S. Nakayama

Gamow-Teller strengths from (t,3He) charge-exchange reactions on light nuclei

Abstract Zero-degree cross sections have been measured for (t, 3 He) reactions on the light target nuclei 9 Be, 10 B, 11 B, 12 C, and 13 C at a bombarding energy of 127 MeV/A. The triton beam was produced by ( 4 He,t+p) breakup on a Be production target using a 4 He beam of 155 MeV/A. Dispersion matching was applied to obtain good energy resolution ( Δ E/E=0.002, Δ E=780 keV). The zero-degree (t, 3 He) cross sections are used to discuss Gamow-Teller strengths in 10 Be and 11 Be.

Isospin decomposition of the Gamow-Teller strength in Cu-58

Abstract The Gamow-Teller (GT) strength excited by a (p, n)-type reaction on a nucleus with isospin T0 and N > Z is shared by isospin components T0 − 1, T0 and T0 + 1. A good energy resolution (3He, t) reaction on 58Ni revealed the fine structure of the GT strength in 58Cu. The isospin of each level constituting the fine structure was assigned by comparing to results from inelastic electron and proton scatterings and (n, p)-type reactions, thus resolving the isospin structure of the GT strength in 58Cu. The ratio of the summed GT strengths among the three isospin components is well described by a shell-model calculation.

Gamma rays and neutrons following preequilibrium 165Ho(p, xcnypγ) reactions at Ep = 60 MeV

Abstract Neutrons and γ-rays following 60 MeV proton bombardment of 165Ho were measured in coincidence with discrete γ-rays characteristic of the reaction channels (residual nuclei). The cross sections for the (p, xcnγ) reactions with x = 2–6, the γ-ray multiplicities, and the energy and angular distributions of the emitted neutrons were analyzed in terms of the preequilibrium and equilibrium deexcitation processes. Characteristic behaviours of the preequilibrium process were found in the (p, 2nγ) and (p, 4nγ) reactions where the sum ET, = ∑xEi of the energies Ei of the emitted neutrons was large, while those of the equilibrium process were typical for the (p, 6ny) reaction with small ET. The reactions are well reproduced by the expression σ. ≈0.35∑xσ (2, x−2) + 0.4∑xσ(1, χ−1)+ 0.25∑xσ(0, x), where σ(np, nc) stands for emission of np neutrons at the preequilibrium stage followed by evaporation of nc neutrons at the equilibrium stage.

Extraction of weak transition strengths via the (He-3, t) reaction at 420 MeV

Differential cross sections for transitions of known weak strength were measured with the (3He, t) reaction at 420 MeV on targets of 12C, 13C, 18O, 26Mg, 58Ni, 60Ni, 90Zr, 118Sn, 120Sn, and 208Pb. Using these data, it is shown that the proportionalities between strengths and cross sections for this probe follow simple trends as a function of mass number. These trends can be used to confidently determine Gamow-Teller strength distributions in nuclei for which the proportionality cannot be calibrated via beta-decay strengths. Although theoretical calculations in the distorted-wave Born approximation overestimate the data, they allow one to understand the main experimental features and to predict deviations from the simple trends observed in some of the transitions.

In-beam conversion electrons from 141Pr, 143Pm, 144Pm and 145Pm excited by (α, xn) and (p, n) reactions

Abstract Electromagnetic transitions between low-lying states in the odd proton nuclei 59 141 Pr 82 , 61 143 Pm 82 , 61 144 Pm 83 and 61 145 Pm 84 with neutron numbers N ≈ 82 were studied by means of in-beam electron-gamma spectroscopy. The triple-focusing electron spectrum selector (TESS) was used to measure in-beam conversion electrons. The TESS, giving very good electron line spectra with small background, was found to be very powerful for in-beam spectroscopy. Accurate values for the internal conversion coefficients (ICC) were obtained by measuring simultaneously both conversion electrons and γ-rays. M2, E3 and some other multipolarities were uniquely assigned from the ICC for the γ-transitions between low-lying states. Properties of these transitions and energy levels are discussed. Analyses of the M2 and E3 transition rates gave, respectively, an isospin-spin (magnetic) core polarization effect and an octupole core polarization effect. The l -forbidden M1 transitions were analyzed in terms of the tensor terms.

Spin-isospin resonances in nuclei

Spin-isospin excitations in nuclei have been investigated via the (He-3,t) reaction at 450 MeV. The volume integrals of the effective interactions J(sigma tau) and J(tau) for the (He-3,t) reaction at 450 MeV have been empirically determined to be 172+/-17 MeV-fm(3) and 53+/-5 MeV-fm(3), respectively. These values are consistent with those obtained for the (p,n) reaction at the same bombarding energy per nucleon. The Gamow-Teller strength can be deduced with the same reliability as the (p,n) reaction at the bombarding energy above 100 MeV. The neutrino absorption cross sections by Ga-71 is obtained on basis of the present (He-3,t) measurement. The new SNU value obtained for Ga-71 is 130.3+/-1.6(stat.)+/-16.8(syst.). The spin-isospin responses for the Sn isotopes, Cd-116 and Mo-100 are presented and are discussed in relation with double beta-decay. A measurement of decay protons from the spin-isospin resonances in N-13 is presented. A new possibility of studying the spin-isospin responses on the beta(+) side via the (t,He-3) reaction using a dispersion matching technique is discussed.

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.

(3He,t) Charge-Exchange Reactions at E(3He) = 450 MeV, θ = 0°

Spin-isospin excitations in nuclei have been studied by means of the (3He,t) reactions at 450 MeV on the targets sBe, natC, *%ii, 58Ni, 62Ni, seZr, l18Sn, **OSn, 124Sn, and 154Sm. The new spectrometer “Grand Raiden” was employed for the (3He,t) study for the first time. Observed (3He,t) spectra at 0’ show remarkable similarity with those from the (p,n) results at intermediate energies, suggesting a simple direct reaction mechanism for the (3He,t) reaction at 450 MeV. The GamowTeller (GT) resonances and the spin-flip AL=1 resonances are excited very strongly. A fine structure of the GT resonances in medium-heavy nuclei is observed with an energy resolution of 210 keV. It is demonstrated that the (3He,t) reaction at 450 MeV is a powerful tool to study the spin-isospin excitations in nuclei.

Triple-focussing electron spectrum selector for in-beam conversion electron spectroscopy

Abstract A triple-focussing electron spectrum selector (TESS) is design. It is a non-dispersive (achromatic) magnet with intermediate double focussing. Electrons with a definite momentum range Δp / pπ 50% are selected by proper slits at the intermediate double focussing plane, and are focussed triply (radial, axial and momentum coordinates) at a small cooled Si detector, where the energy is analyzed. It is used for studying conversion electrons following nuclear reactions. The background is extremely small. It can be used for measurements of angular distributions and transition times.

Spin-isospin Responses of ^{71}Ga for Solar Neutrinos Studied by ^{71}Ga(^{3}He,t

Abstract The charge-exchange 71Ga(3He,tγ)71Ge reaction was used to study the spin-isospin (Gamow Teller: GT) excitations relevant to the axial-vector charged-weak response for solar neutrinos. High-lying GT states in 71Ge with excitation energies up to one MeV above the neutron threshold were found to have finite γ-decay branches to low-lying states. The GT strengths for relevant states and absorption rates of solar neutrinos through the 71Ga(ν, e)71Ge and 71Ga(ν,eγ)71Ge reactions were derived.