Supernova Inelastic Neutrino-Nucleus Cross Sections from High-Resolution Electron Scattering Experiments and Shell-Model Calculations

Highly precise data on the magnetic dipole strength distributions from the Darmstadt electron linear accelerator for the nuclei 50Ti, 52Cr and 54Fe are dominated by isovector Gamow-Teller-like contributions and can therefore be translated into inelastic total and differential neutral-current neutrino-nucleus cross sections at supernova neutrino energies. The results agree well with large-scale shell-model calculations, validating this model.Comment: 5 pages, 4 figures, RevTeX 4, version accepted in Phys. Rev. Letter

Magnetic dipole probes of the sd and pf shell crossing in the A=36,38 argon isotopes

We have calculated the M1 strength distributions in the A=36,38 argon isotopes within large-scale shell model studies which consider valence nucleons in the sd and pf shells. While the M1 strength in 36Ar is well reproduced within the sd shell, the experimentally observed strong fragmentation of the M1 strength in 38Ar requires configuration mixing between the sd and the pf shells adding to our understanding of correlations across the N=20 shell gap.Comment: 14 pages, 8 figure

First Measurement of Collectivity of Coexisting Shapes based on Type II Shell Evolution: The Case of 96^{96}Zr

Background: Type II shell evolution has recently been identified as a microscopic cause for nuclear shape coexistence. Purpose: Establish a low-lying rotational band in 96-Zr. Methods: High-resolution inelastic electron scattering and a relative analysis of transition strengths are used. Results: The B(E2; 0_1^+ -> 2_2^+) value is measured and electromagnetic decay strengths of the secdond 2^+ state are deduced. Conclusions: Shape coexistence is established for 96-Zr. Type II shell evolution provides a systematic and quantitative mechanism to understand deformation at low excitation energies.Comment: 5 pages, 4 figure

Two-phonon 1- state in 112Sn observed in resonant photon scattering

Results of a photon scattering experiment on 112Sn using bremsstrahlung with an endpoint energy of E_0 = 3.8 MeV are reported. A J = 1 state at E_x = 3434(1) keV has been excited. Its decay width into the ground state amounts to Gamma_0 = 151(17) meV, making it a candidate for a [2+ x 3-]1- two-phonon state. The results for 112Sn are compared with quasiparticle-phonon model calculations as well as the systematics of the lowest-lying 1- states established in other even-mass tin isotopes. Contrary to findings in the heavier stable even-mass Sn isotopes, no 2+ states between 2 and 3.5 MeV excitation energy have been detected in the present experiment.Comment: 10 pages, including 2 figures, Phys. Rev. C, in pres

Twist Mode in Spherical Alkali Metal Clusters

A remarkable orbital quadrupole magnetic resonance, so-called twist mode, is predicted in alkali metal clusters where it is represented by $I^{\pi}=2^-$ low-energy excitations of valence electrons with strong M2 transitions to the ground state. We treat the twist by both macroscopic and microscopic ways. In the latter case, the shell structure of clusters is fully exploited, which is crucial for the considered size region ($8\le N_e\le 1314$). The energy-weighted sum rule is derived for the pseudo-Hamiltonian. In medium and heavy spherical clusters the twist dominates over its spin-dipole counterpart and becomes the most strong multipole magnetic mode.Comment: 8 pages, 4 figures, to be published in Phys. Rev. Lett., v.85, n.15, 200

Electric and magnetic dipole strength in ⁵⁸Ni from forward-angle proton scattering

Background: Electric and magnetic dipole strengths in nuclei at excitation energies well below the giant resonance region are of interest for a variety of nuclear structure problems including a possible electric dipole toroidal mode or the quenching of spin-isospin flip modes. Purpose: The aim of the present work is a state-by-state analysis of possible ⁢1 and ⁢1 transitions in 58Ni with a high-resolution (,′) experiment at 295 MeV and very forward angles including 0∘ and a comparison to results from studies of the dipole strength with the (,′) and (,′) reactions. Methods: The ⁢1 and ⁢1 cross sections of individual peaks in the spectra are deduced with a multipole decomposition analysis (MDA). They are converted to reduced ⁢1 and spin ⁢1 transition strengths using the virtual photon method of relativistic Coulomb excitation and the unit cross-section method, respectively. The experimental ⁢1 strength distribution is compared to large-scale shell-model calculations with the effective GXPF1A and KB3G interactions. Results: In total, 11 ⁢1 and 26 ⁢1 transitions could be uniquely identified in the excitation energy region 6–13 MeV. In addition, 22 dipole transitions with preference for either ⁢1 or ⁢1 multipolarity and 57 transitions with uncertain multipolarity were found. Despite the high level density good agreement is obtained for the deduced excitation energies of =1 states in the three types of experiments indicating that the same states are excited. The ⁡(⁢1) and ⁡(⁢1) strengths deduced in the (,′) experiments are systematically smaller than in the present work because of the lack of information on branching ratios to lower-lying excited states and the competition of particle emission. Fair agreement with the ⁡(⁢1) strengths extracted from the (,′) data is obtained after removal of ⁢1 transitions uniquely assigned in the present work belonging to a low-energy toroidal mode with unusual properties mimicking ⁢1 excitations in electron scattering. The shell-model calculations provide a good description of the isospin splitting and the running sum of the ⁢1 strength. A quenching factor 0.74 for the spin-isospin part of the ⁢1 operator is needed to attain quantitative agreement with the data. Conclusions: High-resolution forward-angle inelastic proton scattering experiments at beam energies of about 300 MeV are a highly selective tool for an extraction of resolved ⁢1 and ⁢1 strength distributions in medium-mass nuclei. Fair agreement with results from electron scattering experiments is obtained indicating a dominance of spin contributions to the ⁢1 strength. Shell-model calculations are in good agreement with gross properties of the ⁢1 strength distribution when a quenching factor for the spin-isospin part comparable to the one needed for a description of Gamow-Teller (GT) strength is included

Strong fragmentation of low-energy electromagnetic excitation strength in 117^{117}Sn

Results of nuclear resonance fluorescence experiments on $^{117}$Sn are reported. More than 50 $\gamma$ transitions with $E_{\gamma} < 4$ MeV were detected indicating a strong fragmentation of the electromagnetic excitation strength. For the first time microscopic calculations making use of a complete configuration space for low-lying states are performed in heavy odd-mass spherical nuclei. The theoretical predictions are in good agreement with the data. It is concluded that although the E1 transitions are the strongest ones also M1 and E2 decays contribute substantially to the observed spectra. In contrast to the neighboring even $^{116-124}$Sn, in $^{117}$Sn the $1^-$ component of the two-phonon $[2^+_1 \otimes 3^-_1]$ quintuplet built on top of the 1/2$^+$ ground state is proved to be strongly fragmented.Comment: 4 pages, 3 figure