8Be核自发裂变的微观研究

应用双阱集团壳模型,提出⁸Be 自发裂交静力学位垒计算中扣除碎片相对动能的一种方式,计算了⁸Be 自发裂变位能曲面,并对库仑位及质心运动伪态的修正作了估算.定性地解释了⁸Be 的亚稳机制,并提供了从微观方式描述变裂的一种理论方式。     

氢硼聚变三体级联衰变连续能谱的Monte-Carlo模拟与分析

¹¹B(p,α1)⁸Be^*(1)(2α)三体级联衰变中居间核8Be⁽¹⁾衰变成两个α粒子的连续能谱呈马鞍形分布.编写了模拟三体级联衰变连续能谱的Monte-Carlo程序,计算结果表明:沿居间核8Be⁽¹⁾运动方向α发射强度增强的各向异性分布,能较好地解释实验结果,揭示p+¹¹B核反应中⁸Be⁽¹⁾核α衰变各向异性发射.     

垒下熔合裂变中观察到的异常

测量了¹⁶O+²³²Th、²³⁸U和¹⁹F+²³²Th近垒和垒下熔合裂变截面以及碎片角分布.包含靶核静态形变的耦合道理论解释了垒下熔合截面增强,预言了复合核系统自旋分布展宽.而实验上观察到的碎片角分布各向异性明显与裂变统计理论的预言不一致.     

12C+209Bi和14N+Pb裂变反应碎片角分布和激发函数测量

本工作采用固体核径迹探测器(天然白云母)和金硅面垒型半导体探测器测量了¹²C+²⁰⁹Bi和¹⁴N+Pb裂变反应的碎片角分布和激发函数.由碎片角分布各向异性提取的鞍点有效转动惯量J₀/J_eff与转动有限力程模型(RFRM)做了比较.用统计蒸发模型计算的裂变激发函数拟合实验数据,研究了裂变位垒的角动量效应.     

裂变位垒的角动量效应测量

本工作采用金硅面垒型半导体探测器和天然白云母固体核径迹探测器测量了¹²C+²⁰⁹Bi和¹⁴N+Pb反应的裂变截面σ_f和蒸发余核截面σ_er.由蒸发余核截面提取角动量l_er,基于统计模型导出的裂变宽度与蒸发中子宽度比(Γ_f/Γ_n)_l的表达式计算给定角动量l_er的裂变位垒E_f(l),从实验上研究裂变位垒的角动量效应.     

推广的液滴模型对超重核288115及其α衰变链性质的研究

运用推广的液滴模型来确定新核素²⁸⁸115及其α衰变链上核的衰变位垒, 采用量子力学中处理α衰变的WKB方法, 对该链上各原子核的α衰变半寿命进行了研究．计算结果表明推广的液滴模型结合WKB方法可以很好地在超重区符合α衰变半寿命的实验值. 同时把推广的液滴模型的计算结果和采用密度相关的M3Y微观核力的结果做了详细的比较, 宏观模型(推广的液滴模型)和微观模型(密度相关的M3Y微观核力)计算的α衰变半寿命以及实验值三者之间的符合是对新核素²⁸⁸115及其α衰变链上核半寿命很好的检验.     

用多中心变分模型研究Be8和Li6的集团结构

为了研究原子核的集团情况,我们建立了多中心变分的轻核模型。本文计算了Be⁸和Li⁶的结合能,分析了这两个核的集团结构,得到和实验所揭示出的现象相一致的结果。     

原子核高自旋态的半径典研究

用推广的汤马斯-费米近似和Skyrme势研究了原子核的高自旋态,应用推转模型给出了含转动的能量密度公式,并计算了¹¹⁶Sn,¹⁵⁸Yb和²⁴⁰Pu这三个典型核在不同角动量下的核形变与转动能.计算结果定性地与液滴模型相似,在数值上则有一定差别.计算结果表明,对转动能的量子修正,即使不考虑壳修正,也不能忽略.     

中子分布弥散度对反应总截面的影响

在引入库仑场修正的Glauber模型基础上,区分入射弹核和靶核中的中子、质子,并考虑有限力程相互作用,发展了一个计算核反应总截面的微观修正模型.如果引入一个核内中子分布弥散度随中子分离能的变化关系,则能较好地解释奇异核(如⁸He,¹¹Li,¹¹Be)反应总截面的反常增加.同时,该修正模型还能在低能到高能的范围内较好地拟会稳定核的反应总截面.     

