A. Makishima

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 .

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.

A graphical analysis of decay curves measured by the Doppler-shift recoil distance method

The present method of analyzing the decay curves is a direct extension of the method of linearizing an exponential decay using a semilog plot. On semilog graph paper experimental points of a decay curve can be approximated using a smooth curve having a slowly varying slope with recoil distance. In addition, the lifetime of an excited state can be expressed as the product of two factors: the reciprocal of the slope of a semilog graph and a correction factor for feeding time. These factors can be evaluated from two semilog graphs for the successive transitions, feeding and de-exciting a given excited state. This factorization simplifies the analysis of the decay curves and brings the method a considerable tolerance as regards their experimental errors. The present method has been applied to the decay curves measured in 124,126Ce and 132,134Nd.

Nuclear isomerism in 100Sn neighbors

Data on B(E2), B(M1) and B(E1) were obtained from lifetime measurements in 103, 105, 107In, 105-108Sn and 109Sb. These data helped us to assign the nuclear configurations to the involved states. The experimental B(E2) and B(M1) values in the Sn isotopes worked as litmus paper to test the wave functions calculated on the basis of the shell model. The present calculation gave a qualitative description of M1 transitions in the Sn isotopes but has not yet succeeded in quantitative estimation of B(M1). Calculated B(E2) values were far from the reality since 100Sn was assumed there to be inert against excitation.

High-spin states in107Sn

High-spin states in107Sn were studied using the reaction54Fe(56Fe, 2pn)107Sn. An odd-parity band was identified as based on thevh11/2 orbit by measuring theγ-ray linear polarization. Also were confirmed the excited states based on thevd5/2 andvg7/2 orbits. The halflives of the 17/2+ and 11/2− states were measured to be 0.25(4) ns and 28(11) ps, respectively. The experimental level structure and its interpretation were in good agreement with a shell model calculation.

In-beam study of105In and103In

High spin states in the isotopes105,103In have been studied by in-beamγ-ray techniques. The reaction51V(58Ni, 2p 2n) populated those states in105In. A measurement of the linear polarizations ofγ-rays has revealed a negative-parity band in105In. The reaction48Ti(58Ni,p 2n) produced the isotope103In, which has showed an yrast spectrum similar to that observed in105In. The lifetimes of the 19/2+ and 17/2+ states in107,105In have been measured by the recoil distance method;T1/2(19/2+)=27±4 ps andT1/2(17/2+)=1.1−0.3+0.6 ns for107In,T1/2(19/2+)=4±2 ps andT1/2(17/2+)=0.33±0.10 ns for105In.

Z dependence of the N=152 deformed shell gap: In-beam {gamma}-ray spectroscopy of neutron-rich {sup 245,246}Pu

We have measured in-beam {gamma} rays in the neutron-rich {sup 246}Pu{sub 152} and {sup 245}Pu{sub 151} nuclei by means of {sup 244}Pu({sup 18}O, {sup 16}O){sup 246}Pu and {sup 244}Pu({sup 18}O, {sup 17}O){sup 245}Pu neutron transfer reactions, respectively. The {gamma} rays emitted from {sup 246}Pu ({sup 245}Pu) were identified by selecting the kinetic energy of scattered {sup 16}O ({sup 17}O) detected by Si {delta}E-E detectors. The ground-state band of {sup 246}Pu was established up to the 12{sup +} state. We have found that the shell gap of N=152 is reduced in energy with decreasing atomic number by extending the systematics of the one-quasiparticle energies in N=151 nuclei into those in {sup 245}Pu. This reduction of the shell gap clearly affects the 2{sup +} energy of the ground-state band of {sup 246}Pu.

Core-excited states in the doubly magic 68Ni and its neighbor 69Cu

The (νg9/22νp1/2−2)8+ isomer with T1/2=23(1) ns at 4208 keV in 68Ni was found by deep-inelastic collisions of 70Zn(8 MeV/nucleon)+198Pt and the νg9/2 E2 effective charge was determined to be 1.5(1)e. In 69Cu, the (πp3/2νg9/22νp1/2−2)19/2− isomer with T1/2=22(1) ns at 3691 keV was identified and its decay data were calculated quite accurately by a parameter-free shell model calculation using experimental level energies. Proton 2p-1h excitation, fed by another T1/2=39(6) ns isomer, induces large collectivity in 69Cu.

Subnanosecond isomers in 105, 107Sn

The lifetimes of excited states in 105,107Sn were measured using a plunger improved to cover a recoil distance up to 9.5 mm. In 105Sn, T1/2 (17/2+) = 0.38(5) ns and T1/2 (7/2+) = 0.33(8) ns. In 107Sn, T1/2 (17/2+) = 0.40(6) ns and T1/2 (7/2+) = 0.9(2) ns. The 7/2+ → 5/2+ M1 transitions are l-forbidden by a factor of about 102: B(M1) = 0.008 W.u. in 105Sn and B(M1) = 0.007 W.u. in 107Sn. From B(M1, 17/2+ → 15/2+) values major configurations of the 17/2+ and 15/2+1, 2 states were deduced as follows: | 17/2+ > ≈ vd5/2(vg7/2)26., | 15/2+1 > ≈ vg7/2(vd5/2)24. + some admixture of vd5/2(vg7/2)26., | 15/2+2 > ≈ [vd5/2(vg7/2)26.]15/2.

Nuclear structure of the yrast 6+, 10+ and 14− states in106Sn

The nuclear precession of106Sn in the 6+ state was measured relative to that of108Sn by the IPAD method: θ(6+;106Sn)/θ(6+;108Sn)=1.06(33). Available data on the 6+ state in108Sn giveg (6+;106Sn)=− 0.14(9), which is consistent with the seniority ν=2 configuration: ({ie133-01}). B(E2; 10+→ 8+) was measured to be 6.4(10) W.u. for106Sn and 0.62(5) W.u. for108Sn. This difference could not be explained by the ν≦4 configurations. The {ie133-02}→13− transition turned out to be an M1/ E2 mixed transition and so the yrast I=14 state was identified with a state of the νh11/2 band.