从四维Langevin方程的Monte Carlo模拟计算裂变碎片动能分布

本文将一个关于对称裂变的两个集体坐标(碎片质心距和颈部参数)及其共轭动量的四维Langevin方程作为描述布朗粒子从鞍点下降到断点这一扩散过程的动力学方程.通过计算机模拟随机力冲量,用Monte Carlo方法求解了Langevin方程组.计算了322/A<40区间的八个核裂变碎片的平均动能、动能涨落宽度,结果与实验值相符.     

中子诱发232Th裂变初始碎片质量及动能分布Monte-Carlo研究

随着第四代反应堆以及先进核能利用系统的发展,对中子核数据提出了高精度、多核素、宽能区的新要求.目前,中国核数据评价库(CENDL库)中相关核裂变的数据较缺失,不足以满足当前核能发展的需求.因此,建立面向中子核数据需求的可靠计算方法和工具变得极为重要.本文基于Monte-Carlo方法建立了裂变碎片质量动能计算模型,研究了中低能中子诱发²³²Th(n,f)反应发射中子前裂变碎片的分布特性.对于裂变碎片质量分布,本模型计算结果与实验值最大偏差约1%,与GEF, TALYS程序计算结果 (与实验值最大偏差约2%)相比具有一定优势.对于发射中子前裂变碎片动能分布,本模型计算结果与实验数据一致.结果表明,所发展的计算模型能够较好地预测²³²Th(n,f)反应发射中子前裂变碎片数据,为中子诱发锕系核裂变反应计算提供一种新思路.     

Isoscaling of the Fission Fragments with Langevin Equation

The Langevin equation is used to simulate the fission process of ^112Sn ^112Sn and ^116Sn ^116Sn. The mass distribution of the fission fragments are given by assuming the process of symmetric fission or asymmetric fission with the Gaussian probability sampling. The isoscaling behaviour has been observed from the analysis of fission fragments of both the reactions, and the isoscaling parameter α seems to be sensitive to the width of fission probability and the beam energy.     

耦合系统的朗之万动力学产生法

提出一种朗之万动力学方法获取处于热平衡态耦合系统内部振子坐标,数值模拟了单端固定简谐振子链的时间演化行为,并将其平衡性质与解析解进行了比较.结果表明了朗之万动力学方法的有效性.推广应用于非简谐四次方型耦合系统,模拟得到振子的四次方均坐标,与理论值验证;以模拟结果作为样本点计算哈密顿量,其能量分布与Boltzmann分布相符.     

中能质子引起散裂反应的碎片质量分布研究

对 SHIELD程序进行了部分检验,利用SHIELD程序计算质子入射铅的薄靶产生的靶碎片的截面及质量分布和激发函数 ,计算结果较好地再现了实验数据 .并计算了 1 .6Ge V的质子轰击长 60 cm和直径20cm的铅靶的碎片质量分布. The test of part benchmark of SHIELD code was performed. The cross section, mass distribution and excitation function of the fragments (including residual nuclei) in the proton induced spallation reaction on thin Pb target at intermediate energy have been calculated by SHIELD code. And the results are in good agreement with experimental data. The fragment mass distributions from proton induced spallation reaction on thick Pb traget at incident energy 1.6 GeV were also presented.     

中能质子引起散裂反应的碎片质量分布研究

对 SHIELD程序进行了部分检验,利用SHIELD程序计算质子入射铅的薄靶产生的靶碎片的截面及质量分布和激发函数 ,计算结果较好地再现了实验数据 .并计算了 1 .6Ge V的质子轰击长 60 cm和直径20cm的铅靶的碎片质量分布. The test of part benchmark of SHIELD code was performed. The cross section, mass distribution and excitation function of the fragments (including residual nuclei) in the proton induced spallation reaction on thin Pb target at intermediate energy have been calculated by SHIELD code. And the results are in good agreement with experimental data. The fragment mass distributions from proton induced spallation reaction on thick Pb traget at incident energy 1.6 GeV were also presented.     

中高能质子入射碎片质量分布研究

中高能质子轰击靶的碎片的质量分布是了解散裂中子靶的放射性累积和对靶材料性能的影响的一个重要方面.利用在级联-激子模型(CEM)的基础上发展起来的“多步动力学”模型(MSDM,Many Stage Dynamical Model)分别研究了入射质子能量为334,572,775,1200,1500和2600MeV轰击Nb靶,329,555,760,1200,1500和2600MeV轰击¹⁹⁷Au靶,160,230,271,322,533,660,759,1400,1600和2600MeV轰击Pb靶引起的碎片的质量分布,并和实验测量数据进行了比较.     

近垒及垒下能区28Si+174Yb体系的裂变碎片质量分布

使用两块大面积平行板雪崩计数器,测量了近垒及垒下28Si+174Yb熔合裂变碎片质量-角度关联分布及质量分布。质量-角度二维图显示在该反应能量区间内并没有发现明显的准裂变成分。运用单高斯曲线拟合了裂变碎片的质量分布并抽取了质量宽度。结果表明:在库仑势垒以上,质量宽度随着反应能量的升高而增大;在库仑势垒以下能区,发现质量宽度随着反应能量的降低而呈现轻微增大的趋势。运用鞍点统计模型对本次实验数据拟合发现,需用比文献给出的更大的刚度系数才能重现实验结果,其物理原因还需要进一步研究。In order to study the fission mechanism, the mass distributions and mass-angle distributions of the fission fragments for 28Si+174Yb were measured near the Coulomb barrier. The mass-angle correlations do not indicate any possible quasi-fission events in this bombarding energy range. The mass distributions were fitted by the single Gaussian distribution and then the mass widths were extracted. At above-barrier energies, the mass widths were found to increase with the energy. However, the mass widths show slight increase with decreasing energy below the barrier. Additionally, the mass widths can not be reproduced in the framework of the saddle statistic model with the given parameter, so further study is still needed.     

