163 research outputs found
Nuclear pairing at finite temperature and angular momentum
An approach is proposed to nuclear pairing at finite temperature and angular
momentum, which includes the effects of the quasiparticle-number fluctuation
and dynamic coupling to pair vibrations within the self-consistent
quasiparticle random-phase approximation. The numerical calculations of pairing
gaps, total energies, and heat capacities are carried out within a doubly
folded multilevel model as well as several realistic nuclei. The results
obtained show that, in the region of moderate and strong couplings, the sharp
transition between the superconducting and normal phases is smoothed out,
causing a thermal pairing gap, which does not collapse at a critical
temperature predicted by the conventional Bardeen-Cooper-Schrieffer's (BCS)
theory, but has a tail extended to high temperatures. The theory also predicts
the appearance of a thermally assisted pairing in hot rotating nuclei.Comment: 4 pages, 1 figure, To appear in the Proceedings of the First Workshop
on State of the Art in Nuclear Cluster Physics, Strasbourg 13 - 16 May, 200
Canonical and microcanonical ensemble descriptions of thermal pairing within BCS and quasiparticle RPA
We propose a description of pairing properties in finite systems within the
canonical and microcanonical ensembles. The approach is derived by solving the
BCS and self-consistent quasiparticle random-phase approximation with the
Lipkin-Nogami particle-number projection at zero temperature. The obtained
eigenvalues are embedded into the canonical and microcanonical ensembles. The
results obtained are found in quite good agreement with the exact solutions of
the doubly-folded equidistant multilevel pairing model as well as the
experimental data for Fe nucleus. The merit of the present approach
resides in its simplicity and its application to a wider range of particle
number, where the exact solution is impracticable.Comment: 10 pages, 2 figures, accepted for publication in Phys. Rev.
Pairing effect on the giant dipole resonance width at low temperature
The width of the giant dipole resonance (GDR) at finite temperature T in
Sn-120 is calculated within the Phonon Damping Model including the neutron
thermal pairing gap determined from the modified BCS theory. It is shown that
the effect of thermal pairing causes a smaller GDR width at T below 2 MeV as
compared to the one obtained neglecting pairing. This improves significantly
the agreement between theory and experiment including the most recent data
point at T = 1 MeV.Comment: 8 pages, 5 figures to be published in Physical Review
Damping of giant dipole resonance in hot rotating nuclei
The phonon damping model (PDM) is extended to include the effect of angular
momentum at finite temperature. The model is applied to the study of damping of
giant dipole resonance (GDR) in hot and noncollectively rotating spherical
nuclei. The numerical results obtained for Mo88 and Sn106 show that the GDR
width increases with both temperature T and angular momentum M. At T > 4 MeV
and M<= 60 hbar the increase in the GDR width slows down for Sn106, whereas at
M<= 80 hbar the GDR widths in both nuclei nearly saturate. By adopting the
nuclear shear viscosity extracted from fission data at T= 0, it is shown that
the maximal value of the angular momentum for Mo88 and Sn106 should be around
46 and 55 hbar, respectively, so that the universal conjecture for the lower
bound of the specific shear viscosity for all fluids is not violated up to T= 5
MeV.Comment: 19 pages, 6 figures, accepted in Phys. Rev.
Specific shear viscosity in hot rotating systems of paired fermions
The specific shear viscosity of a classically rotating system of
nucleons that interact via a monopole pairing interaction is calculated
including the effects of thermal fluctuations and coupling to pair vibrations
within the selfconsistent quasiparticle random-phase approximation. It is found
that increases with angular momentum at a given temperature .
In medium and heavy systems, decreases with increasing at
2 MeV and this feature is not affected much by angular momentum. But in
lighter systems (with the mass number 20), increases with
at a value of close to the maximal value , which is defined as
the limiting angular momentum for each system. The values of
obtained within the schematic model as well as for systems with realistic
single-particle energies are always larger than the universal lower-bound
conjecture up to =5 MeV.Comment: 19 pages, 7 figures, accepted for publication in Phys. Rev.
Pairing reentrance in hot rotating nuclei
The pairing gaps, heat capacities and level densities are calculated within
the BCS-based quasiparticle approach including the effect of thermal
fluctuations on the pairing field within the pairing model plus noncollective
rotation along the z axis for Ni and Ge nuclei. The analysis of
the numerical results obtained shows that, in addition to the pairing gap, the
heat capacity can also serve as a good observable to detect the appearance of
the pairing reentrance in hot rotating nuclei, whereas such signature in the
level density is rather weak.Comment: 19 pages, 4 figures, accepted in Phys. Rev.
- …