Pairing and the structure of the pf-shell N ~ Z nuclei

The influence of the isoscalar and isovector L=0 pairing components of the effective nucleon-nucleon interaction is evaluated for several isobaric chains, in the framework of full pf shell model calculations. We show that the combined effect of both isospin channels of the pairing force is responsible for the appearance of T=1 ground states in N=Z odd-odd nuclei. However, no evidence is found relating them to the Wigner energy. We study the dependence of their contributions to the total energy on the rotational frecuency in the deformed nucleus 48Cr. Both decrease with increasing angular momentum and go to zero at the band termination. Below the backbending their net effect is a reduction of the moment of inertia, more than half of which comes from the proton-neutron channel.Comment: 5 pages, RevTeX, 5 figure

Band Crossing and Signature Splitting in Odd Mass fp Shell Nuclei

Structure of two sets of mirror nuclei: 47V - 47Cr and 49Cr - 49Mn, as well as 49V and 51Mn, is studied using the projected shell model. Their yrast spectra are described as an interplay between the angular momentum projected states around the Fermi level which carry different intrinsic K-quantum numbers. The deviations from a regular rotational sequence are attributed to band crossing and signature splitting, which are usually discussed in heavy nuclear systems. Our results agree reasonably with experimental data, and are comparable with those from the full $pf$ shell model calculations.Comment: 3 figures, submitted to Nucl. Phys.

Neutrino absortion cross sections in supernova environment

We study charged-current neutrino cross sections on neutronrich nuclei in the mass $A\sim60$ region. Special attention is paid to environmental effects, i.e. finite temperature and density, on the cross sections. As these effects are largest for small neutrino energies, it is sufficient to study only the Gamow-Teller (GT) contributions to the cross sections. The relevant GT strength distributions are derived from large-scale shell model calculations. We find that the low-energy cross sections are enhanced at finite temperatures. However, for $(\nu_e,e^-)$ reactions Pauli blocking of the electrons in the final state makes the cross sections for low-energy neutrinos much smaller than for the competing inelastic scattering on electrons at moderate and large densities. Absorption cross sections for low-energy antineutrinos are strongly enhanced at finite temperatures.Comment: 11 pages, 4 figure

Shell model study of the isobaric chains A=50, A=51 and A=52

Shell model calculations in the full pf-shell are carried out for the A=50, 51 and 52 isobars. The most frequently used effective interactions for the pf-shell, KB3 and FPD6 are revisited and their behaviour at the N=28 and Z=28 closures examined. Cures to their -relatively minor- defaults are proposed, and a new mass dependent version called KB3G is released. Energy spectra, electromagnetic transitions and moments as well as beta decay properties are computed and compared with the experiment and with the results of the earlier interactions. A high quality description is achieved. Other miscellaneous topics are addressed; the Coulomb energy differences of the yrast states of the mirror pair 51Mn-51Fe and the systematics of the magnetic moments of the N=28 isotones.Comment: 45 pages, 34 figures, Latex. Submitted for publicatio

Neutral-current neutrino reactions in the supernova environment

We study the neutral-current neutrino scattering for four nuclei in the iron region. We evaluate the cross sections for the relevant temperatures during the supernova core collapse and derive Gamow-Teller distributions from large-scale shell-model calculations. We show that the thermal population of the excited states significantly enhances the cross sections at low neutrino energies. Calculations of the outgoing neutrino spectra indicate the prospect of neutrino upscattering at finite temperatures. Both results are particularly notable in even-even nuclei.Comment: 14 pages, 4 figures, accepted in Phys. Lett. B

Quasi-SU(3) truncation scheme for even-even sd-shell nuclei

The Quasi-SU(3) symmetry was uncovered in full pf and sdg shell-model calculations for both even-even and odd-even nuclei. It manifests itself through a dominance of single-particle and quadrupole-quadrupole terms in the Hamiltonian used to describe well-deformed nuclei. A practical consequence of the quasi-SU(3) symmetry is an efficient basis truncation scheme. In a recent work was shown that when this type of Hamiltonian is diagonalized in an SU(3) basis, only a few irreducible represntations (irreps) of SU(3) are needed to describe the Yrast band, the leading S = 0 irrep augmented with the leading S = 1 irreps in the proton and neutron subspaces. In the present article the quasi-SU(3) truncation scheme is used, in conjunction with a "realistic but schematic" Hamiltonian that includes the most important multipole terms, to describe the energy spectra and B(E2) transition strengths of 20-Ne, 22-Ne, 24-Mg and 28-Si. The effect of the size of the Hilbert space on both sets of observables is discussed, as well as the structure of the Yrast band and the importance of the various terms in the Hamiltonian.Comment: 30 pages, 8 figures. Submited to Nucl. Phys.

Isovector pairing in odd-odd N=Z 50Mn

High-spin states in the odd–odd N=Z nucleus 5025Mn have been investigated. A sequence of states up to Jπ=6+ has been assigned as the T=1 analogue of the yrast band in 5024Cr for the first time. The differences in energy between levels in these bands are interpreted in terms of rotational alignments and the effect they have on the Coulomb energy of the nucleus. Comparisons with shell model calculations show that the Coulomb energy difference between the T=1 analogue structures is an important indicator of the competition between isovector pairing modes in N=Z nuclei and their isobars