T. Motobayashi

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.

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.

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.

DALI2: A NaI(Tl) detector array for measurements of

A NaI(Tl) detector array called DALI2 (Detector Array for Lo w Intensity radiation 2) has been constructed for in-beam γ-ray spectroscopy experiments with fast radioactive isotope (R I) beams. It consists typically of 186 NaI(Tl) scintillator s covering polar angles from∼15◦ to ∼160◦ with an average angular resolution of 6 ◦ in full width at half maximum. Its high granularity (good angular resolution) enables Doppler-shift correcti ons that result in, for example, 10% energy resolution and 20 % full-energy photopeak e fficiency for 1-MeVγ rays emitted from fast-moving nuclei (velocities of v/c ≃ 0.6). DALI2 has been employed successfully in numerous experiments using fast RI beams wi th velocities ofv/c = 0.3 − 0.6 provided by the RIKEN RI Beam Factory.

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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.

rays from fast nuclei

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.

Anomalously hindered E2 strength B(E2;2+(1)-->0+) in 16C.

Excited states in (38,40,42) Si nuclei have been studied via in-beam γ-ray spectroscopy with multinucleon removal reactions. Intense radioactive beams of ^{40}S and (44)S provided at the new facility of the RIKEN Radioactive Isotope Beam Factory enabled γ-γ coincidence measurements. A prominent γ line observed with an energy of 742(8) keV in (42) Si confirms the 2(+) state reported in an earlier study. Among the γ lines observed in coincidence with the 2^{+} → 0+ transition, the most probable candidate for the transition from the yrast 4(+) state was identified, leading to a 4(1)+) energy of 2173(14) keV. The energy ratio of 2.93(5) between the 2(1)+ and 4(1)(+) states indicates well-developed deformation in (42) Si at N = 28 and Z = 14. Also for 38,40)Si energy ratios with values of 2.09(5) and 2.56(5) were obtained. Together with the ratio for (42)Si, the results show a rapid deformation development of Si isotopes from N = 24 to N = 28.

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

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.

Well Developed Deformation in 42Si

Abstract The Coulomb excitation of a halo nucleus 11 Be to its first bound excited state (E1: 1 2 + → 1 2 − ) has been studied using a 64 MeV/u 11 Be radioactive beam incident on a 208 Pb target. The electric dipole strength deduced by a first-order perturbation analysis is in good agreement with the result of previous life-time measurements of the 11 Be 1 2 − state.

Unbound excited states in 19,17C

Abstract The cross section and analysing powers Ay, Axx and Ayy of the charge-exchange reaction 1 H ( d , 2 p ) n have been measured at Ed = 200 and 350 MeV up to excitation energies (Ex) of 8 MeV in the final pp system. The good agreement observed between the experimental dependence of the cross section and analysing powers upon Ex and the predictions of an impulse approximation calculation improves with increasing beam energy. The large experimental figures of merit found for Axx and Ayy demonstrate that this reaction can be used as the basis of a new deuteron tensor polarimeter in the 200–400 MeV energy range.