No abstract

No abstract

We have made a thorough study of the low-energy behaviour of the γ-ray strength function within the framework of the shell model. We have performed large-scale calculations spanning isotopic and isotonic chains over several mass regions, considering 283 nuclei in total, with the purpose of studying the systematic behavior of the low-energy enhancement (LEE) for M1 transitions. There are clear trends in the calculations: From being nearly absent in the lowest mass region, the LEE becomes steeper and more pronounced as the mass number increases, and for a given mass region it further increases towards shell closures. Moreover, the LEE is found to be steeper in regions near doubly-magic nuclei where proton particles couple to neutron holes. These trends enable us to consolidate several previous works on the LEE into a single, consistent concept. We compare the inferred trends to the available experimental data from the Oslo method, and find support for the systematic behaviour. Lastly we have compared the calculations to strength functions compiled from discrete, experimental lifetimes, and find excellent agreement; the discrete data are consistent with an LEE, and indicate that the slope varies as function of mass number.

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