N. Aoi

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.

Evidence for particle stability of 31 F and particle instability of 25 N and 28 O

Abstract The neutron drip line up to fluorine has been investigated by the projectile fragmentation of a 94.1A MeV 40 Ar beam at the fragment separator RIPS at RIKEN. A new neutron-rich isotope, 31 F, has been observed for the first time while clear evidence for the particle instability of 24,25 N, 27,28 O and 30 F has been obtained. The sudden change in stability from oxygen to fluorine may demonstrate the onset of deformation for the neutron-rich fluorine isotopes.

Experimental study of 8He + p elastic and inelastic scattering

Abstract An experimental study of 8He + p elastic scattering and a search for 8He∗ as observed in two independent experimental methods. The obtained parameters of the excited state are E 8 He ∗ = 3.57±0.12 MeV , Γ =0.5±0.35 MeV, Γ 4 He+4n /Γ 6 He+n+n ⩽ 5% . Angular distributions for elastic and inelastic scattering are measured. The inelastic angular distribution is consistent with Jπ = 2+ for 8He3.55∗.

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.

Fusion and breakup at the barrier with 11Be

Abstract The cross-section excitation functions for various neutron-evaporation channels have been measured in the 11,10,9 Be+ 209 Bi systems around the Coulomb barrier, for energies ranging from 35 to 70 MeV, in order to see any eventual effect from the expected breakup of the 11 Be unstable halo nucleus. Within the simplified reaction process considered, the data are consistent with a breakup of 11 Be at the top of the Coulomb barrier contributing to less than 25% to the total fusion cross-section; however contributions from other type of breakup mechanisms cannot be excluded.

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.

One- and two-neutron removal reactions from the most neutron-rich carbon isotopes

The structure of 19,20,22C has been investigated using high-energy (around 240 MeV/nucleon) one- and two-neutron removal reactions on a carbon target. Measurements were made of the inclusive cross sections and momentum distributions for the charged residues. Narrow momentum distributions were observed for one-neutron removal from 19C and 20C and two-neutron removal from 22C. Two-neutron removal from 20C resulted in a relatively broad momentum distribution. The results are compared with eikonal-model calculations combined with shell-model structure information. The neutron removal cross sections and associated momentum distributions are calculated for transitions to both the particle-bound and particle-unbound final states. The calculations take into account the population of the mass A−1 reaction residues A−1C and, following one-neutron emission after one-neutron removal, the mass A−2 two-neutron removal residues A−2C. The smaller contributions of direct two-neutron removal, that populate the A−2C residues in a single step, are also computed. The data and calculations are shown to be in good overall agreement and consistent with the predicted shell-model ground-state configurations and one-neutron overlaps with low-lying states in 18−21C. These suggest significant νs1/22 valence neutron configurations in both 20C and 22C. The results for 22C strongly support the picture of 22C as a two-neutron halo nucleus with a dominant νs1/22 ground-state configuration.

Analyzing power in elastic scattering of 6He from a polarized proton target at 71 MeV/nucleon

The vector analyzing power has been measured for the elastic scattering of neutron-rich {sup 6}He from polarized protons at 71 MeV/nucleon making use of a newly constructed solid polarized proton target operated in a low magnetic field and at high temperature. Two approaches based on local one-body potentials were applied to investigate the spin-orbit interaction between a proton and a {sup 6}He nucleus. An optical model analysis revealed that the spin-orbit potential for {sup 6}He is characterized by a shallow and long-ranged shape compared with the global systematics of stable nuclei. A semimicroscopic analysis with an {alpha}+n+n cluster folding model suggests that the interaction between a proton and the {alpha} core is essentially important in describing the p+{sup 6}He elastic scattering. The data are also compared with fully microscopic analyses using nonlocal optical potentials based on nucleon-nucleon g matrices.

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.

Determination of the astrophysical S factor of the 8B(p,γ)9C capture reaction from the d(8B,9C)n reaction

The asymptotic normalization coefficients for the virtual decay 9 C → 8 B + p have been determined by measuring the crosssection of the 8 B(d, n) 9 C reaction in inverse kinematics at 14.4 MeV/u, using the RIPS facility. The deduced astrophysical S factor S18 of the 8 B(p ,γ ) 9 C capture reaction in the center of mass energy range 1–100 keV is S18 = 45 ± 13 eV b.  2001