Discovery of 157W and 161Os

Abstract

The nuclides 157W and 161Os have been discovered in reactions of 58Ni ion beams with a 106Cd target. The 161Os α-decay energy and half-life were 6890 ± 12 keV and 640 ± 60 μs. The daughter 157W nuclei β-decayed with a half-life of 275 ± 40 ms, populating both low-lying α-decaying states in 157Ta, which is consistent with a 7 / 2- ground state in 157W. Fine structure observed in the α decay of 161Os places the lowest excited state in 157W with Iπ = 9 / 2- at 318 ± 30 keV. The branching ratio of 5.5- 2.2+ 3.1 % indicates that 161Os also has a 7 / 2- ground state. Shell-model calculations analysing the effects of monopole shifts and a tensor force on the relative energies of 2 f7 / 2 and 1 h9 / 2 neutron states in N = 83 isotones are presented. © 2010 Elsevier B.V. All rights reserved.