Isomer Decay Spectroscopy ofSm164andGd166

Abstract

Excited states in the N ¼ 102 isotones 166 Gd and 164 Sm have been observed following isomeric decay for the first time at RIBF, RIKEN. The half-lives of the isomeric states have been measured to be 950(60) and 600(140) ns for 166 Gd and 164 Sm, respectively. Based on the decay patterns and potential energy surface calculations, including β 6 deformation, a spin and parity of 6 − has been assigned to the isomeric states in both nuclei. Collective observables are discussed in light of the systematics of the region, giving insight into nuclear shape evolution. The decrease in the ground-band energies of 166 Gd and 164 Sm (N ¼ 102) compared to 164 Gd and 162 Sm (N ¼ 100), respectively, presents evidence for the predicted deformed shell closure at N ¼ 100. In the exploration of the nuclear landscape, it is evident that the neutron-rich side of stability contains a vast unknown territory, where approximately half of all the bound nuclides remain to be identified. Furthermore, this is the domain of rapid-neutron-capture (r process) nucleosynthesis, which is poorly understood and yet is key to the creation of chemical elements from iron to uranium (Z ¼ 26-92) in stellar environments [1]. With the advent of the current generation of radioactive-beam facilities, it is now possible to address some of the open questions PRL 113, 262502 (2014) P H Y S I C A L

Review of metastable states in heavy nuclei

“…It is worth noting that important information relevant to the r-process has already been obtained from isomer spectroscopy, see for example Refs. [380,381,382]. With the development of the new rare-isotope beam facilities and improved experimental techniques, the discovery of neutron-decaying isomers [90] could also be within reach in the foreseeable future.…”

Insight into nuclear midshell structures by studying K isomers in rare-earth neutron-rich nuclei

“…The results with ε 6 = 0 are displayed to examine the high-order deformation effect. In general, compared to the ε 6 = 0 calculations, the non-zero ε 6 results reproduce the experimental multi-particle state energies better, except for the 1.486 MeV 6 − state in 164 Sm and the 1.350 MeV 4 + state in 166 Gd [11].…”

“…The nuclear structure inputs of the rare-earth nuclei can lead to an improved understanding of the r−process nucleosynthesis [3]. However, due to their neutron excess, detailed structure informations are very difficult to be revealed by the experiment in these rare-earth neutron-rich nuclei.The recent experimental progresses in the neutron-rich A = 150 − 170 region [4][5][6][7][8][9][10][11][12][13] are attributed to a great extent to the existence of the nuclear isomeric state. A nucleus can be "traped" in an aligned spin orientation relative to its symmetric axis to form the K isomeric state (or K-isomer), where K is a quantum number representing the projection of the total nuclear spin along the symmetry axis of the nucleus.…”

“…Investigations of K-isomers have been reviewed recently in Refs. [14][15][16][17][18].An impressive progress has been achieved in experimental studies of samarium and gadolinium isotopes very recently [4,5,8,9,11,12]. The most exiting discoveries include: 160 Sm became the lightest nucleus with a known four-quasiparticle K-isomer [8], K-isomers were firstly observed in very neutron-rich nuclei 164 Sm and 166 Gd [11] and in odd-A samarium isotopes [4,5].…”

Systematic study of multi-quasiparticleK-isomeric bands in tungsten isotopes by the extended projected shell model

“…Recent theoretical calculation predicts a deformed shell closure at N = 100 [53]. This has also been experimentally verified recently [54] for Sm and Gd nuclei. This scenario may get changed if one moves towards the higher side of the rare-earth region.…”