5,152 research outputs found
Level Densities by Particle-Number Reprojection Monte Carlo Methods
A particle-number reprojection method is applied in the framework of the
shell model Monte Carlo approach to calculate level densities for a family of
nuclei using Monte Carlo sampling for a single nucleus. In particular we can
also calculate level densities of odd-even and odd-odd nuclei despite a new
sign problem introduced by the projection on an odd number of particles. The
method is applied to level densities in the iron region using the complete
-shell. The single-particle level density parameter and the
backshift parameter are extracted by fitting the microscopically
calculated level densities to the backshifted Bethe formula. We find good
agreement with experimental level densities with no adjustable parameters in
the microscopic calculations. The parameter is found to vary smoothly with
mass and does not show odd-even effects. The calculated backshift parameter
displays an odd-even staggering effect versus mass and is in better
agreement with the experimental data than are the empirical values.Comment: To be published in the proceedings of the Tenth International
Symposium on Capture Gamma-Ray Spectroscopy and Related Topics, S. Wender,
ed., AIP Conference Proceedings (2000
Collectivity in Heavy Nuclei in the Shell Model Monte Carlo Approach
The microscopic description of collectivity in heavy nuclei in the framework
of the configuration-interaction shell model has been a major challenge. The
size of the model space required for the description of heavy nuclei prohibits
the use of conventional diagonalization methods. We have overcome this
difficulty by using the shell model Monte Carlo (SMMC) method, which can treat
model spaces that are many orders of magnitude larger than those that can be
treated by conventional methods. We identify a thermal observable that can
distinguish between vibrational and rotational collectivity and use it to
describe the crossover from vibrational to rotational collectivity in families
of even-even rare-earth isotopes. We calculate the state densities in these
nuclei and find them to be in close agreement with experimental data. We also
calculate the collective enhancement factors of the corresponding level
densities and find that their decay with excitation energy is correlated with
the pairing and shape phase transitions.Comment: 6 pages, 3 figures, to be published in the Proceedings of the Fourth
International Workshop on Compound-Nuclear Reactions and Related Topics
(CNR*13
Crossover from vibrational to rotational collectivity in heavy nuclei in the shell-model Monte Carlo approach
Heavy nuclei exhibit a crossover from vibrational to rotational collectivity
as the number of neutrons or protons increases from shell closure towards
midshell, but the microscopic description of this crossover has been a major
challenge. We apply the shell model Monte Carlo approach to families of
even-even samarium and neodymium isotopes and identify a microscopic signature
of the crossover from vibrational to rotational collectivity in the
low-temperature behavior of , where is the total spin
and is the temperature. This signature agrees well with its values
extracted from experimental data. We also calculate the state densities of
these nuclei and find them to be in very good agreement with experimental data.
Finally, we define a collective enhancement factor from the ratio of the total
state density to the intrinsic state density as calculated in the
finite-temperature Hartree-Fock-Bogoliubov approximation. The decay of this
enhancement factor with excitation energy is found to correlate with the
pairing and shape phase transitions in these nuclei.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. Let
Signatures of phase transitions in nuclei at finite excitation energies
The mean-field approximation predicts pairing and shape phase transitions in
nuclei as a function of temperature or excitation energy. However, in the
finite nucleus the singularities of these phase transitions are smoothed out by
quantal and thermal fluctuations. An interesting question is whether signatures
of these transitions survive despite the large fluctuations. The shell model
Monte Carlo (SMMC) approach enables us to calculate the statistical properties
of nuclei beyond the mean-field approximation in model spaces that are many
orders of magnitude larger than spaces that can be treated by conventional
diagonalization methods. We have extended the SMMC method to heavy nuclei and
used it to study the transition from vibrational (spherical) to rotational
(deformed) nuclei in families of rare-earth isotopes. We have calculated
collective enhancement factors of level densities as a function of excitation
energy and found that the decay of the vibrational and rotational enhancements
is well correlated with the pairing and shape phase transitions, respectively.Comment: 8 pages, 3 figures, to be published in the Proceedings of Beauty in
Physics: Theory and Experimen
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