14.7 MeV中子引起~(238)U裂变的电荷分布

在14.7兆电子伏中子引起~(238)U裂变中,用放射化学方法测定了~(82)Br、~(132)I、~(136)Cs、~(140)La4个独立产额以及~(95)Zr、~(97)Zr、~(99)Mo、~(131)I、~(133)I、~(134)I、~(135)I、~(138)Cs、~(142)La等9个累积产额。实验测定的独立产额和其他文献报道的~(96)Nb、~(139)Cs、~(139)Ba一共7个独立产额用高斯电荷分布曲线进行了关联,计算中采用了各种质量公式、电荷分布假设和中子发射函数。结果指出,使用不考虑壳效应的Grenn质量公式的最小位能假设以及考虑壳效应的Coryell质量公式的等电荷位移假设给出几乎同样的最佳拟合,得到的电荷分布曲线以及实验的最可几电荷Z_p对恒电荷密度的迁移ΔZ都和~(235)U的热中子裂变实验结果相一致。     

基于Potential-driving模型的中子诱发~(239)Pu裂变产物计算与模拟研究

当前新型核能利用系统及核数据评价的发展对快中子诱发~(239)Pu裂变核数据提出了更高的精度需求。本工作基于已提出并构建的Potential-driving模型,通过中子诱发~(239)Pu(n,f)裂变驱动势研究,计算了几个典型能量中子诱发~(239)Pu(n,f)反应发射中子前裂变碎片质量分布,并与实验数据进行了对比。结果显示:Potential-driving模型计算数据能够很好地与实验数据符合。将Potential-driving模型植入GEANT4程序,开展了快中子诱发~(239)Pu(n,f)反应相关的模拟研究,给出了14 Me V中子诱发~(239)Pu(n,f)反应的裂变碎片独立产额质量分布和电荷分布、累积产额质量分布和电荷分布、动能分布、裂变中子能谱以及~(239)Pu(n,f)反应裂变碎片平均总动能随入射中子能量的变化等数据,并与GEANT4程序原有的参数化裂变模型(G4Para Fission Model)模拟结果、ENDF/B-VII.1库评价数据以及实验数据进行了比较。结果显示:所发展的Potential-driving模型能很好地预测快中子诱发~(239)Pu(n,f)反应裂变产物数据,为快中子诱发~(239)Pu(n,f)反应裂变产物核数据的研究提供了一种更可靠的计算方法。     

中能^12C离子同厚铋靶反应近靶产物的产额测定

用放射化学分离及γ谱学技术测量了入射能量为４７ＭｅＶ／ｕ＾１２Ｃ离子轰击厚靶反应产物的平均产额，获得了近１００个核素的平均生成截面。基于这些数据并借助于电荷分布母子体关系的修正，从许多累积及部分累积产额获得了独立产额同时得到了比靶核少３、４、５个电荷数的产物的同位素分布曲线，由此得到了中能较轻重离子引起的溶部非弹转移反应产生丰中子同位素的有效产额。     

~(235,238)U和~(239)Pu中子诱发裂变独立产额的系统学研究和数据评价

裂变产额在核工程中有重要的应用,例如反应堆工程中衰变热、裂变毒物的计算。独立产额是裂变产额数据的重要组成部分。本工作评价给出235,238U、239Pu的热能点、裂变谱中子和14MeV中子裂变的所有质量链(A=66～172)的独立产额。100多个质量链1000多种产物核的独立产额是不可能完     

裂变产额的Zp模型评价方法及燃耗不确定度分析

裂变产额在核科学技术和核工程中有着重要的应用,发展可靠、高效的产额评价方法和相应燃耗计算不确定度分析方法,对于建立高质量的产额数据库具有重要的意义。本文根据裂变产物核衰变模式和衰变分支比,建立独立产额与累积产额的转换矩阵,用于Zp模型的扩展,使之适用于独立产额和累积产额的统一描述,并以此建立了用于产额统一评价的拟合程序ZpFit。把ZpFit程序应用于中子诱发235U裂变产物产额评价,获得了自洽的独立产额、累积产额和相应的协方差数据,并建立ENDF格式的中子诱发235U裂变的产额数据库。在此基础上,计算了UAM燃耗基准题的TMI 1栅元的kinf、重要核素原子核密度的不确定度,比对了本工作评价的产额数据和ENDF/B Ⅷ0评价库中产额数据传递给响应量的相对不确定度,结果基本一致,差异不大。     

53 ^235，238U和^239Pu中子诱发裂变独立产额的系统学研究和数据评价

裂变产额在核工程中有重要的应用，例如反应堆工程中衰变热、裂变毒物的计算。独立产额是裂变产额数据的重要组成部分。本工作评价给出^235，238U和^239Pu的热能点、裂变谱中子和14MeV中子裂变的所有质量链（A=66～172）的独立产额。     

