24 research outputs found
Effect of deformation on two-neutrino double beta decay matrix elements
We study the effect of deformation on the two-neutrino double beta decay for
ground state to ground state transitions in all the nuclei whose half-lives
have been measured. Our theoretical framework is a deformed QRPA based in
Woods-Saxon or Hartree-Fock mean fields. We are able to reproduce at the same
time the main characteristics of the two single beta branches, as well as the
double beta matrix elements. We find a suppression of the double beta matrix
element with respect to the spherical case when the parent and daughter nuclei
have different deformations
Parity-Dependence in the Nuclear Level Density
Astrophysical reaction rates are sensitive to the parity distribution at low
excitation energies. We combine a formula for the energy-dependent parity
distribution with a microscopic-macroscopic nuclear level density. This
approach describes well the transition from low excitation energies, where a
single parity dominates, to high excitations where the two densities are equal.Comment: 4 pages, 3 figures; contribution to Nuclei In The Cosmos VIII, to
appear in Nucl. Phys.
Gamow-Teller strength distributions in 76Ge and 76Se from deformed quasiparticle random-phase approximation
10 pages, 4 tables, 8 figures.-- PACS nrs.: 23.40.Hc; 21.60.Jz; 27.50.1e.-- ArXiv pre-print available at: http://arxiv.org/abs/nucl-th/0303053v1We study Gamow-Teller strength distributions of 76Ge and 76Se within a deformed quasiparticle random-phase approximation formalism, which includes residual spin-isospin forces in the particle-hole and particle-particle 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.This work was supported by Ministerio de Ciencia y TecnologĂa (Spain) under Contract Nos. PB98/0676 and BFM2002-03562 and by International Graduiertenkolleg GRK683, by the "Land Baden-Wuerttemberg" within the "Landesforschungsschwerpunkt: Low Energy Neutrinos," and by the DFG under 436SLK 17/2/98.Peer reviewe
A deformed QRPA formalism for single and two-neutrino double beta decay
We use a deformed QRPA formalism to describe simultaneously the energy
distributions of the single beta Gamow-Teller strength and the two-neutrino
double beta decay matrix elements. Calculations are performed in a series of
double beta 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 particle-hole and particle-particle channels. We
discuss the sensitivity of the parent and daughter Gamow-Teller strength
distributions in single beta decay, as well as the sensitivity of the double
beta 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 beta decay. The double beta 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 the importance of a proper simultaneous description of both
double beta 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.Comment: 33 pages, 19 figures. To be published in Phys. Rev.
Proton-neutron pairing in the deformed BCS approach
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
Large-scale prediction of the parity distribution in the nuclear level density and application to astrophysical reaction rates
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.Comment: 15 pages, 17 figures; corrected/updated references in v2; additional
material can be found at http://nucastro.org/adndt.html#parit
Nuclear deformation and the two neutrino double-\beta decay in ^{124,126}Xe,^{128,130}Te, ^{130,132}Ba and ^{150}Nd isotopes
The two neutrino double beta decay of Xe,Te, Ba and Nd isotopes is studied in the Projected
Hartree-Fock-Bogoliubov (PHFB) model. Theoretical 2
half-lives of Te, and Nd isotopes, and 2, 2 and 2 for Xe and Ba nuclei are presented. Calculated quadrupolar
transition probabilities B(E2: ), static quadrupole moments and
factors in the parent and daughter nuclei reproduce the experimental
information, validating the reliability of the model wave functions. The
anticorrelation between nuclear deformation and the nuclear transition matrix
element is confirmed.Comment: 19 page