由飞行时间质谱峰形获取光解碎片平动能分布

由于在魔角条件下测量的飞行时间（TOF）质谱峰形与光解碎片的角分布无关,因而便于用含待定参数的平动能分布函数去拟合质谱峰形来获取光解碎片的平动能分布.与通常的拟合法不同,提出一种峰形参量法,只需通过测量TOF峰形的半高宽(t^1/2)、四 分之一高宽(t^1/4)和四分之三高宽(t^3/4),便可借助文中提供 的曲线图或 解析式得出碎片平均平动能,平动能分布特征宽度以及描述实验装置本身的特征峰宽等.处 理过程中,平动能分布及实验装置峰形均采用相应的高斯分布函数描述.关键词：魔角TOF质谱平动能分布     

n+233U裂变碎片质量分布的唯象模型研究

本模型基于裂变多通道无规颈断裂模型,考虑宏观液滴能、壳效应能以及壳效应与温度的关系,得到参数化的势能表示形式。通过拟合不同测量方法得到的实验数据(经过评价) 获得3 组模型参数。3 组参数计算的碎片质量分布均很好地再现了不同能点的实验数据,除了基于动能法实验数据得到的参数外,其14 MeV的计算结果与实验数据符合较差。研究发现,不同入射中子能量的裂变碎片质量分布有4 个主要交叉点,在叉点之上的产额随入射中子能量增加减少,之下的产额则随入射中子能量上升;内侧(或外侧) 的两个交叉点质量数之和近似等于裂变复合核的质量数;不同裂变系统的重峰左侧的交叉点都保持在132 附近。Present model was based upon Brosa model, where macro energy,shell effect energy and its decreasing with system temperature were considered and parameterized. 3 sets of model parameters were obtained by fitting to 3 groups of evaluated experimental data, respectively, where the groups were classified by measured method: radiochemistry (RA) method, double kinetic energy (KE) method, and both (ALL). All the 3 sets of parameters could well reproduce themeasured mass distributions on different energies, except the KE set on 14 MeV. The result shows the mass distributions mainly had 4 trend turning points, above which the yield decreasing and below which the yield increasing with energy.The sum of the 1st and 4th (or 2nd and 3rd) point positions was equal to the compound nuclei mass approximately. And the 3rd point was kept at ~132 constantly for different fission systems n+233 U, n+235U, n+233Pu.     

Temperature dependent fission fragment distribution in the Langevin equation

The temperature dependent width of the fission fragment distributions was simulated in the Langevin equation by taking two-parameter exponential form of the fission fragment mass variance at scission point for each fission event. The result can reproduce experimental data well, and it permits to make reliable estimate for unmeasured product yields near symmetry fission.     

25MeV/u 40)Ar+209Bi裂变反应研究

实验对25MeV/u ⁴⁰Ar+²⁰⁹Bi体系的裂变反应,利用线性动量转移的分窗选择不同的激发能,研究裂变动能分布和质量分布与热核初始激发能的关系.实验证实激发能小于380MeV时裂变总动能分布与低激发能复合核相似.激发能大于380MeV时,最可几动能呈现出随激发增加而增加,并出现高能非对称性,而且质量分布宽度随激发能增加而迅速增大.     

Thin-film breakdown counters and their applications (review)

Experimental techniques based on the use of thin-film breakdown counters fabricated at Khlopin Radium Institute are reviewed.     

Recent radiochemical studies of the fission process

This article summarizes the recent radiochemical investigations on mass, charge kinetic energy and fragment angular distributions in low energy fission of actinides.     

核裂变——未尽的探索

核裂变的发现深刻地影响了人类社会。核裂变的研究还在不断深入,一方面核裂变有新的应用需求,另一方面核裂变是一个复杂的量子多体动力学过程。近年来,核裂变理论和实验研究有很大进展,人们对核裂变几率、裂变产物和裂变机制都获得了新的认识,这有助于澄清一些唯象模型的经验假设。此外,机器学习的应用为发掘利用不精确不完整的核数据提供了可能。期待未来更精确更自洽的核裂变理论可以更好地支撑应用创新。     

Radiochemical studies on fission of actinides

Since its discovery in 1939, nuclear fission has been extensively studied by various experimental as well as theoretical groups in several countries leading to an understanding of major aspects of this important and complex nuclear reaction. In Trombay, studies have been carried out in the last 25 years using both physical and radiochemical methods and significant contributions have been made towards a better understanding of this reaction. This paper presents highlights of radiochemical studies on fission of actinides, particularly mass, kinetic energy and charge distribution and fragment angular momentum. Results of these studies brought out the important role played by deformation energy surface, spherical and deformed nuclear shells and nucleon pairing.     

平均最后通过时间和热核裂变速率

提出利用平均最后通过鞍点时间计算热核裂变速率,结果表明平均最后通过鞍点时间的倒数比平均首次通过断点时间的倒数更接近朗之万数值模拟值. The mean last passage time is introduced to instead of the mean first passage time for studying the decay of an induced-fissioning system. The stationary fission rate determined by the inverse of mean last passage time across the saddle point is agreement with the resulting rate of Langevin simulation and better than that of mean first passage time arriving at the scission point.