α decay studies of the neutron-deficient uranium isotopes 215-217U

Abstract

The α-decay study is an important tool for obtaining information on the nuclear structure of heavy neutron-deficient nuclei. For the N=126 and 124 isotones with even proton number, an intruder state (πh9/2 πf7/2)8+ based on the attractive interaction of f7/2 protons and f5/2 neutrons was observed, which increases the binding energy of this configuration with increasing proton number. Additionally, taking into account the octupole correlations, a large-scale shell model calculations predict that the (πh9/2 πf7/2)8+ state lies close or below the (Πh9/3Πf7/2) 6+ state in the 216Th and 218U. To extend the systematic of the Iπ=8+ isomeric state in N=124 isotones and search for the most neutron-deficient uranium isotopes, an experiment for the α-decay studies of 215-217U was performed at the gas-filled recoil separator SHANS at the Heavy Ion Research Facility in Lanzhou (HIRFL). Heavy ion induced fusion evaporation reactions of the 180W(40Ar, xn)220-xU (x=3-5) were used to synthesize these neutron-deficient uranium isotopes. The 40Ar beam was delivered by the sector focusing cyclotron of the HIRFL. The evaporation residues recoiled from the thin target were separated in-flight from the primary beam particles by the gas-filled recoil separator SHANS. After passing through two multi-wire proportional counters, the residues were planted into a focal plane detection system. Based on the energy-position-time correlation measurement, the isotopes 215-217U were identified. At a beam energy of 187.2 MeV, two α-decaying states were observed in 216U with Eα=8.384(30) MeV, T1/2=+4.72-1.57 ms for the ground state and Eα=10.582(30) MeV and 8.182(30) MeV, T1/2=+1.80-0.48ms for an isomeric state. Through the analysis of the α-decay reduced width, the isomeric state is suggested to have the configuration (πh9/2 πf7/2)8+. At the beam energy of 205.5 MeV, two decay chains of 215U were observed and they were assigned to the decays from ground state to ground state transitions. The α-particle energy and half-life of 215U were determined to be 8.428(30) MeV and +1.33-0.29ms, respectively. In addition, two new α-decay transitions of 217U with α-particle energy of 8.210(30) MeV and 8.098(30) MeV were also observed in this experiment, and they were assigned to the decay of 217U populating the ground state and one low-lying excited state in 213Th, respectively.