230 research outputs found
Random-matrix approach to the statistical compound nuclear reaction at low energies using the Monte-Carlo technique
Using a random-matrix approach and Monte-Carlo simulations, we generate
scattering matrices and cross sections for compound-nucleus reactions. In the
absence of direct reactions we compare the average cross sections with the
analytic solution given by the Gaussian Orthogonal Ensemble (GOE) triple
integral, and with predictions of statistical approaches such as the ones due
to Moldauer, to Hofmann, Richert, Tepel, and Weidenm\"{u}ller, and to Kawai,
Kerman, and McVoy. We find perfect agreement with the GOE triple integral and
display the limits of validity of the latter approaches. We establish a
criterion for the width of the energy-averaging interval such that the relative
difference between the ensemble-averaged and the energy-averaged scattering
matrices lies below a given bound. Direct reactions are simulated in terms of
an energy-independent background matrix. In that case, cross sections averaged
over the ensemble of Monte-Carlo simulations fully agree with results from the
Engelbrecht-Weidenm\"{u}ller transformation. The limits of other approximate
approaches are displayed
Emission of Scission Neutrons in the Sudden Approximation
At a certain finite neck radius during the descent of a fissioning nucleus
from the saddle to the scission point, the attractive nuclear forces can no
more withstand the repulsive Coulomb forces producing the neck rupture and the
sudden absorption of the neck stubs by the fragments. At that moment, the
neutrons, although still characterized by their pre-scission wave functions,
find themselves in the newly created potential of their interaction with the
separated fragments. Their wave functions become wave packets with components
in the continuum. The probability to populate such states gives evidently the
emission probability of neutrons at scission. In this way, we have studied
scission neutrons for the fissioning nucleus U, using two-dimensional
realistic nuclear shapes. Both the emission probability and the distribution of
the emission points relative to the fission fragments strongly depend on the
quantum numbers of the pre-scission state from which the neutron is emitted. In
particular it was found that states with = 1/2+ dominate the
emission. Depending on the assumed pre- and post-scission configurations and on
the emission-barrier height, 30 to 50% of the total scission neutrons are
emitted from 1/2+ states. Their emission points are concentrated in the region
between the newly separated fragments. The upper limit for the total number of
neutrons per scission event is predicted to lie between 0.16 and 1.73
(depending on the computational assumptions).Comment: 31 pages, 16 figures, 2 table
Correlated Prompt Fission Data in Transport Simulations
Detailed information on the fission process can be inferred from the
observation, modeling and theoretical understanding of prompt fission neutron
and -ray~observables. Beyond simple average quantities, the study of
distributions and correlations in prompt data, e.g., multiplicity-dependent
neutron and \gray~spectra, angular distributions of the emitted particles,
-, -, and -~correlations, can place stringent
constraints on fission models and parameters that would otherwise be free to be
tuned separately to represent individual fission observables. The FREYA~and
CGMF~codes have been developed to follow the sequential emissions of prompt
neutrons and -rays~from the initial excited fission fragments produced
right after scission. Both codes implement Monte Carlo techniques to sample
initial fission fragment configurations in mass, charge and kinetic energy and
sample probabilities of neutron and ~emission at each stage of the
decay. This approach naturally leads to using simple but powerful statistical
techniques to infer distributions and correlations among many observables and
model parameters. The comparison of model calculations with experimental data
provides a rich arena for testing various nuclear physics models such as those
related to the nuclear structure and level densities of neutron-rich nuclei,
the -ray~strength functions of dipole and quadrupole transitions, the
mechanism for dividing the excitation energy between the two nascent fragments
near scission, and the mechanisms behind the production of angular momentum in
the fragments, etc. Beyond the obvious interest from a fundamental physics
point of view, such studies are also important for addressing data needs in
various nuclear applications. (See text for full abstract.)Comment: 39 pages, 57 figure files, published in Eur. Phys. J. A, reference
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Time-dependent properties of proton decay from crossing single-particle metastable states in deformed nuclei
A dynamical study of the decay of a metastable state by quantum tunneling
through an anisotropic, non separable, two-dimensional potential barrier is
performed by the numerical solution of the time-dependent Schrodinger equation.
Initial quasi- stationary proton states are chosen in the framework of a
deformed Woods-Saxon single-particle model. The decay of two sets of states
corresponding to true and quasi levels-crossing is studied and the evolution of
their decay properties as a function of nuclear deformation is calculated
around the crossing point. The results show that the investigation of the
proton decay from metastable states in deformed nuclei can unambiguously
distinguish between the two types of crossing and determine the structure of
the nuclear states involved.Comment: 15 pages, 9 figures, submitted to Phys. Rev.
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