arXiv subjects
Publications and source records attributed to S. Shimoura.
The neturon rich nucleus 13B was studied via the proton transfer reaction 4He(12Be,13B γ) at 50AMeV. The known 4.83-MeV excited state was strongly populated and its spin and parity were assigned to 1/2+ by comparing the angular differential cross section data with DWBA calculations. This low-lying 1/2+ state is interpreted as a proton intruder state and indicates a deformation of the nucleus.
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 19C, while three peaks with energies of 2.20(3), 3.05(3), and 6.13(9) MeV have been observed in 17C. Deduced cross sections are compared with microscopic DWBA calculations based on p-sd shell model wave functions and modern nucleon-nucleus optical potentials. Jpi assignments are made for the four observed states as well as the ground states of both nuclei.
Inspired by the recent measurement of the B(E2;$0_{2}^{+} \to 2_{1}^+$) and B(E0;$0_{2}^{+} \to 0_{1}^+$) values in $^{12}$Be, we give an interpretation of available spectroscopic data on both $^{12}$Be and $^{11}$Be, using a simple model which contains the essential feature of these two nuclei; the presence of weakly-bound neutron(s) in deformed potentials. The agreement of the calculated results with observed data is surprisingly good, including well-known strong E1 transitions in both nuclei.
Breakup reactions of the one-neutron halo nucleus 11Be on Pb and C targets at about 70 MeV/u have been investigated by measuring the momentum vectors of the incident 11Be, outgoing 10Be, and neutron in coincidence. The relative energy spectra as well as the angular distributions of the 10Be+n center of mass have been extracted for both targets. For the breakup on Pb target, the selection of forward scattering angles is found to be effective to extract almost purely the first-order E1 Coulomb breakup component, and to exclude the nuclear contribution and higher-order Coulomb breakup components. This angle-selected energy spectrum is thus used to deduce the spectroscopic factor for the 10Be(0+) 2s_1/2 configuration in 11Be which is found to be 0.72+-0.04 with B(E1) up to Ex=4 MeV of 1.05+-0.06 e2fm2. The energy weighted E1 strength up to Ex=4 MeV explains 70+-10% of the cluster sum rule, consistent with the obtained spectroscopic factor. The non-energy weighted sum rule is used to extract the root mean square distance of the halo neutron to be 5.77(16) fm, consistent with previously known values. In the breakup with C target, we have observed the excitations to the known unbound states in 11Be at Ex=1.78 MeV and 3.41 MeV. Angular distributions for these states show the diffraction pattern characteristic of L=2 transitions, resulting in J^pi =(3/2,5/2)+ assignment for these states. We finally find that even for the C target the E1 Coulomb direct breakup mechanism becomes dominant at very forward angles.
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 lifetime is 75 +- 23 ps, corresponding to a B(E2;2_1+ -> 0^+) value of 0.63 +- 0.19 e2fm4, or 0.26 +- 0.08 Weisskopf units. The transition strength is found to be anomalously small compared to the empirically predicted value.