The effect of deformation on the two-neutrino double decay (2νββ-decay) for ground state transition 76 Ge → 76 Se is studied in the framework of the deformed QRPA with separable Gamow-Teller residual interaction. A new suppression mechanism of the 2νββ-decay matrix element based on the difference in deformations of the initial and final nuclei is included. An advantage of this suppression mechanism in comparison with that associated with ground state correlations is that it allows a simultaneous description of the single β and the 2νββ-decay. By performing a detail calculation of the 2νββ-decay of 76 Ge, it is found that the states of intermediate nucleus lying in the region of the Gamow-Teller resonance contribute significantly to the matrix element of this process.

We use a deformed quasiparticle random phase approximation formalism to describe simultaneously the energy distributions of the single ␤ Gamow-Teller strength and the two-neutrino double ␤ decay matrix elements. Calculations are performed in a series of double ␤ decay partners with A = 48, 76, 82, 96, 100, 116, 128, 130, 136, and 150, using deformed Woods-Saxon potentials and deformed Skyrme Hartree-Fock mean fields. The formalism includes a quasiparticle deformed basis and residual spin-isospin forces in the particlehole and particle-particle channels. We discuss the sensitivity of the parent and daughter Gamow-Teller strength distributions in single ␤ decay, as well as the sensitivity of the double ␤ decay matrix elements to the deformed mean field and to the residual interactions. Nuclear deformation is found to be a mechanism of suppression of the two-neutrino double ␤ decay. The double ␤ decay matrix elements are found to have maximum values for about equal deformations of parent and daughter nuclei. They decrease rapidly when differences in deformations increase. We remark on the importance of a proper simultaneous description of both double ␤ decay and single Gamow-Teller strength distributions. Finally, we conclude that for further progress in the field, it would be useful to improve and complete the experimental information on the studied Gamow-Teller strengths and nuclear deformations.

A generalized method to calculate the excitation-energy dependent parity ratio in the nuclear level density is presented, using the assumption of Poisson distributed independent quasi particles combined with BCS occupation numbers. It is found that it is crucial to employ a sufficiently large model space to allow excitations both from low-lying shells and to higher shells beyond a single major shell. Parity ratios are only found to equilibrate above at least 5-10 MeV of excitation energy. Furthermore, an overshooting effect close to major shells is found where the parity opposite to the ground state parity may dominate across a range of several MeV before the parity ratio finally equilibrates. The method is suited for large-scale calculations as needed, for example, in astrophysical applications. Parity distributions were computed for all nuclei from the proton dripline to the neutron dripline and from Ne up to Bi. These results were then used to recalculate astrophysical reaction rates in a Hauser-Feshbach statistical model. Although certain transitions can be considerably enhanced or suppressed, the impact on astrophysically relevant reactions remains limited, mainly due to the thermal population of target states in stellar reaction rates.

We examine isovector and isoscalar proton-neutron pairing correlations for the ground state of even-even Ge isotopes with mass number A=64-76 within the deformed BCS approach. For N=Z 64Ge the BCS solution with only T=0 proton-neutron pairs is found. For other nuclear systems (N>Z) a coexistence of a T=0 and T=1 pairs in the BCS wave function is observed. A problem of fixing of strengths of isoscalar and isovector pairing interactions is addressed. A dependence of number of like and unlike pairs in the BCS ground state on the difference between number of neutrons and protons is discussed. We found that for nuclei with N much bigger than Z the effect of proton-neutron pairing is small but not negligible.Comment: 24 pages, 6 figure

We study Gamow-Teller strength distributions of 76 Ge and 76 Se within a deformed quasiparticle randomphase approximation formalism, which includes residual spin-isospin forces in the particle-hole and particleparticle channels. We consider two different methods to construct the quasiparticle basis, a self-consistent approach based on a deformed Hartree-Fock calculation with density-dependent Skyrme forces and a more phenomenological approach based on a deformed Woods-Saxon potential. Both methods contain pairing correlations in the BCS approach. We discuss the sensitivity of Gamow-Teller strength distributions to the deformed mean field and residual interactions.