Gammasphere上裂变丰中子核核结构若干前沿领域的新进展(II:第三和第四部分)

原子核物理评论 , 2010, 27(4): 363-389.

Gammasphere上裂变丰中子核核结构若干前沿领域的新进展(II:第三和第四部分)

罗亦孝 ^1, , J.H.Hamilton ^2, , J.O.Rasmussen ^3, , A.V.Ramayya ^4, , C.Goodin ^5, , A.V.Daniel ^6, , N.J.Stone ^7, , 朱胜江 ^8, , J.K.Hwang ^9, , 刘少华 ^10, , C.J.Beyer ^11, , 李科 ^12, , H.L.Crowell ^13, , D.Almehed ^14, , S.Frauendorf ^15, , A.Covello ^16, , V.Dimitrov ^17, , 张敬业 ^18, , 车兴来 ^19, , 姜卓 ^20, , D.Fong ^21, , A.Gelberg ^22, , I.Stefanescu ^23, , A.Gargano ^24, , E.F.Jones ^25, , P.M.Gore ^26, , I.Y.Lee ^27, , G.M.Ter-Akopian ^28, , Yu.Ts.Oganessian ^29, , M.A.Stoyer ^30, , R.Donangelo ^31, , 马文超 ^32, , J.D.Cole ^33, , J.Kormicki ^34, , 张学谦 ^35, , S.C.Wu ^36, , J.Gilat ^37, , T.N.Ginter ^38, , S.J.Asztalos ^39,

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基金项目: 合同项目(W-7405-ENG48;DE-AC03-76SF00098;DE-AC07-991D13727;DE-AC07-761D01570) 美国能源部基金(DE-FG05-88ER40407;DE-FG02-95ER40934;DE-FG02-95ER40939;DE-FG05-87ER40311) 中国国家自然科学基金(19775028;10375032;10775078) 国家重点基础研究发展规划资助项目(2007CB815005;G2000077400)

关键词：形状过渡 , 轴对称形状 , 三轴形变 , 形状突变 , 带交叉 , 奇-质子 , 八极激发 , B(E1)/B(E2)比值 , 电二极矩D0 , 二极矩的下降 , 八极振动 , Cs,La同位素

New Insights into Frontier of Nuclear Structure of Neutron-rich Nuclei by Means of Prompt Fission γ Ray Measurements at Gammasphere(II: Section 3 & Section 4)

Keywords： shape transition , axially-symmetric shape , triaxial deformation , onset of deformation , band-crossing , odd-Z , octupole excitations , B(E1)/B(E2) values , electric dipole moment D0 , sharp drop of dipole moments , octupole vibrations , Cs,La isotopes

使用Gammasphere多探测器系统对252Cf裂变源瞬发7射线进行γ-γ-γ和γ-γ(θ)符合测量,A～100(Z～40,N＞56)丰中子核区关于形状过渡、形状共存和形状突变的深入研究获得了新的进展.奇-Z核的系统研究揭示了从Z=39(Y)同位素的轴对称大四极形变到Z=43(Tc),44(Ru)和45(Rh)同位素中具有中等四极形变之大三轴形变的过渡.Nb(Z=41)同位素具有过渡核的特征.根据邻近同位素和同中素带交叉信息的系统性及推转壳模型(CSM)计算,Tc和Rh偶-N同位素中观察到的带交叉产生于一对h11/2中子的转动顺排.首次观察到100Nb(Z=41,N=59)的高自旋能级纲图和形变态,从而证实了100Nb中的形状共存,并确认,在Nb同位素链上,基态形状突变发生于N=58(球形)到N=59(大形变);而在Sr(Z=38),Y(Z=39)和Zr(Z=40)各同位素链上,基态形状突变均发生于N=59(球形)到N=60(大形变).100Nb具有大形变基态,同现有理论预言相矛盾,其大形变的基态及转动带所具有的很大的和不规则的转动惯量具有重要的理论意义. 对奇-质子Cs(Z=55)和La(Z=57)丰中子同位素八极激发的详细研究,发现了141,143,144Cs同位素链上电二极矩D.随中子数增加而减小的急剧变化,此前在145,147La中观察到的电二极矩Do的较缓和的减小似乎亦很陡地下延至148La.在Cs和La同位素中观察到的电二极矩D.的急剧下降很可能同偶-质子核146Ba和224Ra中观察到的电二极矩D.的急剧下降相类似,后者已获反射非对称平均场壳修正理论的成功解释.实验证明,141Cs具有很大电二极矩D.141Cs和142Xe中观测到的很大的电二极矩D.和仅在141Cs中观察到的电二极矩D.的simplex量子数相关性需要进一步的理论研究.根据转动频率比值ω-(I)/ω+(I)的检定,同Xe同位素一样,认定Cs同位素中包括号143Cs的八极激发为八极振动,而在La同位素中存在八极形变.

By means of γ-γ-γ and γ-γ(θ) measurements of prompt fission γrays from 252Cf using Gammasphere, new insights have been gained into the A～100 (Z～40, N>56) neutron-rich region, which has long been of interest for shape transitions, shape coexistence and onset of deformations. The systematic investigations of odd-Z isotopes have revealed a shape transition from axially symmetric shapes with very large quadrupole deformations in Z=39 (Y) to near maximum triaxial shapes with moderate quadrupole deformations in Z=43(Tc), 44 (Ru),45(Rh)isotopes. The Nb(Z=41)isotopes are found to be transitional with regard to this shape transition. The band-crossings observed in the even-N Tc and Rh isotopes are attributed to alignment of a pair of h11/2 neutrons. The long-expected sudden shape transition in Nb isotopic chain is now observed. The long-sought high-spin level scheme and deformed states in 100Nb (N=59) were identified. The ground state of 100Nb was found to be strongly deformed, in disagreement with existing theory. The'shape coexistence was confirmed in the nucleus, and the sudden shape transiti0n in the Nb isotopic chain was seen to occur between N=58 (spherical) to N=59 (strongly deformed), in contrast to the sudden shape transition from N=59 (spherical) to N=60 (strongly deformed) in Sr (Z=38), Y (Z=39) and Zr (Z=40) isotopic chains. The observation of strongly deformed ground state in 100Nb and its very large and irregular moments of inertia make 100Nb an especially interesting case for further theoretical studies. Recent detailed studies of octupole excitations in odd-Z Cs (Z=55) and La (Z=57) isotopes have discovered a sharp drop in electric dipole moments Do in the Cs isotopic chain 141,143. 144Cs,and, most likely, the previously reported moderate drop in Do in 145, 147La is extended sharply down in 148La. The sharp drops of Do in Cs and La are most likely analogous to those reported in 146Ba and 224Ra, which were accounted for by the reflection-asymmetric mean field shell correction theory. The very large Do in both 141Cs and 142Xe and the simplex-dependent Do deduced in 141Cs but not in 143Cs need further theoretical studies. Based on rotational frequency ratios,ω- (I)/ω+(I), octupole vibrations are proposed in Cs isotopes, like the cases in Xe isotopes, while octupole deformations are suggested in La isotopes.

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