S. Shimoura

Measurement of interaction cross sections using isotope beams of Be and B and isospin dependence of the nuclear radii

Abstract Interaction radii for 11,12,14 Be and 8,12,13,14,15 B have been determined by the measurement of interaction cross sections at 790 MeV/nucleon. Effective root-mean-square (RMS) radii of nucleon distribution of these nuclei have been deduced using a Glauber-model calculation. Isospin dependence of nuclear radii are presented for isobars of mass numbers from 6 to 12, and compared with theoretical predictions by the droplet model and by a Hartree-Fock (HF) model. The HF model with the Skyrme-III potential, which includes a strong density-dependent force, reproduce the observed isospin dependence.

The RIKEN radioactive beam facility

Abstract A radioactive beam facility based on projectile fragmentation, RIPS, has been constructed at RIKEN to enable radioactive-beam experiments. The facility is characterized by the production of high-intensity beams and spin-polarized beams. Since the beginning of 1990, experiments on exotic nuclei have been extensively performed exploiting these useful features. The characteristics and the present status of the RIPS are described.

Revelation of thick neutron skins in nuclei

Abstract Thick neutron skins (∼ 0.9 fm) have been verified for the first time in 6He and 8He nuclei from a combined analysis of the interaction and the fragmentation cross sections of 4,6,8He incident reactions at 800A MeV. A relativistic mean field (RMF) model was applied to 6He and 8He nuclei and shown to reproduce the neutron skin thicknesses very well. It is also shown that the RMF model predicts a gross linear dependence of the neutron skin thickness on the difference between the proton and the neutron Fermi energy in a wide range of nuclei. Possible observations of thick neutron skins in other nuclei, in particular in Na isotopes, are also discussed.

Coulomb dissociation of a halo nucleus 11Be at 72A MeV

Abstract We have measured the excitation energy spectrum of the 10 Be+n system in the Coulomb dissociation of the one-neutron-halo nucleus 11Be on a Pb target at 72A MeV. We have determined the dipole strength distribution dB (E1) /dEx which shows a strong peak at about 800 keV excitation energy. The spectral shape is consistent with a direct breakup model. The Coulomb post accelaration effect has been observed also to support the direct breakup mechanism. The E1 dominance of the Coulomb dissociation is confirmed by observed angular distributions of 10Be in the 11Be rest frame.

Electromagnetic dissociation and soft giant dipole resonance of the neutron-dripline nucleus 11Li

Abstract The electromagnetic dissociation (EMD) cross sections of light neutron-dripline projectile 11Li have been inferred from the target dependence of the interaction cross section, σ1, and the two-neutron-removal cross section, σ−2n, at an incident energy of 0.8 GeV/nucleon. The EMD cross sections of 11Li projectiles on a lead target were found to be quite large; σ I EMD ( 11 Li + Pb )=1.72 ±0.65 b and σ −2 n EMD ( 11 Li + Pb )=0.89 ±0.10 b , which are approximately 80 times larger than those of 12C projectiles after scaling the cross section by Zproj2. The large EMD cross section is related to the possible existence of a soft mode of the giant dipole resonance at low excitation energy in the extremely neutron-rich nuclei.

Neutron halo in 11Be studied via reaction cross sections

Abstract Reaction cross sections (σR) and fragmentation cross sections (σF) of reactions of a 11Be beam on C, Al targets were measured by a transmission method at 33A MeV. The σR have been determined to be 1.56±0.03 b for 11 Be + 12 C , and 2.27±0.05 b for 11 Be + 27 Al . These values are extremely large compared with the systematics of stable nuclei. A large σF has also been observed for the 11Be→10Be channel. Model calculations using present and 790A MeV data confirm the long neutron tail (neutron halo) in the 11Be density distribution.

Determination of the density distribution and the correlation of halo neutrons in 11Li

Interaction cross sections (σ1) and transverse momentum Pt distributions of 9Li fragments in 11Li reactions were measured 1467 using p and d targets at 800A MeV, and Be and C targets at 400A MeV. The density distribution of the 11Li nucleus has been determined for the first time from these σIs. It is confirmed that only a distribution with a long tail (halo) consistently reproduces the observed data. From a combined analysis of Pt distributions of 9Li and of neutrons, the momentum correlation of the two halo neutrons was extracted. It suggests that these neutrons have a strong tendency to move in the same direction.

Determination of the astrophysical 13N (p, γ) 14O cross section through the Coulomb dissociation method☆

Abstract The Coulomb dissociation reaction 208Pb(14O, 13N p)208Pb was measured at Ein = 87.5 MeV/u to determine the radiative width Γγ of the 1− state in 14O at Eex = 5.17 MeV. This state dominates the radiative capture process 13N(p, γ) 14O, a key reaction in the hot CNO cycle of hydrogen burning in stars. We have deduced Γγ = 3.1 ± 0.6 eV with the help of a coupled channel calculation. The validity of the technique was confirmed in the case of the 208Pb(13N, 12C p)208Pb reaction exciting the 1 2 + state in 13N whose width is known.

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.