基于傅里叶级数展开的裂变位能曲面与碎片质量分布计算

裂变位能曲面是裂变核结构性质和裂变动力学研究的基础。本文使用傅里叶级数展开方法描述裂变过程中原子核的形状,采用基于Lublin Strasbourg Drop(LSD)宏观模型和Yukawa Folded微观模型计算了236U多维裂变位能曲面,研究了位能曲面随不同集体自由度(原子核拉长形变、左右碎片质量不对称度以及颈部宽度)的变化情况以及温度对位能曲面的影响。在位能曲面基础上采用基于Born Oppenheimer近似的三维集体模型描述原子核裂变过程,计算了236U裂变碎片质量分布,计算结果与实验数据符合较好,特别是质量分布的峰位,同时分析了核温度、零点能和颈部断裂概率半宽度对裂变碎片质量分布计算结果的影响。     

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%。     

中能质子入射的裂变产物质量分布计算

采用量子分子动力学和统计衰变模型以及半经验的多模裂变模型等方法,分别对由200 MeV左右质子入射锕系和非锕系重核引起裂变的裂变产物质量分布进行了计算,通过拟合190 MeV p ~(208)Pb和170 MeV p ~(238)U的裂变产物质量分布的实验数据,对锕系和非锕系重核分别给出了用于多模裂变模型的两组拟合参数。并采用这两组参数计算了190 MeV p ~(197)Au,184 MeV p ~(209)Bi和190 MeV p ~(232)Th的裂变产物质量分布,与实验值比较可以看出,采用这个方法能够得到符合实验数据的计算值,这两组参数也可以推广用于处于这一能区的重核裂变产物质量分布的描述。     

^238U、^239Pu裂变质量分布数据的评价

评价了~(238)U、~(239)Pu裂变质量分布数据,其中包括En=1.5,5.5,8.3,11.3,14.9,22.0,27.5,50.0,99.5,160 MeV中子诱发~(238)U裂变和Ep=20.0,60.0 MeV质子诱发~(238)U裂变,及En=0.17,7.9,14.5 MeV中子诱发~(239)Pu裂变。除从国际EXFOR库中检索实验数据外,还从最新出版物和私人通信中得到了一些重要数据。对搜集到的每家数据进行了分析、取舍和必要的修正及误差调整和处理。     

TALYS程序的开发和裂变碎片质量分布的理论计算研究

TALYS程序是近年发展起来的基于多通道随机颈断裂模型的可用于计算很宽能区裂变碎片质量分布的微观理论模型程序，是目前国际上仅有的一个可以用于计算很宽能区（1keV-200MeV）的裂变碎片质量分布的微观理论计算程序。     

5MeV中子诱发~(238)U裂变时几个核素累计产额的绝对测定

引言 对14MeV中子诱发~(238)U裂变的质量分布已做了很多研究,这是因为氘-氚聚变反应(D T→n α)提供了一个极好的14MeV的单能中子源。但对其它单能中子诱发~(238)U裂变的质量分布研究得就不够充分,已报道的结果中有些彼此分歧较大。我们用放化法测量了D(d,n)~3He反应产生的5MeV中子诱发~(238)U裂变时几个核素的绝对产额。     

22MeV中子诱发~(235)U裂变的质量分布

用γ射线能谱法测定了22MeV中子诱发^235U裂变产额。裂变率用双裂变室测定，得到了29个质量链的产额。     

热中子诱发239Pu裂变初级裂变产物质量分布测量

原子核裂变是最复杂的物理过程之一,至今仍缺乏可以统一描述裂变前和裂变后过程的理论。中子诱发239Pu裂变产额数据是重要的核数据,完整的初级裂变产物质量分布数据有助于完善裂变理论模型并提高产额评价数据的质量。本文研制了初级裂变产物鉴别谱仪（FFIS）,通过屏栅电离室和微通道板时间探测器分别测量裂变碎片的动能和飞行时间,基于动能 速度关联的方法直接获得碎片放中子后的质量分布,在BNCT医院中子照射器（IHNI 1）上开展了热中子诱发239Pu裂变初级裂变产物的质量分布测量。测量结果表明,对轻峰碎片质量分辨约为1 amu,对重峰碎片质量分辨约为15 amu。239Pu(nth,f)初级裂变产物质量分布的精确测量可为裂变产额理论计算和评价提供重要的实验数据。     

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。     

裂变碎片瞬发中子数随质量分布的研究

基于TALYS程序计算结果，我们研发了利用放中子前后裂变碎片的质量分布计算裂变碎片瞬发中子数随碎片质量分布的方法。裂变碎片瞬发中子数随裂变碎片质量分布的形状依赖于裂变产生的激发能如何在两个碎片中分配。这个问题一直以来没有得到很好的解决。     

