M. Ogawa

Particle hole yrast states in146Gd and147Gd and the Z=64 shell closure

The level structures of146Gd and147Gd have been investigated by in-beamγ-ray ande− spectroscopy with (α, xn) reactions on enriched Sm targets. Detailed level schemes up to ∼4 MeV, which differ radically from earlier schemes, are reported. The energy levels are characterized as particle-hole excitations using empirical single particle energies and two nucleon interactions. Analysis of pure 1p 1h proton excitations demonstrates that theZ=64 andN=82 energy gaps are about equally large.

The 3− state in the doubly closed shell nucleus 64 146 Gd82

Inbeam γ-ray angular distribution measurements establishIπ = 3− for the 1579 keV first excited state in146Gd.

Properties of the 3− first excited state in the doubly closed shell nucleus146Gd

Conversion electron measurements confirm the 3− assignment recently proposed for the 1579 keV first excited state in146Gd. Fromγγ- ande−γ-coincidence data the half life of that state has been determined asT1/2=1.06±0.12 ns.

Isomer-scope: A new instrument for in-beam γ-ray spectroscopy through deep inelastic collisions

Abstract A new instrument, isomer-scope, has been constructed to observe the ns-μs isomers produced by deep inelastic collisions. It consists of a Si detector, a γ-absorber and Ge detectors. The Si detector provides the energy and timing signals of projectile-like fragments(PLF) for the coincidence measurement and also stores the isomers of PLF for the Ge detectors. The γ-absorber shields the Ge detectors from the strong γ-radiation generated in the target. Thus, the isomer-scope greatly intensifies the γ-rays from the isomers of PLF in the PLF-γ(-γ) coincidences. In the reaction 635 MeV 76 Ge + 198 Pt , a new isomer of 69 Cu was found to be excited at 2741 keV. It decayed with T 1 2 = 0.36(3) μ s via transition cascade 190-680-1871 and via cascade 73-486-471-1711. The cascade 486-471-1711 was fed in part from another state; either the 486 keV γ-ray or a side-feeding γ-ray unobserved was an isomeric transition of T 1 2 = 25(5) ns .

Stopping power of dense helium plasma for fast heavy ions

The interaction process between fast heavy ions and dense plasma was experimentally investigated. We injected 4.3-MeV/u or 6.0-MeV/u iron ions into a z -pinch-discharge helium plasma and measured the energy loss of the ions by the time of flight method. The energy loss of 4.3-MeV/u ions fairly agreed with theoretical prediction when the electron density of the target was on the order of 10 18 cm −3 . With increasing electron density beyond 10 19 cm −3 , the difference between the experiment and the theory became remarkable; the experimental energy loss was 15% larger than the theoretical value at the peak density. For 6.0-MeV/u ions, the deviation from the theory appeared even at densities below 10 19 cm −3 . These discrepancies indicated that density effects such as ladderlike ionization caused the enhancement of the projectile mean charge in the target.

High-spin isomer and level structure of 145Sm

Abstract High-spin states of 145 Sm were studied by using the reactions 20 Ne( 136 Xe,α7n), 16 O( 136 Xe,7n), 139 La( 10 B,4n) and 138 Ba( 13 C,6n). The level scheme was extended up to the state at 14.6 MeV. The excitation energy and the spin-parity of the 0.96 μs high-spin isomer were determined to be 8.8 MeV and ( 49 2 + ), respectively. Experimental results were discussed comparing with the deformed independent particle model calculation. High-spin isomers observed in N = 83 isotones were interpreted to be caused by the sudden shape change from near spherical to oblate. The Z = 64 shell gap energy was found to decrease from 2.4 to 2.0 MeV as the proton number decreases from Z = 64 to 61.

Measurement of stopping power of 240 MeV argon ions in partially ionized helium discharge plasma

An energy loss of 240 MeV argon ions in a Z -pinch helium plasma has been for the first time observed throughout the entire pinching process. Standard Stark broadening analysis gives an electron density ranging from 4 to 6 × 10 17 cm −3 during the pinch. To deduce stopping power from the energy loss, the target thickness of the helium plasma has been evaluated assuming the mean charge of helium based on thermal equilibrium. The observed electron density and the mean charge of helium give a target thickness of 30 ± 3 μg cm −2 from 1 μs to 1.8 μs after the discharge ignition. The measured stopping power exceeds a tabulated value for cold helium gas by a factor of 2 to 3 around the time of the first pinch. The experimental stopping power is compared with theoretical values calculated using an equation of stopping power for a partially ionized plasma.

In-beam γ-ray study of the neutron-rich nuclei 240U, 246Pu, and 250Cm produced by the (18O, 16O) reaction

We have measured deexcitation γ rays in the neutron-rich nuclei of 240U, 246Pu, and 250Cm produced by the (18O, 16O) two-neutron transfer reactions, in coincidence with the 16O particles using Si ΔE−E detectors. The γ rays in these nuclei were identified by selecting the kinetic energies of 16O particles, which correspond to the excitation energies in the residual nuclei below the neutron separation energies. The ground-state bands of 240U, 246Pu, and 250Cm were established up to 12+ states and the Kπ = 0− octupole band of 240U was established up to 9− states. The systematics of the moments of inertia of the ground-state bands for actinide nuclei shows that the deformed subshell closure at N = 152 is sustained for 96Cm isotopes and that it disappears for 94Pu isotopes.

Coulomb excitation of 174Hf K-isomer. γ-ray spectroscopy with high-spin isomer beam

Abstract A new experimental technique utilizing a high-spin isomer beam (HSIB) has been developed. The HSIB of 174Hf was produced by the 9Be(170Er, 5n)174Hf reaction in inverse kinematics. By using the HSIB, the Coulomb excitation of the Kπ = 8− isomer in 174Hf was measured. The γ-rays from the 9− level, which is the first excited state built on the isomer, were observed as well as the cascade γ-rays of the ground-state rotational band. From the measured γ-ray yield, a value of 2 ± 1 e2b2 was extracted for the B(E2: 8− → 9−). This is the first experiment of γ-ray spectroscopy by means of the HSIB-induced secondary reactions.

High-spin isomeric state in144Pm

A new high-spin isomeric state with a half-life of ≥2μs was found in144Pm by observing the14N(136Xe,6n)144Pm reactions. The excitation energy and spin of the isomer were estimated to be ∼7.4 MeV and ∼27+, respectively. Filling gas in a recoil ion separator was used as the target material.