87 research outputs found
Astrophysical Reaction Rates From Statistical Model Calculations
Theoretical reaction rates in the temperature range 0.01*10^9<=T[K]<=10.*10^9
are calculated in the statistical model (Hauser-Feshbach formalism) for targets
with 10<=Z<=83 (Ne to Bi) and for a mass range reaching the neutron and proton
driplines. Reactions considered are (n,gamma), (n,p), (n,alpha), (p,gamma),
(p,alpha),(alpha,gamma), and their inverse reactions. Reaction rates as a
function of temperature for thermally populated targets are given by analytic
seven parameter fits. To facilitate comparison with experimental rates, the
stellar enhancement factors are also tabulated. Two complete sets of rates have
been calculated, one of which includes a phenomenological treatment of shell
quenching for neutron-rich nuclei. These extensive datasets are provided
on-line as electronic files, while a selected subset from one calculation is
given as printed tables. A summary of the theoretical inputs and advice on the
use of the provided tabulations is included.Comment: 22 pages of text and 1 table; accepted by Atomic Data Nuclear Data
Tables; a preprint is also available from
http://quasar.physik.unibas.ch/~tommy/adndt.htm
Pairing correlation in nuclear matter from Skyrme force
The properties of pairing correlation in nuclear matter are investigated by
using various versions of Skyrme forces. Truncation of states involving pairing
correlation, necessary due to zero range nature of the Skyrme force, is
discussed in detail. A plateau appears in pairing gap versus cutoff for each
force. We propose to choose the cutoff parameter in the middle of the plateau
so that the parameterization is independent of nuclides.Comment: Latex, Submitted to Phys. Lett. B, 8 pages, 1 table and 3 uuencoded
postscript figures (modified) included, UT-Komaba 94-
Isospin dependence of nuclear matter symmetry energy coefficients
Generalized symmetry energy coefficients of asymmetric nuclear matter are
obtained as screening functions. The dependence of the isospin symmetry energy
coefficient on the neutron proton (n-p) asymmetry may be determined unless by a
constant (exponent) which depend on microscopic properties. The dependence
of the generalized symmetry energy coefficients with Skyrme forces on the n-p
asymmetry and on the density, only from .5 up to 1.5 , are investigated
in the isospin and scalar channels. The use of Skyrme-type effective forces
allows us to obtain analytical expressions for these parameters as well as
their dependences on the neutron-proton (n-p) asymmetry, density and even
temperature. Whereas the density dependence of these coefficients obtained with
Skyrme forces is not necessarily realistic the dependence on the n-p asymmetry
exhibit a more consistent behaviour. The isospin symmetry energy coefficient
(s.e.c.) increases as the n-p asymmetry acquires higher values whereas the
isoscalar s.e.c. decreases. Some consequences for the Supernovae mechanism are
discussed.Comment: 17 pages (latex) plus four figures in two eps files. To be published
in Nucl. Phys.
Deformed Hartree-Fock Calculation of Proton-Rich Nuclei
We perform Hartree-Fock+BCS calculations for even-even nuclei with 2 <= Z <=
82 and N ranging from outside the proton drip line to the experimental frontier
on the neutron-rich side. The ground state solutions are obtained for 737
nuclei, together with shape-coexistence solutions for 480 nuclei. Our method
features the Cartesian-mesh representation of single-particle wavefunctions,
which is advantageous in treating nucleon skins and exotic shapes. The results
are compared with those of the finite-range droplet model of Moller et al. as
well as the experimental values.Comment: 7 pages Latex, 5 postscript figures appended as uufil
Competition Between T=0 and T=1 Pairing in Proton-Rich Nuclei
A cranked mean-field model with two-body T=1 and T=0 pairing interactions is
presented. Approximate projection onto good particle-number is enforced via an
extended Lipkin-Nogami scheme. Our calculations suggest the simultaneous
presence of both T=0 and T=1 pairing modes in N=Z nuclei. The transitions
between different pairing phases are discussed as a function of neutron/proton
excess, T, and rotational frequency, . The additional binding
energy due to the T=0 -pairing correlations, is suggested as a possible
microscopic explanation of the Wigner energy term in even-even nuclei.Comment: 8 RevTeX pages, 3 uuencoded POSTSCRIPT figures include
The puzzle of the synthesis of the rare nuclide 138La
The calculations of the p-process in the O/Ne layers of Type II supernovae
are quite successful in reproducung the solar system content of p-nuclides.
