The Decay of a Microsecond Seniority Isomer in 155Hf

Abstract

A search for an isomeric state has been conducted at Jyväskylä Accelerator Laboratory for the even-odd neutron-deficient nuclide 155H f . The proton-rich N = 82 and 83 isotones below Z = 72 have been investigated by McNeill et al., who identified seniority isomers with half-lives in the microsecond range and compared their measured properties with shell-model calculations [1]. Even though systematics would suggest such an isomer exists in 155H f , they found no evidence for such an isomer. A 1 mg/cm2 thick target of 102Pd was bombarded by a beam of (293 − 315) MeV 58Ni ions to produce the nuclei of interest via a 2p3n evaporation channel. The Mass Analysing Recoil Apparatus (MARA) was utilized to separate fusion-evaporation reaction products in flight, after which they were implanted into the Double-Sided Silicon Strip Detector (DSSD) at its focal plane. The DSSD was surrounded by germanium detectors to measure delayed gamma rays. The Recoil Decay Tagging (RDT) method was applied using the alpha decays of its daughter 155Lu. An isomeric state, which decay through gamma emissions has been observed for the first time. It has been assigned a spin and parity of 27/2− based on the πh11/2 ⊗ν f7/2 configuration, seniority υ = 3. Using a gamma-gamma coincidence analysis, the decay level scheme was constructed. The isomeric state has an excitation energy of 2764(4) keV and a half-life of 509 (22) ns. The B(E2) value was measured for the 105.4 keV (27/2− −→ 23/2−) isomeric transition and was found to be 0.44(4) W.u.. The value can be attributed to the known hindrance in N = 83 and N = 82 isotones which is responsible for the parabolic behaviour of B(E2) values as a function of Z.