丰中子核145,147La的八极形变

通过对²⁵²Cf自发裂变产生的瞬发γ谱的实验研究,建立与扩展了丰中子核^145,147La的高自旋态能级图,最高自旋态达到(41/2⁺)与(43/2^－).测得的交叉相联的相反宇称带及带间的强化E1跃迁说明在^145,147La中形成强的八极形变.对¹⁴⁵La的研究表明,具有轴对称形状的四极形变与具有反演不对称形状的八极形变在同一核中产生共存与竞争.在πh_11/2带中观测到的带交叉位于ω≈0.26—0.30MeV附近,由推转壳模型计算表明,它是由一对i_13/2中子的顺排所引起的.     

90Mo电磁跃迁几率的测量

利用束流能量为116MeV的⁵⁹Co(³⁵Cl,2p2n)⁹⁰Mo反应布居⁹⁰Mo的高自旋态.用10台反康普顿HPGe探测器组成的探测阵列进行γγ符合测量.通过多普勒展宽峰的形状分析测定⁹⁰Mo高自旋态的寿命.在正宇称衰变系观察到增强的M1跃迁,推断I=13以上是扁椭形变.负宇称高自旋态具有大的B(E2)值,并随自旋增大而起伏变化.正、负宇称态之间的跃迁显示增大的E1跃迁,似有八极关联的可能.但是,⁹⁰Mo并不处在理论预言的存在八极形变的核区内.     

丰中子核区奇A核八极形变及A=130区一些缺中子核高自旋态研究

对近几年来清华大学物理系核物理研究组原子核高自旋态实验研究的一些新进展进行了介绍.内容包括两个部分:第一部分介绍通过国际合作,利用测量裂变瞬发γ谱方法对A=140丰中子核区的奇A核^143,145Ba,^145,147La等的极形变研究的进展.第二部分简述对A=130缺中子核区的^134,137,138Ce及¹²²Ba等核高自旋态研究,包括形状驱动、八极关联、集体回弯等效应.     

Rn-Th偶偶核的八级形变研究

用推转壳模型在多维形变空间系统研究了Rn-Th偶偶核的基态性质及高自旋情形下的性质, 其中选取的形变自由度为β₂, β₃, β₄和β₅. 计算结果很好的再现了实验提取的转动惯量值. 势能面的研究表明, 随中子数的增加, 基态形状由近球形(N≈130)逐渐演变为八极形变(N≈136)后又发展成为稳定的四极形变(N≥140). 推转计算表明Rn-Th偶偶核的八极形变非常软, 在推转到高自旋情形下, 逐渐变为反射对称形变.     

丰中子奇A核145,147Ce的集体带结构

通过对²⁵²Cf自发裂变产生的瞬发γ谱的测量,对丰中子奇A核^145,147Ce的高自旋态进行了研究,首次识别了¹⁴⁵Ce的集体带结构,扩展了¹⁴⁷Ce的集体带结构,扩展了¹⁴⁷Ce的能级,并新识别了3个边带,粒子–转子模型的计算指出,^145,147Ce的晕带可能来自于νi_13/2轨道的耦合,¹⁴⁵Ce基态起源于(νh_9/2νf_7/2)组态混合,而¹⁴⁷Ce基态则起源于νh_9/2轨道.在这两个核中未观测到明显的八极形变带,然而,在¹⁴⁷Ce中观测到的一个边带,可能显露出八极关联的迹象.     

反射不对称壳模型及Ra八极形变带的描述

应用变分法,建立了反射不对称壳模型(RASM)的基本理论框架.通过计算RASM的本征方程可以得到原子核的八极转动带.在RASM框架下,再现了典型八极形变偶偶核Yrast带的一些基本特征.计算了典型八极形变偶偶同位素^222—230Ra的Yrast带,理论与实验符合得很好.     

146Ce核的八级形变带和准γ带

通过测量²⁵²Cf自发裂变所产生的瞬发γ射线, 对¹⁴⁶Ce核的高自旋结构进行了重新研究, 结果更新了以前报道的能级纲图, 把八级形变集体带扩展到更高的自旋, 并且重新构建了可能的准γ带结构. 此外, 用反射不对称壳模型(RASM)对¹⁴⁶Ce核的八级形变带进行了计算, 低自旋处的计算结果与实验数据符合得很好.     

超重核熔合中的靶核形变效应

利用额外推力模型研究了⁴⁸Ca+²³⁸U俘获和熔合过程中的靶核形变效应.计算表明,在近垒和垒下能区,靶核形变使俘获截面和熔合截面增强,形成的复合核自旋分布展宽.     

A=190区奇奇核超形变带顺排涌动量的相加性

利用推转玻尔–莫特森模型指定的能级自旋,研究了A=190区奇奇核¹⁹⁴Tl和¹⁹⁶Bi超形变带的顺排角动量的相加性.¹⁹⁴Tl的6条超形变带中,有4条满足顺排角动量的相加性规则,而¹⁹⁶Bi的超形变带不满足这个规则.     

