High-spin Level Structure In ⁷⁴As

In the A～70 mass region, proton number and neutron number are larger than the magic number 28 while smaller than the magic number 50. There are many nucleon outside the 28 closed shell. Therefore, the nuclei shape is the transformed from spherical shape to deformed shape. And in this region, the competition between single-particle and collective degrees of freedom is stiff. A lot of complicated phenomenon of nuclear structure was found in this mass region, such as large shape deformation, shape coexistence, shape transition, signature splitting, signature inversion and so on. Furthermore, the proton number is close to neutron number. The nuclei shape change much while the proton number or neutron number changes.The high-spin states in 74As were populated via heavy ion fusion evaporation 70Zn(7Li,3n)74As at a bombarding energy of 30 MeV. The 7Li beam was delivered by HI-13 tandem accelerator at the China Institute of Atomic Energy (CIAE). The target consisted of 70Zn with the thickness of 2.15 mg/cm2 evaporated on 0.93 mg/cm2 of gold backing. 12 Compton suppressed HPGe detectors and 2 planar HPGe detectors were employed to detect the de-excitedγrays from the reaction residues. 92×106 two foldsγ-γcoincidence events were collected. Gain matching and relative efficiency calibration were performed by using standard 60Co, 152Eu and 133Baγrays sources. They were sorted offline into symmetrical matrices and directional correlation of oriented states (DCO) matrices. These matrices were analyzed by Radware programs and Gaspware programs based on a Linux-PC computer. New level scheme of 74As shown has been constructed on the basis of coincidence relationship. All the transitions of 74As observed in the previous study have been confirmed. 43 newγrays and 28 new levels belonging to 74As have been identified. 2 rotational bands have been constructed. The positive-parity yrast band has been extended to I=13? at the energy of 4006.4 keV and the negative-parity band has been extended to I=12 ? at the energy of 3171.8 keV. Furthermore, 4 sidebands have been constructed too. The spin and parity of energy levels in 74As haven been assigned on the basis of the DCO ratios, information in the new 74As level scheme and systematic comparison.Configuration of band1 has been determined according to the kinematic of inertia J(1) and systematic comparison with neighbouring nucleus. And the configuration of band1 has also been confirmed by the relation between the spin and rotational frequency. The signature inversion in the positive-parity band and the negative-parity band indicated by the experimental Routhian. And the signature in these bands can be seen more clearly by plotting the experimental quantity (EI-EI-1)/2I as a function of spin. The mechanism of signature inversion has been explained by two quasiparticles plus rotor model. Total Routhian Surface has also been calculated for 74As. It's shown that triaxial shapes exists in low spin region and the shapes is relative stable. Shape transition has been found from prolate sphere in low spin to non-collecitve oblate sphere in high-spin states by the TRS calculation.