T. Gomi

Isomeric 0+ state in 12Be

An isomeric 0+ state at E x = 2.24 MeV in 12Be was found in the projectile fragmentation of 18O at 100 A MeV on a Be target. The excitation energy and the spin assignment were deduced by the measurements of the energies and the angular correlation function of the coincident two γ-rays corresponding to the 0+ → 2+ → 0+ decay, respectively.

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.

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.

Isobaric analog state of 14Be

The halo structure of the neutron-dripline nucleus 14Be has been investigated by various approaches, such as the interaction cross section measurement, dissociation reaction and 14Be fragmentation. In the recent plausible view, the mixing of s and d-wave components in the halo wave function has been suggested. In the present study, the halo structure of 14Be was investigated by measuring its Isobaric Analog State (IAS). Since the wave function of the IAS is essentially the same as that of its isobaric partner, possible exotic natures of 14Be may be traced in properties of the IAS such as the Coulomb displacement energy (ΔE C) and the width of the IAS. The IAS of the 14Be was excited by the charge-exchange reaction which was successfully applied to the neutron-dripline nucleus 11Li and obtained the halo properties [1, 2]. In the present study, applying this reaction to 14Be, we have measured the 14Be(p,n)14B* reaction in inverse kinematics at E in = 74 A MeV [3].

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)

Astrophysical reaction rate for the Li-8(n,gamma)Li-9 reaction

An attempt was made to measure the excitation function of the cross section for the Li-8(n,gamma)Li-9 reaction by performing the inverse reaction Li-9(gamma,n)Li-8, with the equivalent photons in the electric field of nuclei in a Pb target providing the gamma rays for the reaction. The energy spectrum of lithium nuclei in coincidence with neutrons had no discernible peak where any beam-velocity Li-8`s would be located. Statistically, a Gaussian-shaped Li-8 peak could have been present with 30+/-29 counts, which we interpreted as consistent with zero, with a two-standard-deviation upper limit of 87 counts. Using the fact that neutron capture on Li-8 must be dominantly s-wave capture, and applying detailed balance, we obtained, with E in eV, sigma(n,gamma)>930E(-1/2) mu b. The corresponding limit on the astrophysical reaction rate is >790 cm(3) mol(-1) s(-1). Theoretical predictions of the reaction rate have exceeded our upper limit by factors of 3-50.

Molecular states in neutron-rich beryllium isotopes

Excited states in 1 2 Be and 1 4 Be were studied by using an exotic beam of 1 4 Be at 75 A MeV. A new excited state in 1 2 Be was observed at 11.8 MeV. The angular-correlation analysis showed that the spin for the new level is tentatively assigned to be 0. According to the energy-spin systematics, the observed state may be a member of the rotational band of the 6 He- 6 He cluster structure. Three excited states in 1 4 Be were also found at 10.8 MeV, 11.6 MeV, and 15.5 MeV.

Search for low lying dipole strength in the neutron rich nucleus 26Ne

Coulomb excitation of the exotic neutron-rich nucleus 26Ne on a natPb target was measured at 58 A.MeV in order to search for low-lying E1 strength above the neutron emission threshold. Data were also taken on an natAl target to estimate the nuclear contribution. The radioactive beam was produced by fragmentation of a 95 A.MeV 40Ar beam delivered by the RIKEN Research Facility. The set-up included a NaI gamma-ray array, a charged fragment hodoscope and a neutron wall. Using the invariant mass method in the 25Ne+n channel, we observe a sizable amount of E1 strength between 6 and 10 MeV. The reconstructed 26Ne angular distribution confirms its E1 nature. A reduced dipole transition probability of B ( E 1 ) = 0.49 ± 0.16 e2fm2 is deduced. For the first time, the decay pattern of low-lying strength in a neutron-rich nucleus is obtained. The results are discussed in terms of a pygmy resonance centered around 9 MeV.

Inelastic Scattering of 12Be with 4He

. Inelastic scattering of the neutron-rich nucleus 1 2 Be on a liquid-He target has been studied by measuring de-excitation γ rays in coincidence with scattered 1 2 Be. In the angular distribution, a difference between the 2 + 1 and the 1- state in 1 2 Be was cleary seen.