双奇核172Re高自旋态转动带结构实验研究

利用重离子熔合蒸发反应¹⁴⁹Sm(²⁷Al,4nγ)¹⁷²Re布居了形变双奇核¹⁷²Re的高自旋态,用12套带有BGO反康普顿抑制的高纯锗探测器阵列进行了在束γ实验测量,首次建立了形变双奇核¹⁷²Re由3个转动带构成的高自旋态能级纲图.研究和讨论了3个转动带的结构特征,基于已有的高自旋态核结构知识并通过系统学比较和分析指出它们的准粒子组态分别为πh_11/2⊙νi_13/2,πh_9/2⊙νi_13/2和π1/2^－[541]⊙ν1/2^－[521].发现前两个转动带在自旋小于18.5h时其转动能级呈现反常的旋称劈裂.     

Possible evidence for static octupole deformation in128,129Ba

It is demonstrated that recent experimental results on high spin states of^128,129Ba offer substantial new evidence for static octupole deformation in these nuclei.     

Complex shape effects in nuclear rotational spectra

We study the evolution of collectivity in the structure of nuclear rotational bands based on complex quadrupole-octupole shapes. We apply an extended version of a quadrupole-octupole rotation model capable of reproducing both the low-lying states of alternating parity bands interpreted on the basis of octupole vibrations and the higher spin states considered as members of a single octupole rotational band. In such a way, the complicated odd-even staggering effects observed in light actinide nuclei are described successfully. The implemented model analysis suggests a unified mechanism in which the octupole band structure is formed as the result of the transition from an octupole vibration (soft) mode to a rotation of a shape with a stable quadrupole-octupole deformation.     

Octupole correlations at medium spin in odd-proton153Eu nucleus

Medium-spin experimental data on alternating parity bands in¹⁵³Eu are analysed and compared with results of cranking calculations assuming no octupole deformation, dynamic octupole deformation and static octupole deformation. It is concluded that the existence of static octupole deformation is plausible in this nucleus at medium spin.     

Yrast structure of 62 139 Sm77:γ-softness of the 62 140 Sm78 core

A general survey of the octupole instability of high spin superdeformed (2∶1) and hyperdeformed (3∶1) nuclei has been performed in the cranked Nilsson-Strutinsky model. The octupole degree of freedom is predicted to play an important role in several already observed superdeformed nuclei around¹⁵²Dy, and in particular in N=114 isotones in the Hg-Pb-region. High spin reflection asymmetric hyperdeformed nuclei are predicted.     

Rotational consequences of stable octupole deformation in nuclei

The characteristic features of high-spin spectra of octupole-deformed nuclei are demonstrated by means of Woods-Saxon-Bogolyubov cranking calculations. The rotational spectra of Ra and Th nuclei are studied. The experimental data suggests shape changes with increasing neutron number from N ? 130 (nearly spherical shapes) through N ? 134 (octupole-deformed shapes) to N ? 140 (well-deformed reflection-symmetric shapes). The octupole mixing between the high-j intruder states and normal-parity orbitals leads to specific patterns of quasiparticle spectra characterised by a quantum number referred to as simplex. The influence of octupole deformation on high-spin properties of nuclear spectra like spin alignment, band interaction, etc. are discussed.     

Search for octupole deformation in neutron-rich Xe isotopes

A search for octupole deformation in neutron-rich Xe isotopes has been conducted through prompt gammaray spectroscopy of secondary fragments produced in the spontaneous fission of²⁴⁸Cm. The spectrometer consisted of the Eurogam 1 array and a set of 5 LEPS detectors. Level schemes were constructed for Xe isotopes with mass number ranging from 140 to 144 and excited states for^143,144Xe nuclei were observed for the first time. None of the level schemes exhibit an alternating parity quasimolecular band, a feature usually expected in nuclei in which octupole correlation effects are strong enough to produce stable octupole deformation. For several isotopes, structures observed in the level schemes are consistent with an octupole softness of the nuclei.     

Clustering aspects of nuclei in highly deformed states

The potential energies, the moments of inertia, and the quadrupole and octupole moments of dinuclear systems are compared with corresponding values for the highly deformed nuclear states. The idea is advocated that hyperdeformed states of nuclei are close to near-symmetric dinuclear systems. The superdeformed states are considered as asymmetric dinuclear systems. The cluster superdeformed and hyperdeformed states have quite a large octupole deformation. Measurement of octupole deformations of highly deformed nuclei can answer the question of whether these nuclei exist in cluster configurations.     

围绕Z=56和N=88丰中子核区八极形变的研究

介绍了通过测量重核裂变产生的瞬发γ谱方法,对围绕Ｚ＝５６、Ｎ＝８８丰中子核区高自旋态八极形变的研究,并对Ｘｅ、Ｂａ、Ｃｅ和Ｌａ等丰中子核链的八极形变特性进行了讨Through study of the prompt γ rays in spontaneous fission, the octupole deformation in neutron rich Xe, Ba, Ce and La isotopes around Z =56, N =88 region was assigned. The properties of the octupole deformation in these nuclei were also discussed. 论．