T. K. Onishi

Anomalously hindered E2 strength B(E2;2+(1)-->0+) in 16C.

The electric quadrupole transition from the first 2(+) state to the ground 0(+) state in 16C is studied through measurement of the lifetime by a recoil shadow method applied to inelastically scattered radioactive 16C nuclei. The measured mean lifetime is 77+/-14(stat)+/-19(syst) ps. The central value of mean lifetime corresponds to a B(E2;2+(1)-->0(+)) value of 0.63e(2) fm(4), or 0.26 Weisskopf units. The transition strength is found to be anomalously small compared to the empirically predicted value.

Unbound excited states in 19,17C

The neutron-rich carbon isotopes 19,17C have been investigated via proton inelastic scattering on a liquid hydrogen target at 70 MeV/nucleon. The invariant mass method in inverse kinematics was employed to reconstruct the energy spectrum, in which fast neutrons and charged fragments were detected in coincidence using a neutron hodoscope and a dipole magnet system. A peak has been observed with an excitation energy of 1.46(10) MeV in C, while three peaks with energies of 2.20(3), 3.05(3), and 6.13(9) MeV have been observed in C. Deduced cross sections are compared with microscopic DWBA calculations based on p-sd shell model wave functions and modern nucleon-nucleus optical potentials. Jπ assignments are made for the four observed states as well as the ground states of both nuclei.

Lifetime measurements of first excited states in C 16 , 18

The electric quadrupole transition from the first 2{sup +} state to the ground 0{sup +} state in {sup 18}C was studied through a lifetime measurement by an upgraded recoil shadow method applied to inelastically scattered radioactive {sup 18}C nuclei. The measured mean lifetime is 18.9{+-}0.9(stat){+-}4.4(syst) ps, corresponding to a B(E2;2{sub 1}{sup +}{yields}0{sub g.s.}{sup +}) value of 4.3{+-}0.2{+-}1.0 e{sup 2} fm{sup 4}, or about 1.5 Weisskopf units. The mean lifetime of the first 2{sup +} state in {sup 16}C was remeasured to be 18.3{+-}1.4{+-}4.8 ps, about four times shorter than the value reported previously. The discrepancy between the two results was explained by incorporating the {gamma}-ray angular distribution measured in this work into the previous measurement. These transition strengths are hindered compared to the empirical transition strengths, indicating that the anomalous hindrance observed in {sup 16}C persists in {sup 18}C.

One-neutron knockout reaction of 17C on a hydrogen target at 70 MeV/nucleon

Abstract First experimental evidence of the population of the first 2 − state in 16 C above the neutron threshold is obtained by neutron knockout from 17 C on a hydrogen target. The invariant mass method combined with in-beam γ -ray detection is used to locate the state at 5.45(1) MeV. Comparison of its populating cross section and parallel momentum distribution with a Glauber model calculation utilizing the shell-model spectroscopic factor confirms the core-neutron removal nature of this state. Additionally, a previously known unbound state at 6.11 MeV and a new state at 6.28(2) MeV are observed. The position of the first 2 − state, which belongs to a member of the lowest-lying p – sd cross shell transition, is reasonably well described by the shell-model calculation using the WBT interaction.

14Be(p,n)14B reaction at 69 MeV in inverse kinematics

Y. Satou, ∗ T. Nakamura, Y. Kondo, N. Matsui, Y. Hashimoto, T. Nakabayashi, T. Okumura, M. Shinohara, N. Fukuda, T. Sugimoto, H. Otsu, Y. Togano, T. Motobayashi, H. Sakurai, Y. Yanagisawa, N. Aoi, S. Takeuchi, T. Gomi, M. Ishihara, S. Kawai, H.J. Ong, T.K. Onishi, S. Shimoura, M. Tamaki, T. Kobayashi, Y. Matsuda, N. Endo, and M. Kitayama Department of Physics and Astronomy, Seoul National University, 599 Gwanak, Seoul 151-742, Republic of Korea Department of Physics, Tokyo Institute of Technology, 2-12-1 Oh-Okayama, Meguro, Tokyo 152-8551, Japan RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Department of Physics, Rikkyo University, 3 Nishi-Ikebukuro, Toshima, Tokyo 171-8501, Japan Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan Center for Nuclear Study (CNS), University of Tokyo, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Department of Physics, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan (Dated: January 12, 2013)

Proton-rich nuclear structure and mirror asymmetry investigated by β-decay spectroscopy of 24Si

In order to investigate the characteristic properties of proton-rich nuclei, β-decay spectroscopy was carried out on 24Si. The Gamow-Teller transition strength B(GT) of 24Si to low-lying states in 24Al was determined. Considering in particular the breaking of mirror symmetry for energy levels and B(GT), we discuss the behavior of a weakly-bound s-orbital proton by incorporating a recent experimental result.

Unbound states of the drip-line nucleus 24O

The characteristics of the new N = 16 shell gap at the neutron drip-line can be deduced from the neutron excitations of 24O. An experiment was carried out to investigate the unbound excited states of 24O using the proton elastic and inelastic proton scattering. It was performed in the BigRIPS line and combines the unique intensities of the RIBF 24O beam with the state-of-the-art particle detector array MUST2. The method is explained.

Mirror asymmetry for B(GT) of 24Si induced by Thomas‐Ehrman shift

We carried out the β‐decay spectroscopy on 24Si in order to investigate a change in configuration in the wave function induced by Thomas‐Ehrman shift from a perspective of mirror asymmetry of B(GT). We observed two β transitions to low‐lying bound states in 24Al for the first time. In this proceeding, the B(GT) of 24Si is compared with that of the mirror nucleus 24Ne, and the mirror asymmetry of B(GT) is determined. Then the origin of the B(GT) asymmetry is discussed through the comparison with theoretical calculations.

Gamow‐Teller transition of the proton‐rich nucleus 24Si

The Gamow‐Teller transition strength B(GT) of 24Si was measured to investigate characteristic structure of proton‐rich nuclei induced by a weakly‐bound proton. By combining the results of the delayed γ‐ray and proton measurements, the entire decay scheme of 24Si was reconstructed, then the B(GT) of 24Si was deduced. Focusing to the low‐lying states, the B(GT) of 24Si to the 11+ and 12+ states in 24Al were smaller than that of the mirror nucleus 24Ne by 22% and 10%, respectively. The mirror asymmetry of B(GT) indicates the configuration change in the wave function. Through the comparison with theoretical shell‐model calculations, it is implied that the configuration change is attributed to the lowering of the 1s1/2 orbital. The applicability of the theoretical calculations was checked using overall B(GT) distribution.

Low-lying states in {sup 32}Mg studied by proton inelastic scattering

Proton inelastic scattering on the neutron-rich nucleus {sup 32}Mg has been studied at 46.5 MeV/nucleon in inverse kinematics. Populated states were identified by measuring de-excitation {gamma} rays, in which five new states were found by {gamma}-{gamma} coincidence analyses. By analyzing the angular differential cross sections via coupled-channel calculations, their spins and parities were constrained and the amplitudes for each transition were extracted. The spin and parity of the 2321-keV state was assigned as 4{sub 1}{sup +}. The ratio between the energies of the 2{sub 1}{sup +} and 4{sub 1}{sup +} states indicates that {sup 32}Mg is a transitional nucleus rather than an axially deformed rigid rotor. The collectivities in the nucleus {sup 32}Mg with N=20 are discussed based on the results obtained in the present experiment.