They predict, however, a significant underproduction of the rare odd-odd
nuclide 138La. A model for the explosion of a 25 Mo star with solar metallicity
is used to suggest that electron neutrino captures on 138Ba may well be its
most efficient production mechanism. The responsibility of an inadequate
prediction of the 138La and 139La photodisintegration rates in the too low
production of 138La is also examined quantitatively. A detailed discussion of
the theoretical uncertainties in these rates suggest that the required rate
changes are probably too high to be fully plausible. Their measurement would be
most welcome. They would help disentangling the relative contributions of
thermonuclear and neutrino processes to the 138La production.Comment: 4 pages to be published by A&A Letter
Effect of nuclear periphery on nucleon transfer in peripheral collisions
A comparison of experimental heavy residue cross sections from the reactions
86Kr+64Ni,112,124Sn with the model of deep-inelastic transfer (DIT) is carried
out. A modified expression for nucleon transfer probabilities is used at
non-overlapping projectile-target configurations, introducing a dependence on
isospin asymmetry at the nuclear periphery. The experimental yields of
neutron-rich nuclei close to the projectile are reproduced better and the trend
deviating from the bulk isospin equilibration is explained. For the
neutron-rich products further from the projectile, originating from hot
quasiprojectiles, the statistical multifragmentation model reproduces the mass
distributions better than the model of sequential binary decay. In the reaction
with proton-rich target 112Sn the nucleon exchange appears to depend on isospin
asymmetry of nuclear periphery only when surface separation is larger than 0.8
fm due to the stronger Coulomb interaction at more compact di-nuclear
configuration.Comment: LaTeX, 13 pages, 7 figures, to appear in Nuclear Physics
Study of Superdeformation in Non-rotating States using the Skyrme-Hartree-Fock Method
The superdeformation (SD) in non-rotating states is studied with the HF+BCS
method using the Skyrme interaction. In applying the BCS theory, the seniority
pairing force is employed, of which strengths are determined in order to
reproduce the empirical pairing gap formula, MeV,
through a smooth level density obtained in the Thomas-Fermi approximation.
Properties of superdeformation are investigated by calculating potential energy
surfaces (PES) for various sets of the pairing force strengths and the Skyrme
force parameter for 194Hg and 236,238U. The best results are obtained using
both the SkM* force and the pairing force strength determined in this paper. By
making use of this set of forces, a systematic calculation of SD states is
carried out extensively for even-even nuclei for 20 <= Z <= 82. From our
calculation, the barriers preventing the decay into the normally deformed
states are about twice as high as those predicted by Krieger et al., who used
the same Skyrme interaction but a pairing force stronger than ours. The
differences of the present results from the Nilsson-Strutinsky calculation are
analyzed.Comment: 19 pages in LaTex, 11 Postscript figure
Extensive Hartree-Fock + BCS calculation with Skyrme SIII force
We have performed deformed Hartree-Fock+BCS calculations with the Skyrme SIII
force for the ground states of even-even nuclei with 2 <= Z <= 114 and N
ranging from outside the proton drip line to beyond the experimental frontier
in the neutron-rich side. We obtained spatially localized solutions for 1029
nuclei, together with the second minima for 758 nuclei. The single-particle
wavefunctions are expressed in a three-dimensional Cartesian-mesh
representation, which is suitable to describe nucleon skins, halos, and exotic
shapes as well as properties of ordinary stable nuclei. After explaining some
of the practical procedures of the calculations, we compare the resulting
nuclear masses with experimental data and the predictions of other models. We
also discuss the quadrupole (m=0, 2) and hexadecapole (m=0, 2, 4) deformations,
the skin thicknesses, the halo radii, and the energy difference between the
oblate and the prolate solutions. Our results can be obtained via computer
network.Comment: 20 pages in Latex, 11 Postscript figures, uuencode-gzip-tar file, to
appear in Nuclear Physics A. Data tables available at
ftp://nt1.c.u-tokyo.ac.jp/hfs3
Computer program for the relativistic mean field description of the ground state properties of even-even axially deformed nuclei
A Fortran program for the calculation of the ground state properties of
axially deformed even-even nuclei in the relativistic framework is presented.
In this relativistic mean field (RMF) approach a set of coupled differential
equations namely the Dirac equation with potential terms for the nucleons and
the Glein-Gordon type equations with sources for the meson and the
electromagnetic fields are to be solved self-consistently. The well tested
basis expansion method is used for this purpose. Accordingly a set of harmonic
oscillator basis generated by an axially deformed potential are used in the
expansion. The solution gives the nucleon spinors, the fields and level
occupancies, which are used in the calculation of the ground state properties.Comment: 18 pages, LaTex, 6 p.s figures, To appear in Comput. Phys. Commu
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