超重核266Hs形成截面的入射道依赖

计算和比较了²⁶Mg+²⁴⁴Cm, ²⁷Al+²⁴³Am和³²S+²³⁸U3个反应系统的俘获截面和复合核²⁷⁰Hs形成截面. 在俘获截面计算中, 考虑了靶核形变效应. 穿越库仑势垒后, 反应系统由熔合谷进入不对称裂变谷. 只有越过不对称裂变谷中的条件鞍点的事件才进入复合核组态. 我们用考虑中子流动和径向运动的二参量Smoluchowski扩散方程来处理中间阶段的动力学过程. 此外, 还计算了经4n蒸发形成超重核²⁶⁶Hs的截面. 研究表明, 入射道的势垒分布, 中间阶段的条件鞍点高度对俘获截面, 复合核形成几率, 以及最终的超重核形成截面有显著影响.     

MICROSCOPIC STUDY FOR SPONTANEOUS FISSION OF 8Be

On the basis of the cluster shell model of double well,a new method for subs-tracting the kinetic energy of the relative motion of the fission fragments has beenproposed,and in this way the potential energy curve for spontaneous fission of ⁸Be-has been calculated.In the calculation,the effects of the coulomb potential and thespurious states of the motion of the center of mass have been considered properly.The results can qualitatively explain the metastable property of ⁸Be.It seems that the method proposed in this paper might serve as a way to describe the fisson processin microscopic study.     

裂变碎块角分布与鞍点结构

从Jacob和Wick所推导的具有自旋粒子的散射矩阵元一般表示式出发,采用适当近似后,分析了中子引起Th²³⁰,Th²³²,U²³⁴,U²³⁶,U²³⁸裂变的阈能附近碎块角分布的实验数据。分析结果说明:1)阈能附近碎块角分布可以用裂变是通过鞍点上少数转动带(每一带具有确定的宇称)来解释;2)这些转动带与Nilsson的渐近能谱图所预言的基本上一致。同时利用对称裂变鞍点比非对称裂变鞍点高的观点,讨论了裂变碎块角分布各向异性与质量比之间的关系。     

Spurious Center of Mass Effects in Electroweak Decays

The microscopic treatment of the spurious motion of the center of mass in nuclear many-body systems is reviewed. In this talk we present and compare the results of using the counterterms which are needed to fulfill the translational and Galilean invariances, and those obtained by using collective variables. The formalism is applied to the calculation of matrix elements of electroweak operators, as those which are active in the (^-, e^-) conversion process.     

Mass distribution in the bremsstrahlung-induced fission of Th,U and Pu

The yields of various fission products in the 10 MeV bremsstrahlung-induced fission of ²³²Th, ²³⁸U and ²⁴⁰Pu were determined using a recoil catcher and off-line γ-ray spectrometric techniques. From the yield data, mass yield distributions were obtained using charge distribution corrections. The higher yields of fission products around mass numbers 133–135, 138–140, 143–145 and their complementary products in the neutron and bremsstrahlung-induced fission of ²³²Th, ²³⁸U and ²⁴⁰Pu were interpreted based on nuclear structure effects. From the mass yield distribution, the peak-to-valley (P/V$P/V$) ratio was also obtained for the above fissioning systems. The present data, along with data from the literature on different bremsstrahlung- and mono-energetic neutron-induced fissions of ²³²Th and ²³⁸U are interpreted to examine the influence of excitation energy on the peak to valley ratio. For the same compound nucleus ²⁴⁰Pu^?, the data in the 10–30 MeV bremsstrahlung-induced fission of ²⁴⁰Pu were compared with similar data of thermal to 14 MeV neutron-induced fission of ²³⁹Pu and the spontaneous fission of ²⁴⁰Pu to examine the role of excitation energy due to bremsstrahlung radiation and mono-energetic neutrons.     

