Y. Toh

High-spin states in Au-188: Further evidence for nonaxial shape

The high-spin level structure of {sup 188}Au has been investigated via the {sup 173}Yb({sup 19}F,4n{gamma}) reaction at beam energies of 86 and 90 MeV. The previously reported level scheme has been modified and extended significantly. A new I{sup {pi}}=20{sup +} state associated with {pi}h{sub 11/2}{sup -1} x {nu}i{sub 13/2}{sup -2}h{sub 9/2}{sup -1} configuration and two new rotational bands, one of which is built on the {pi}h{sub 9/2} x {nu}i{sub 13/2} configuration, have been identified. The prolate-to-oblate shape transition through triaxial shape has been proposed to occur around {sup 188}Au for the {pi}h{sub 9/2} x {nu}i{sub 13/2} bands in odd-odd Au isotopes. Evidence for {pi}h{sub 11/2}{sup -1} x {nu}i{sub 13/2}{sup -1} structure of nonaxial shape with {gamma}<-70 deg. has been obtained by comparison with total Routhian surface and cranked-shell-model calculations.

Development of a correction method for the time-of-flight prompt gamma-ray analysis

A new analytical technique, time-of-flight prompt gamma-ray analysis, has been developed at the Japan Proton Accelerator Research Complex. In order to apply it to accurate elemental analysis, a set of Fe and Au reference samples were measured to examine the several factors which affect the number of detected events. It was found that major contributing factors included attenuations of neutrons and gamma rays in the sample, live-time fraction and signal pile-up correction. A simulation model was built for the estimation of neutron and gamma-ray attenuations. A simple empirical formula was proposed to calculate the signal pile-up correction factor. The whole correction method has proven to be accurate and reliable.

Precise Measurements of Neutron Capture Cross Sections for LLFPs and MAs

To evaluate the feasibility of development of nuclear transmutation technology and an advanced nuclear system, precise nuclear data of neutron capture cross sections for long-lived fission products (LLFPs) and minor actinides (MAs) are indispensable. In this chapter, we present our research activities for the measurements of neutron capture cross sections for LLFPs and MAs.

Medium-spin states in {sup 70}Ge and the role of the g{sub 9/2} orbital

Medium-spin states of {sup 70}Ge have been studied via the {sup 60}Ni({sup 12}C,2p{gamma}){sup 70}Ge reaction at 45 MeV. The ground-state band and the second 0{sup +} band have been extended to the 12{sup +} and 8{sup +} states, respectively. Two negative-parity bands, one of which has a coupled structure and the other has a decoupled structure, have been observed additionally. Although the latter decoupled structure was known up to the (21{sup -}) state from a previous experiment, the part of the level scheme up to the 15{sup -} state has been largely modified by the present experiment. Backbendings observed in the positive- and negative-parity yrast bands have been compared with those of the neighboring even Ge isotopes. The experimental level structure has been compared with the shell-model calculations in the model space (2p{sub 3/2}, 1f{sub 5/2}, 2p{sub 1/2}, 1g{sub 9/2}) employing two kinds of effective interactions, one of which is an extended P+QQ interaction with monopole interactions and the other is developed from a renormalized G matrix. Microscopic structures of the observed bands have been discussed with the help of the shell-model calculations.

Band properties of the transitional nucleus 189Pt

High‐spin states in 189Pt have been studied with the in‐beam γ‐spectroscopy method via the 176Yb(18O,5n) reaction at beam energies of 88 MeV and 95 MeV. A new level scheme of 189Pt has been established. Rotational bands based on the υi13/2−1, υf5/2(p3/2) and υi13/2−2υf5esol2(p3/2) configurations, as well as several structures with irregular level spacings, have been observed. Properties of rotational bands have been analyzed in the framework of triaxial particle‐rotor model. A γ~−30° triaxial shape and a near prolate shape have been proposed to the υi13/2−1 and υf5/2(p3/2) bands, respectively. Two ΔI = 2 transition sequences with similar energies have been observed, and they have been proposed to be associated with the υi13/2−2υf5/2(p3/2) configuration. According to the relevant Nilsson orbitals, the bands built on the υi13/2−2υf5/2(p3/2) configuration could be interpreted as a pair of pseudo‐spin partner.

Octupole collectivity in 94Zr

The Zr isotopes between 90Zr and 96Zr are expected to be spherical based on the almost complete subshell closures at Z = 40 and N = 50, 56. On the other hand, they have low‐lying 3− states and show the characteristics of low frequency octupole oscillation which arise as a superposition of particle‐hole excitations. A 380 MeV 94Zr beam from the tandem accelerator at the Japan Atomic Energy Agency (JAEA) was excited on a self‐supporting natPb target. The gamma‐ray detector array GEMINI‐II was used to detect deexcitation gamma rays. The scattered beam (94Zr) was detected with a position‐sensitive particle detector system. The gamma‐ray intensities were used as an input to the least‐squares search code GOSIA to determine the E3 matrix element of the first 3− excited state of 94Zr. The B(E3;0+→3−) value of 0.21(6) e2b3 in 94Zr has been obtained by Coulomb excitation experiment.