K. Yuasa

Gross structure of deeply bound hole states in odd tin isotopes observed with the (3He, α) reaction

Abstract A study of the ( 3 He, α) reaction at 110 and 120 MeV on even tin isotopes revealed the presence of a (1g) −1 bump at E X = 6.3–7.0 MeV with the same spectroscopic factor as for the known (1 g 9 2 ) −1 deeply bound hole state.

Observation of two-hole states at high excitation energy in (p, t) reactions

Abstract Bumps of two-hole states at high excitation energies were observed systematically in the triton spectra from (p, t) reactions with 52 MeV protons on nuclei in a broad range of masses. The cross sections of the bumps are almost equal for various targets with the same deep major shells. These cross sections vary discontinuously with variation of the corresponding deep major shell. About 20 to 50 % of the total expected strength is observed experimentally, if the bumps are assumed to arise from two-neutron pickup from the deep major shells. The centres of gravity of the bumps are located at excitation energies of about 7 to 9 MeV in all cases. On the other hand, the widths of the bumps change from about 3 MeV for 66 Zn to about 9 MeV for 230 Th.

Excitation of isovector states by the (7Li,7Be) reaction on 12C and 13C

Abstract Isovector states in 12B and 13B are studied by using the (7Li, 7Be) reaction at a projectile energy of 21 MeV/nucleon. Angular distributions are measured at forward scattering angles of θL ⩽ 10° and compared with one-step distorted-wave Born approximation (DWBA) calculations. The form factors are microscopically evaluated with effective nucleon-nucleon interactions and realistic transition densities. The DWBA calculations reproduce the observed cross sections with normalization factors near to unity. This suggests that the (7Li, 7Be) reaction at 21 MeV/nucleon proceeds mainly through the one-step direct reaction process. The excitation of the giant dipole resonance is confirmed at E x = 7.6 MeV in 12 B and at E x = 9.5 MeV in 13B, from which the isovector collective potential strength is obtained to be V1 ∼ 50 MeV. The isovector octupole strength is observed in three states in 12B, but it is not distinctly observed in 13B.

Dominance of the direct reaction process in the 12C(7Li, 7Be)12B reaction at θL=0° and EL⩾21 MeVA

The 12C(7Li, 7Be)12B reaction is studied at projectile energies of 14, 21 and 26 MeV/A. Emitted 7Be particles are detected at θL∼0° with an energy resolution of about 200 keV which is enough to separate transitions leading to the first excited state of 7Be (7Be1) from ones leading to the ground state of 7Be(7Be0). At incident energies higher than 21 MeV/A, both zero-degree excitation functions for 7Be0 and 7Be1 are well predicted by microscopic distorted-wave Born approximation calculations, but at an incident energy of 14MeV/A there are large contributions from complex reaction processes other than the one-step process.

Isovector giant resonances in light nuclei observed by (7Li, 7Be) reaction

Abstract Isovector giant resonances were studied with the ( 7 Li, 7 Be) reaction on 12 C, 24 Mg, 27 Al, 28 Si and 48 Ti at an incident energy of 150 MeV. Differential cross sections were measured at forward scattering angles including 0°. We observed the octupole resonance at E x ∽ 16 MeV and the giant dipole resonance at E x ∽ 20 MeV in all residual nuclei.

Particle-γ coincidence applied to observation of spin-flip probability in the (7Li, 7Be) reaction

Abstract A particle-γ coincidence technique was applied to separation between the (7Li, 7Beg.s.) and (7Li, 7Be0.43MeV) reaction channels in the 12C(7Li, 7Be)12B reaction, where the two reaction channels have the spin-selection rules of ΔS = 0, 1 and ΔS = 1, respectively. The 7Be particles emitted at θL = 0° were analyzed by means of a magnetic spectrograph with an energy resolution of 200 keV at EL = 26 MeV/A. The γ-ray de-exciting from the 7Be0.43MeV was detected with four NaI(Tl) scintillators (2 in. diameter by 3 in.). From the comparison between the (7Li, 7Beg.s.) and (7Li, 7Be0.43MeV) reaction channels, we can obtain information on the spin-flip probability of the transitions in the (7Li, 7Be) reaction.

Hexadecapole component in the giant quadrupole resonance region of 208Pb

Abstract The L = 4 component located at E x ≈2 h ω in 208Pb was searched for by using inelastic scattering of 3He at 140 MeV. An energy-weighted sum-rule fraction of (23–29)% was found in the giant quadrupole resonance region.

Spin-flip and non spin-flip excitations in the (7Li,7Be) reaction

Abstract Spin-flip (ΔS = 1) and non spin-flip (ΔS = 0) excitations were investigated in the (7Li,7Be) reactions on 12C and 28Si at EL = 26 MeV/A and θL = 0° by measuring separately the (7Li,7Beg.s.) and (7Li,7Be0.43MeV) reaction channels with the 7Be-γ coincidence technique. For both 12C and 28Si, the giant dipole resonance (GDR) is clearly identified in the ΔS = 0 spectrum, and its strength distribution is in good agreement with that observed by the photonuclear reaction. In 12C, the 1− and 2− components of the spin-dipole resonance (SDR) are also observed in the ΔS = 1 spectrum. Not only the magnitudes but also the shapes of the GDR and 1− SDR are found to be very similar in 12C. On the other hand, in 28Si, though a possible 2− component of the SDR is observed, the 1− SDR is not clearly identified as a resonance. Beyond the excitation energy of the GDR for both 12C and 28Si, ΔS = 0 cross sections are enhanced relative to ΔS = 1 ones, which suggests excitation of isovector giant resonances at 2ℏω. In addition, a possible isovector octupole resonance at 1ℏω was observed in the ΔS = 0 spectrum of 28Si.

The 12C(3He, α)11C reaction at 100–140 MeV

Abstract Transitions to negative-parity states in 11C below Ex = 6.48 MeV were studied in the 12 C ( 3 He , α) 11 C reaction at incident energies of 99.4, 119.1 and 139.6 MeV in an angular range of θc.m. ∼ 1°–60°. A magnetic quadrupole spectrometer was used for the measurement at the very forward angle. The observed angular distributions for the transitions to 4.26 ( 5 2 − ) and 6.48 ( 7 2 − ) MeV states in 11C (forbidden transitions in terms of the simple shell model) show a remarkable fall-off at the very forward angle, whereas those to 0.0 ( 3 2 − ), 1.94 ( 1 2 − ) and 4.74 ( 3 2 − ) MeV states transitions) show forward-peaked behaviour. The experimental angular distributions were analysed in terms of the zero-range DWBA including two-step processes via inelastic scattering both in the entrance and exit channels. With the spectroscopic amplitudes of the deformed nucleus model and a normalization constant of D20 = 20 × 104 fm3 · MeV2 which is usually used in the analysis of the ( 3 He , α) reactions on heavier targets, the observed cross sections were considerably well reproduced by the DWBA calculations, especially for the allowed transitions. Some deviation from the DWBA predictions was found in the excitation functions of the cross sections for the forbidden transitions. This suggests that another reaction model is further required in understanding the reaction mechanism in the present incident energy region.