~(252)Cf自发裂变电荷分布研究——~(133m)Te和~(133g)Te分累计产额测定

用放化法测定了~(252)Cf自发裂变中~(133m)Te和~(133g)Te的分累计产额,其值分别为FCY(~(133m)Te)=0.533±0.014,FCY(~(133g)Te)=0.291±0.042。计算得到~(133m)Te与~(133g)Te分独立产额比R=3.5,由此导出~(133)Te初始碎块均方根角动量为8.8h。FCY(~(133m g)Te)值与A.C.Wahl的电荷分布系统学预报值在误差范围内一致。N=82中子壳和质子对效应对A=131～141质量区电荷分布没呈现出明显影响。     

~(252)Cf自发裂变电荷分布研究——~(133m)Te和~(133s)Te分累计产额测定

用放化法测定了~(252)CF自发裂变中~(133m)Te和~(133g)Te的分累计产额,其值分别为FCY(~(133m)Te)=0.533±0.014,FCY(~(133g)Te)=0.291±0.042。计算得到~(133m)Te与~(133g)Te分独立产额比R=3.5,由此导出~(133)Te初始碎块均方根角动量为8.8h。FCY(~(133m g)Te)值与A.C.Wahl的电荷分布系统学预报值在误差范围内一致。N=82中子壳和质子对效应对A=131～141质量区电荷分布没呈现出明显影响。     

~(144)La快速放化分离的方法研究

正~(144)La是高产额的短寿命裂变产物,半衰期为40.8(4)s,是重核裂变的重要裂变监测体。但它的相关衰变数据,如半衰期等,不确定度较大,并缺少独立产额实验数据,不能满足裂变监测和模拟计算对核参数精度的要求。这就需要对~(144)La开展高精度的核数据测量。然而,~(144)La的半衰期太短,常规的化学分离方法难以达到目的。因此,如     

E_p≤300MeV质子-铅核核反应微观截面的理论计算

对５—３００ ＭｅＶ 能区质子与天然铅的各种反应截面进行了计算，在符合多种实验数据的基础上，预言了长寿命放射性核的累积产生截面及其随质子入射能量的变化和中子多重数。在分析实验数据的基础上，给出了中能质子引发天然铅核裂变质量分布的经验公式，计算了电荷分布，并将计算结果与实验数据进行了比较。     

能量到GeV的质子入射铅靶的散裂产物分布研究(英文)

由中高能质子入射靶引起的散裂碎片的分布关系到散裂靶经长期辐照后的放射性废物的累积。在量子分子动力学模型(QMD)中考虑剩余核的裂变过程(FISSION),利用QMD FISSION模型研究了322,660,759 MeV的质子入射铅靶的散裂产物的分布。计算结果很好地再现了实验测量值。     

Nuclear fission in heavy elements by high-energy particles

The results of a radiochemical investigation of nuclear fission in uranium, thorium, and bismuth by protons with an energy of 680 Mev are presented. Using an interpolation method a complete chart of the fission residue products is obtained. It is noted that there is a predominance in the production of nuclei with excess neutrons (58–64%); it is also shown that isotopes with maximum yield lie mainly in the neutron-ex cess region. The probability for symmetric fission is largest in bismuth. The cross sections for fission in uranium and thorium are 55–60% of the geometric cross section; in bismuth it is 5%. The charge distribution of fragments in fission induced by high energy protons is constant and independent of the mass number of the fission fragments and the atomic number of the fissioning nucleus. An analysis of the main features of the fission process seems to indicate that fission in uranium and thorium is due to a combined barrier-emission mechanism.     

基于协变密度泛函理论的258Fm诱发裂变动力学研究

采用基于协变密度泛函理论的含时生成坐标方法研究了258Fm低能诱发裂变动力学性质,重点探讨了裂变位能曲面、裂变碎片总动能分布和碎片质量分布等。研究表明,258Fm位能曲面中存在显著的对称裂变谷,因而其低能裂变碎片总动能分布与质量分布均呈单峰结构,且随剪裂线判据Qn（脖子处粒子数）从4减至1,碎片总动能分布变窄,碎片质量分布的峰值从988%增至1028%。此外,随初态激发能从83 MeV增至173 MeV,碎片质量分布峰值从988%降至855%。     

Improvements in the description of the multiplicities of the prompt neutrons emitted in fission

Various parameterizations of the multiplicity distribution of prompt neutrons from nuclear fission are investigated. The currently accepted formalism is found to fail for those fission processes which release on average a low number of neutrons. Based on realistic assumptions regarding binary fission, an improved multiplicity distribution formula gives much better agreement with the experimental data. Explicit details of the practical usage of the new formalism are presented. Finally, it is shown that for spontaneously fissioning isotopes, it is possible to calculate the variance of the neutron multiplicity distribution from a knowledge of the mass yield and the variances of the neutron multiplicity distributions of the individual fission fragments.     

