using³H(²H,n)⁴He reaction and were used to irradiate the fissile target deposited on the plastic detector. The track densityT, registered on the plastic detector is related to the fission cross sectionσ _f, through the relationT=knσ _føt wheren is the number of fissile atoms per cm² in the deposit, ø is the neutron flux,k is fission track registration efficiency andt is the time of irradiation. The fission cross sectionσ _f of²³²Th, relative to the well measured fission cross section of²³⁸U, was found to be 0.36±0.04 barn.     

~(252)Cf自发裂变中子发射率符合测量的回归分析

~(252)Cf同位素源具有标准的自发裂变中子能谱,但由于其半衰期较短,应用中常需要对源强进行标定修正.随着源年龄增加,来自源中~(250)Cf和~(248)Cm自发裂变的影响愈加凸显,不能简单按~(252)Cf的衰变规律计算源中子发射率,而通过锰浴活化的间接测量方法周期较长,且在源强低于10~4 n/s时误差较大.最近,基于中子多重性计数的源强绝对测量算法已得到验证.本文进一步从点模型假设的测量方程出发,在将符合计数率与总中子计数率关联的基础上,分别对符合计数率随源位置、符合门宽的变化关系进行回归分析,提取变化过程的特征系数,建立了两种避规效率变化的~(252)Cf中子发射率测量方法,并基于JCC-51型中子符合测量装置开展实验验证.结果表明:两种回归分析方法的测量值均与标称值的修正结果在2%的偏差范围内一致;反推求得装置中轴线上的探测效率也与基于MCNPX程序的蒙特卡罗模拟计算值相符.研究结果可为活度信息不明的~(252)Cf源强标定及符合测量装置的效率刻度提供便携准确的实验方法.     

Angular distributions of fragments in the fission of thorium induced by neutrons in the range 12–18 MeV

The angular distributions of fragments from neutron-induced fission of ²³²Th have been measured by means of glass detectors in the range 12.2 MeV ≦ E_n ≦ 18.3 MeV. The behaviour of the angular anisotropy is analysed and the contribution of the (n, 2n′f) process is discussed.     

The discoveries of uranium 237 and symmetric fission - from the archival papers of Nishina and Kimura

Shortly before the Second World War time, Nishina reported on a series of prominent nuclear physical and radiochemical studies in collaboration with Kimura. They artificially produced (231)Th, a member of the natural actinium series of nuclides, by bombarding thorium with fast neutrons. This resulted in the discovery of (237)U, a new isotope of uranium, by bombarding uranium with fast neutrons, and confirmed that (237)U disintegrates into element 93 with a mass number of 237. They also identified the isotopes of several middle-weighted elements produced by the symmetric fission of uranium. In this review article, the highlights of their work are briefly summarized along with some explanatory commentaries.     

Recoil studies of fission products formed in the fission of U by protons of energies 25–85 MeV

The recoil properties of ten fission products with masses ranging from 72 to 136 formed in the fission of ²³⁸U with protons of energies 25–85 MeV have been determined radiochemically by the integral-range method. From the recoil properties of the products and the Monte Carlo cascade calculations the average kinetic energy, cascade deposition energy, and anisotropy parameter for each fission product has been calculated. The kinetic energy and the excitation energy of the primary fragments leading to the observed fission product, and the total kinetic energy and the total excitation energy of the primary fragment pair have also been calculated.

The results indicate that up to a bombarding energy of 40 MeV fission takes place predominantly by the compound nucleus mechanism, with an increasing contribution of the direct interaction mechanism as the bombarding energy increases. The kinetic energy deficit was found to decrease with increasing bombarding energy. The fission products formed from the symmetric mode of fission have a larger separation distance between the charge centres of their respective primary fragments than those for the asymmetric mode of fission.     