234U裂变的五维势能曲面计算

本工作基于连接二次曲面的形状描述+LSD(Lublin Strasbourg drop)公式+折叠汤川势建立了宏观 微观模型。对234U裂变,计算了具有5 906 250个格点的五维势能曲面,然后通过搜索势能曲面得到了对称裂变和非对称裂变两个裂变通道,给出了裂变势垒高度以及鞍点和断点等特殊位置的原子核形状。     

Multi-parametric Measurement of Prompt Neutrons and Fission Fragments for 233U(n th,f)

The multiplicity and the energy of prompt neutrons from the fragments for ²³³U(n _th, f)were measured as functions of fragment mass and total kinetic energy. Average neutron energy against the fragment mass showed a nearly symmetric distribution about the half mass division with two valleys at 98 and 145 U. This shape formed a contrast with a saw-tooth distribution of the average neutron multiplicity. It indicates that the shell-effects, which are pronounced for the fragments having the proton number or neutron number close to the magic-number of 50 or 82, affected the neutron emission process. The slope of the neutron multiplicity with total kinetic energy depended on the fragment mass and showed the minimum at about 130 U. The obtained neutron data were applied to determine the total excitation energy of the system, and the resulting value in the typical asymmetric fission lied between 22 and 25 MeV. The excitation energy agreed with that determined by subtracting the total kinetic energy from the Q-value within 1MeV, thus satisfied the energy conservation. In the symmetric fission, where the mass yield was drastically suppressed, the total excitation energy is significantly large and reaches to about 40MeV: suggesting that fragment pairs are preferentially formed in a compact configuration at the scission point.     

Systematic Analysis of Fission Cross Sections of Actinides by means of Double-Humped Barrier Model

Neutron-induced fission cross sections of 24 actinide nuclei were analyzed in terms of the double-humped fission barrier model to deduce the barrier heights. Good fits were obtained by assuming that the first barrier is mass-symmetric and axially asymmetric, while the second barrier is mass-asymmetric and axially symmetric. Systematic trends were observed in the barrier heights of the actinide nuclei; the first barrier height as a function of neutron number tends to be peaked at V?147, whereas the second barrier height increases linearly as a function of (1 -x)³A^2/3, where x is the fissility. By decomposing the barrier heights into liquid-drop and shell correction parts, the surface energy coefficient was deduced to be 17.55 MeV. This value is consistent with existing values obtained from nuclear mass systematics. This fact corroborates the theoretical conjecture that the shell correction is damped at larger deformation corresponding to the second barrier. Near constancy of the fission barrier heights for actinides (fission barrier anomaly) was interpreted in terms of the threecomponent analysis.     

Prompt fission neutron spectrum evaluation for 252Cf(SF) in the frame of the multi-modal fission model

In the present work, an attempt to improve the evaluation of the prompt fission neutron spectrum of ²⁵²Cf(SF) is made. The multi-modal fission concept is included into the Los Alamos model. A more generalized form of the fission fragment residual nuclear temperature distribution and a possible anisotropy effect of the prompt neutron emission in the center-of-mass system are taken into account, too. The multi-modal fission parameters entering the prompt fission neutron spectrum model are determined on the basis of the experimental data concerning the fission fragment total kinetic energy TKE(A) and mass distribution Y(A) measured at IRMM. The calculated prompt neutron spectrum is obtained in better agreement with the standard point-wise evaluation of Mannhart and compared to other evaluations made with different models.     

Estimation of Neutron Yield from Individual Fragment in Medium-Excitation Fission

A method of calculation is described to estimate the average number of neutrons emitted per fragment in medium-excitation fission from published experimental data on neutron emission in thermal-neutron induced fission, average total kinetic energy as a function of fragment mass and mass yield in low- and medium-excitation fission reactions. Use is made of a relation of fragment excitation energy with internal excitation and deformation energies, and the difference in kinetic energy between the fission reactions at two-excitation energies. A tentative calculation is made for the fission of ²³⁸U induced by 12 MeV protons. The results are in good agreement with experimental data.

The method developed in the present work may make it possible to predict the average number of neutrons emitted from individual fragment in medium-excitation fission which has not yet been measured experimentally.     

Investigating nuclear dissipation properties at large deformations via excitation energy at scission

Using the stochastic Langevin model coupled with a statistical decay model, we study nuclear dissipation properties at large deformations with excitation energy at scission (E*sc) measured in experiments. It is found that the postsaddle dissipation strength required to fit E*sc data is 12 ×1021 s-1 for 254;256Fm and 6 ×1021 s-1 for 189Au, which has a smaller postsaddle deformation than the former heavy nucleus, showing a rise of nuclear dissipation strength with increasing deformation.     

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.