超核激发态的对称性分类

我们把维格纳处理超级多重态的方法,推广用来研究超核激发态的能谱。给出了近似不破坏SU₆=SU₃?SU₂分类的两体相互作用普遍形式,并利用现有的散射实验数据定出了其中的系数。导出了在SU₆分类下,计算能量矩阵元的公式。并以⁹_ΛBe^*,¹⁰_ΛBe^*和¹⁰_ΛΛBe^*为例,进行了具体计算。理论上计算的⁹_ΛBe^*的能量与实验定性符合。     

Microscopic description of fission in neutron-rich plutonium isotopes with the Gogny-D1M energy density functional

The most recent parametrization D1M of the Gogny energy density functional is used to describe fission in the isotopes ^232-280Pu . We resort to the methodology introduced in our previous studies (Phys. Rev. C 88, 054325 (2013) and Phys. Rev. C 89, 054310 (2014)) to compute the fission paths, collective masses and zero point quantum corrections within the Hartree-Fock-Bogoliubov framework. The systematics of the spontaneous fission half-lives t _SF , masses and charges of the fragments in plutonium isotopes is analyzed and compared with available experimental data. We also pay attention to isomeric states, the deformation properties of the fragments as well as to the competition between the spontaneous fission and α-decay modes. The impact of pairing correlations on the predicted t _SF values is demonstrated with the help of calculations for ^232–280Pu, in which the pairing strengths of the Gogny-D1M energy density functional are modified by 5% and 10%, respectively. We further validate the use of the D1M parametrization through the discussion of the half-lives in ^242–262Fm. Our calculations corroborate that, though the uncertainties in the absolute values of physical observables are large, the Gogny-D1M Hartree-Fock-Bogoliubov framework still reproduces the trends with mass and/or neutron numbers and therefore represents a reasonable starting point to describe fission in heavy nuclear systems from a microscopic point of view.     

等电子系列离子的广义振子强度密度之标度关系

利用Born近似可以计算高能电子与原子或离子碰撞激发的微分截面和总截面。微分截面正比于广义振子强度。电子碰撞过程可将靶原子或离子激发到无数个束缚态以及相应的连续态。量子亏损理论能够统一处理这些激发态;因此可定义广义振子强度密度——即每单位激发能内的广义振子强度。我们计算了Li的等电子系列Li,Be⁺,B⁺⁺,C³⁺,Ne⁷⁺,Na⁸⁺,K¹⁶⁺等从基态到S,P,D,F通道的激发的广义振子强度密度,总结了类Li等电子系列离子的广义振子强度密度的标度关系。 关键词：     

Investigation of the electronic structure of Be2+He and Be+He,and static dipole polarisabilities of the helium atom

The potential energy and spectroscopic constants of the ground and many excited states of the Be⁺He van der Waals system have been investigated using a one-electron pseudo-potential approach, which is used to replace the effect of the Be²⁺ core and the electron-He interactions by effective potentials. Furthermore, the core–core interactions are incorporated. This permits the reduction of the number of active electrons of the Be⁺He van der Waals system to only one electron. Therefore, the potential energy of the ground state as well as the excited states is performed at the SCF level and considering the spin–orbit interaction. The core–core interaction for Be²⁺He ground state is included using accurate CCSD (T) calculations. Then, the spectroscopic properties of the Be⁺He electronic states are extracted and compared with the previous theoretical and experimental studies. This comparison has shown a very good agreement for the ground and the first excited states. Moreover, the transition dipole moment has been determined for a large and dense grid of internuclear distances including the spin orbit effect. In addition, a vibrational spacing analysis for the Be²⁺He and Be⁺He ground states is performed to extract the He atomic polarisability.     

Excited baryons in the bag

We calculate the negative parityN ^* andΔ ^* states in the chiral bag model. After projecting out spurious center of mass states we reproduce the known masses reasonably well. Both the colour gluon exchange diagrams and the quark-quark spindependent forces due to the presence of Goldstone pions give important contributions to the ordering of the states. Previous calculations are examined and we conclude that the chiral bag models can sucessfully account for the negative parity states as well as the baryon ground states.     

On the treatment of the center of mass motion in nuclear mean field theories

It is shown how the spurious components due to the center of mass motion can be eliminated from general Hartree-Fock-Bogoliubov quasi-particle configurations with the help of projection techniques. The problem how to restore the additional symmetries being broken by such configurations is discussed. An explicit formulation is given for the spherical Hartree-Fock problem with center of mass momentum projection before the variation. As an example for the application of this method the ground state of⁴He is studied using two different interactions, a microscopic two-body one as well as a phenomenological one including a Skyrme-type three-body force. The results are compared to those of the usual approximate treatment of the center of mass motion in Hartree-Fock calculations. It turns out that, at least for the chosen example, the latter yields a rather reasonable approximation to the correct total energy, single particle energy and even the mass density provided that it is calculated from a translationally invariant density operator.