宏观-微观模型和朗之万方法在低能核裂变研究中的应用

基于三维朗之万模型对低能核裂变动力学过程和断点构型进行了研究,其中位能曲面采用基于双中心壳模型和有限程液滴模型的宏观 微观模型计算得到,质量张量和黏滞张量分别采用Werner Wheeler方法和墙加窗一体模型得到。以14 MeV中子诱发235U裂变为例,分别研究了拉长形变空间和壳衰减因子对裂变碎片质量分布、总动能分布及断点处核拉长与质量非对称度关联的影响,确定了模型计算中拉长形变空间边界至少应为35R0（R0为球形核半径）,以及壳衰减因子的合理取值为60 MeV。基于该模型,计算得到了14 MeV中子诱发233,235U裂变碎片质量分布,与ENDF/B Ⅷ0评价数据符合较好,说明该模型具有定量计算裂变碎片质量分布的能力。     

Product yields for the photo-fission of Bi with 2.5 GeV bremsstrahlung

The mass-yield distribution of fission products in the 2.5 GeV bremsstrahlung-induced fission of ²⁰⁹Bi have been determined by using the recoil catcher and the off-line γ-spectrometry technique in the high energy electron linac at the Pohang Accelerator Laboratory. The mass-yield determination involves the measurements of cumulative yields for 32 fission products and independent yields of 17 fission products in the photo-fission of ²⁰⁹Bi nuclei. It was found that the mass-yield distribution of fission products in ²⁰⁹Bi is symmetric with an average mass of 95 ± 0.5 and a FWHM of 51 ± 2.0 mass units. Present data at 2.5 GeV along with the literature data at 1 GeV, 700-600 MeV, and 85-28 MeV were interpreted from the point of increase of multi-chance fission and multi-nucleon emission probabilities with an increase in excitation energy. It was found that the average mass of the mass-yield distribution of the fission products decreases from 103 ± 0.5 at 28-85 MeV to 95 ± 0.5 at 2.5 GeV. On the other hand, the FWHM of the mass-yield distribution increases from 19 mass units at 28-40 MeV to 51 mass units at 2.5 GeV. It was also found that the nuclear structure effect observed at the photo-fission of ²⁰⁹Bi with 28-85 MeV bremsstrahlung is washed out at higher energy.     

Evaluation of prompt fission gamma rays for use in simulating nuclear safeguard measurements

Nondestructive assay methods that rely on measurement of correlated gamma rays from fission have been proposed as a means to determine the mass of fissile materials. Sensitivity studies for such measurements will require knowledge of the multiplicity of prompt gamma rays from fission; however, a very limited number of multiplicity distributions have been measured. A method is proposed to estimate the average number of gamma rays from any fission process by using the correlation of neutron and gamma emission in fission. Using this method, models for the total prompt gamma ray energy from fission adequately reproduce the measured value for thermal neutron induced fission of ²³³U. Likewise, the average energy of prompt gamma rays from fission has been adequately estimated using a simple linear model. Additionally, a method to estimate the multiplicity distribution of prompt gamma rays from fission is proposed based on a measured distribution for ²⁵²Cf. These methods are only approximate at best and should only be used for sensitivity studies. Measurements of the multiplicity distribution of prompt gamma rays from fission should be performed to determine the adequacy of the models proposed in this article.     

Role of Effective Distance in the Fission Mechanism Study by the Double-energy Measurement for Uranium Isotopes

Fission product kinetic energies were measured by the double-energy method for thermal-neutron fission of ^235,233U and proton-induced fission of ²³⁸U at the 15.8-MeV excitation. From the obtained energy-mass correlation data, the kinetic-energy distribution was constructed from each mass bin to evaluate the first moment of the kinetic energy for a given fragment mass. The resulting kinetic energy was then converted to the effective distance between the charge centers at the moment of scission. The effective distances deduced for the proton-induced fission was concluded to be classified into two constant values, one for asymmetric and the other for symmetric mode, irrespective of the mass though an additional component was further extracted in the asymmetric mass region. This indicates that the fission takes place via two well-defined saddles, followed by the random neck rupture. On the contrary, the effective distances obtained for thermal-neutron induced fission turned out to lie along the contour line at the same level as the equilibrium deformation in the two-dimensional potential map. This strongly suggests that it is essentially a barrier-penetrating type of fission rather than the over-barrier fission.     

The asymmetry of nuclear fission

One of the essential characteristics of nuclear fission and of processes occuring in nuclear reactors is the distribution of fission fragments by mass. According to the drop model of the nucleus, a symmetrical mass distribution of the fragments is most probable, but this, however, is contrary to experiment.To find the mass distribution of the fragments, in this work we calculated the energy of the fissionable nucleus before rapture of the neck, taking into account shell effects, for several nuclei. It was shown that the energy minimum corresponds to asymmetric fission.