M. Ishihara

Large deformation of the very neutron-rich nucleus 32Mg from intermediate-energy Coulomb excitation

Abstract The Coulomb excitation of a very neutron-rich nucleus 32Mg to its 2+ state was studied using an unstable nuclear beam of 32Mg at 49.2 MeV/u with a 208Pb target. The B(E2) value of 454±78 e2fm4 was extracted by a coupled channel analysis. The value is in good agreement with recent predictions. It confirms a large deformation for 32Mg suggested by the low excitation energy of the 2+ state, and points to the vanishing of the N = 20 shell gap.

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.

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.

Coulomb Dissociation of B-8 and the Be-7(p, Gamma)b-8 Reaction At Low Energies

The cross section for Coulomb dissociation of [sup 8]B---the [sup 208]Pb([sup 8]B,[sup 7]Be [ital p])[sup 208]Pb reaction---was measured using a [sup 8]B radioactive beam of 46.5 MeV/nucleon energy, and the cross section for the [sup 7]Be([ital p],[gamma])[sup 8]B capture reaction was deduced at low energies; [ital E][sub c.m.]=0.6[minus]1.7 MeV. The extracted astrophysical [ital S][sub 17] factors were found to be consistent with the values measured by Vaughn [ital et] [ital al]. and Filippone [ital et] [ital al]. This result encourages further experimental studies extended to lower relative energies for a new determination of the [ital S][sub 17] value relevant to the [sup 8]B solar neutrino flux.

Gamma-rays from an incomplete fusion reaction induced by 95 MeV 14N

Abstract Gamma-rays from the 159 Tb ( 14 N , αx n ) 169−x Yb reaction, in which non-evaporation α-particles are emitted, have been identified. Yields of E2 cascade transitions suggest that the angular momentum distribution of the entrance channel leading to this reaction is localized just above the critical angular momentum for complete fusion.

Quadrupole deformation of 12Be studied by proton inelastic scattering

Abstract Inelastic proton scattering exciting the 2 + 1 states in the neutron-rich beryllium isotopes 10,12 Be has been studied in inverse kinematics. From a coupled-channel analysis, the deformation lengths for the 2 + 1 states in 10 Be and 12 Be were determined to be 1.80±0.25 fm and 2.00±0.23 fm respectively, indicating that a tendency towards strong quadrupole deformation is preserved for these nuclei and that the singly-closed shell structure does not prevail in 12 Be. A quantitative analysis based on shell model calculations supports this picture.

Coulomb dissociation reaction and correlations of two halo neutrons in 11Li

Abstract We have performed an exclusive measurement for the Coulomb dissociation reaction, Pb( 11 Li, 9 Li+2n) X at an incident energy of 43 A MeV. The deduced excitation energy spectrum of the 11Li nucleus shows a prominent peak at 1 MeV with a large tail towards higher excitation energies. The relative energy spectra for the n-n and 9Li−2n sub-systems show that the excitation is dominated by the relative motion between 9Li and the center-of-mass of the two neutrons. The above characteristics are explained by a direct Coulomb breakup mechanism with an extended ‘di-neutron’ model.

New aspect of intermediate energy heavy ion reactions. Large spin polarization of fragments

Abstract The spin polarization, P , of 12 B ejectiles has been measured for projectile fragmentation reactions induced by a 40.6 MeV/u 14 N beam. Large values of P were observed, showing a characteristic dependence on the outgoing momentum including a sign change occurring at a momentum corresponding to the beam velocity. This behavior corresponds well with simple kinematical arguments based on projectile fragmentation models.

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.