丰中子区近^(132)Sn原子核的壳演化研究进展

Progress on nuclear shell evolution in neutron-rich region around^(132)Sn

弱束缚区原子核传统幻数的消失和新幻数的产生一直是放射性核束物理的前沿研究方向.近年来,基于世界各地大科学装置的研究已经积累了丰硕的成果.在远离稳定线的轻丰中子区,人们已经发现8,20,28传统中子幻数的消失和新中子幻数14,16,34等的出现.在中重丰中子区,已有的实验和理论研究显示,50和82传统中子闭壳随着中质比的增大,有减弱直至消失的趋势.利用日本理化学研究所的放射性束流装置,我们开展了对^(123)Pd和^(125)Pd核的β衰变实验研究,在衰变子核^(123)Ag和^(125)Ag的低激发能区发现了具有β放射性的同核异能态.利用新发现的同核异能态,讨论了奇质量Ag同位素核中由πg9/2和πp1/2两个轨道形成的Z=40亚闭壳能隙在N=82附近的演化,发现在N=82处,Z=40亚闭壳能隙可能存在明显的减小.为进一步了解壳演化的微观机制,使用包含张量力的壳模型计算了这个质量区单粒子轨道的演化.结果显示,相比于N=50处,Z=40亚闭壳能隙在N=82处存在明显的减小,张量力对Ag同位素中πg9/2和πp1/2轨道以及Z=40亚闭壳能隙在接近N=82时的演化起到非常重要的作用.

The disappearance of traditional magic numbers and the generation of new magic numbers in weakly-bound nuclei have always been the frontier of physics with radioactive beam.In recent years,the research based on large scientific devices around the world has got fruitful results.In the light neutron-rich region far away from theβ-stable line,it has been found that the traditional neutron magic numbers 8,20,28 disappear and the new neutron magic numbers 14,16,34 appear.In the medium and heavy neutron-rich regions,experimental and theoretical studies have shown that the traditional neutron magic numbers 50 and 82 tend to weaken or disappear with the increase of the neutron/proton ratio.For example,recent experimental studies have shown that the N=82 shell closure in^(132)Sn,130 Cd,and128 Pd isotones is still robust,but a significant reduction of the N=82 gap was suggested to occur between Sn and Zr as a consequence of the absent Z=40 subshell gap.With the Z=47,the neutron-rich odd-A Ag isotopes are naturally of great interest.Their valence protons are assumed to fill theπg9/2 andπp1/2 orbitals,between which the Z=40 subshell gap is formed.The energy difference between the lowest lying 9/2^(+)and 1/2-states in these neutron-rich odd-A Ag isotopes provides direct information on the Z=40 subshell gap.Therefore,to explore the N=82 shell evolution in Ag isotopes,theβ-delayedγ-ray spectroscopy of neutronrich^(123,125)Ag isotopes is investigated at the Radioactive Isotope Beam Factory of RIKEN in the framework of the Euroball RIken Cluster Array project.The long-predicted 1/2-β-emitting isomers in^(123,125)Ag are identified for the first time.Shellmodel calculations have been performed using the KSHELL code with the state-of-the-art monopole-based universal interaction VMUplus a spin-orbit force from M3 Y(VMU^(+)LS),and give an overall satisfactory description of experimental levels in^(123,125)Ag,particularly for the low-lying levels.The calculations indicate that,as approaching N=82,the wave functions from theπg9/2 andπp1/2 orbitals dominate low-lying states of Ag isotopes.With the newly observed 1/2-isomeric states in^(123,125)Ag,the systematic energy difference between the lowest 9/2^(+)and 1/2-states along Ag isotopic chain indicates an increasing trend beyond125 Ag,which reveals that the Z=40 subshell gap starts to be restored toward N=82.Extrapolating the trend toward N=82,a considerable diminishment of the Z=40 subshell gap is expected.To get more insight into the microscopic origin of shell evolution in this region,the effective single-particle energies are calculated for the proton orbitals in the region of N=68–82 with the VMU^(+)LS interaction.It is found that,if only the central^(+)spin-orbit parts are considered,theπp1/2 orbital lies below theπg9/2 orbital in the whole region of N=68–82.In contrast,with the inclusion of the tensor part(especially theπg9/2-νh11/2 monopole),theπg9/2 orbital is affected much more than theπp1/2 orbital,and the spacing betweenπg9/2 andπp1/2 orbitals is notably reduced,consequently,resulting in the inversion of these two orbitals at N~74,which is compatible with the experimental inversion position.The tensor force manifests its crucial role in the modification of the order of the proton orbitals and the size of the Z=40 subshell gap in Ag isotopes mainly through theπg9/2-νh11/2 monopole.Vice versa,our calculations indicate that this tensor force will also influence the behavior of theνh11/2 orbital,which is important for the N=82 shell gap.