An EDXRF method for determination of uranium and thorium in AHWR fuel after dissolution

A fast energy dispersive x‐ray fluorescence (EDXRF) method requiring only microgram amounts of analytes, i.e. uranium (U) and thorium (Th), in their mixtures in solution form is described. Calibration solutions and samples covering the fuel composition range (0–5% of U in U + Th) of advanced heavy water reactor (AHWR) were prepared by mixing uranium and thorium solutions. A known fixed amount of internal standard yttrium (Y) was added to these solutions. EDXRF spectra of calibration solutions and samples were measured by taking 20 µl aliquots on 30 mm diameter filter papers, after drying, using a Rh target tube operated at 40 kV and 500 µA. Calibration plots were made by plotting U/Y, U/Th and Th/Y amount ratios against the respective intensity ratios of Th Lα, U Lα and Y Kα. In the first set, U was determined using Y as an internal standard, and for Th determination, U, thus determined, was used as an internal standard since the amounts of Th and Y were kept constant in the calibration solutions and samples. In the second set, both U and Th were varied and determined using Y as internal standard. The results of U and Th determinations showed a precision of about 3% (1s) and the results deviated from the expected values by <3% in most of the cases. This approach has an advantage that it requires only microgram amounts of sample, thus mitigating radiation hazards associated with radioactive samples as well as the amount of radioactive analytical waste generated is quite less. Copyright © 2009 John Wiley & Sons, Ltd.     

Possible reference method of total excitation energy partition between complementary fission fragments

A method of total excitation energy (TXE) partition between fully-accelerated fission fragments based exclusively on the systematic behaviour of experimental ν(A) (νH/νpair as a function of AH being parameterized) is proposed. This TXE partition method has the advantage to be not dependent on models and assumptions made at scission and in this sense it can be taken as a possible reference method. From this TXE partition applied on many fissioning systems a general parameterization of the fragment residual temperature ratio RT=TL/TH as a function of AH is obtained, giving the possibility to predict prompt fission quantities as a function of fragment. Other TXE partition methods (based on models and assumptions at scission or on equal residual temperatures of complementary fragments at full acceleration) are compared to this one, revealing limits of applicability as well as the sensibility of prompt neutron emission models to the TXE partition. The behaviour of experimental ν(A) with the increase of incident energy consisting in the multiplicity increase for heavy fragments only, is argued and entirely supported by the quantitative Point-by-Point model results of ν(A) describing very well the experimental ν(A) of ²³⁷Np(n, f) at 0.8 and 5.5 MeV incident energies.     

Neutron-induced fission cross sections of short-lived actinides with the surrogate reaction method

Neutron-induced fission cross sections for ^242,243Cm and ²⁴¹Am have been obtained with the surrogate reaction method. Recent results for the neutron-induced cross section of ²⁴³Cm are questioned by the present data. For the first time, the ²⁴²Cm cross section has been determined up to the onset of second-chance fission. The good agreement at the lowest excitation energies between the present results and the existing neutron-induced data indicates that the distributions in spin and parity of states populated with both techniques are similar.     

Correlation between fractional independent yields and neutron-to-proton ratio of fission products in low energy fission

Fractional independent yields of fission products in the thermal neutron-induced fission of²³³U,²³⁵U,²³⁹Pu,²⁴¹Pu and in the spontaneous fission of²⁵²Cf have been correlated with the neutron-to-proton ratio of the fission products. The yields of the products from a fissioning system, when plotted as a function of neutron-to-proton (N/Z) ratio of fission preducts, fall on two Gaussian distribution corresponding to light and heavy fission products. The centroids of the distribution or the most probable value of neutron-to-proton ratio is found to be very close to theN/Z of the fissioning nucleus. From the most probable value ofN/Z the various parameters of charge distribution e.g. most probable massA _p, most probable chargeZ _p, the mass dispersionσ _Aand the charge dispersionσ _Zhave been obtained and are in good agreement with the experimental values ofA _pandZ _p.     

Systematics on fission fragment mass distribution of neutron induced 235U fission

Based on the neutron induced fission fragment mass distribution data up to