Program on Nuclear Reaction Phenomenology
Investigator
Constance
Kalbach Walker
(Senior Research Scientist)
Physics Department – Box 90305
Duke University
Durham NC 27708-0305
USA
Email: cwalker@tunl.duke.edu
Program Overview
The TUNL Program on Nuclear Reaction
Phenomenology develops models to describe preequilibrium nuclear
reactions. In these reactions, particle
emission occurs before the energy brought into the system by the bombarding
particle has become randomly distributed throughout the nucleus. The program is built around the exciton
model, first proposed by J. J. Griffin in 1966, though this is supplemented by
a number of auxiliary models. The models
are implemented in the computer code PRECO, which is being constantly
updated. Versions of the code, written
in Fortran-77, are periodically released to the public along with extensive
documentation of the physics and the code itself. In recent years, much of this work was
conducted as part of the U.S. Nuclear
Data Program, administered through the National Nuclear Data Center at
Brookhaven National Laboratory, though this funding has been terminated. The work is carried out at the Triangle Universities Nuclear Laboratory
or TUNL
The exciton model is a simple, statistical model
for describing the continuum energy differential and double differential cross
sections of particles emitted during energy equilibration in light particle
induced reactions at incident energies of around 14 to 200 MeV. Because of its simplicity, its physical
transparency, its utility, and its adaptability, the exciton model continues to
be used in spite of the development of more microscopic and quantum mechanical
models. In the PRECO code, the exciton
model is supplemented by a number of smaller models treating direct reaction
mechanisms not described by the exciton model.
These direct reactions are particularly important for reactions with
complex particles (deuterons, tritons, He-3, and alpha particles) in the
entrance and/or exit channel. Model and
code development uses simple physical concepts and relies on available data
both to direct choices between alternative formulations of a model concept and
to provide values for key model parameters that cannot be obtained from
independent sources.
The code PRECO has been used around the world,
either alone or in Hauser-Feshbach model codes, for both basic and applied
research. The models, code and global
input set are being refined and benchmarked against a broad range of energy
spectra from the literature in order to allow the reliable calculation of
unmeasured or unmeasurable spectra without the use of adjustable
parameters. For many years the primary
focus was on reactions with only nucleons (neutrons and protons) in the
entrance and exit channels. More
recently that focus has shifted to reactions involving complex particles. The description of these reactions was
significantly updated in PRECO-2000 and again in PRECO-2006. One additional mechanism, projectile breakup,
is still undergoing study and is not yet fully implemented.
Code Features
Both PRECO-2000 and PRECO-2006 permit the
simultaneous inclusion of shell structure, pairing, isospin conservation, and
finite potential well depth effects in a two-component version of the exciton
model. The finite well depth corrections
include additional limitations that apply when the initial target-projectile
interaction occurs primarily near the nuclear surface. Primary preequilibrium emission of all light
particles from nucleons through alpha particles is considered, while secondary
emission is only calculated for neutrons and protons.
Additional models in the code give a
phenomenological statistical description of the direct transfer of one or more
nucleons (stripping, pickup and exchange reactions) and of direct knockout and
inelastic scattering involving cluster degrees of freedom. The direct transfer model has been
extensively updated for PRECO-2006. Excitation of strong collective states --
both discrete spectroscopic states and giant resonance states -- is also
included, along with a highly over-simplified model for elastic
scattering. Angular distributions at
specific emission energies are calculated using global, empirical systematics. A model for projectile the breakup is not yet
included but will be important for a complete description of reactions induced
by deuterons, tritons, He-3 ions, and, at higher incident energies, alpha
particles.
Further details about the code are available in
the extensive users manual which describes all of the incorporated physics as
well as the structure and use of the code.
Ongoing Work
Additional work is being conducted to refine a
preliminary model for deuteron breakup and to extend it to incident tritons,
He-3 and alpha particles. Additional
detailed benchmarking of the exciton model at incident energies above 60 MeV is
also needed, as new data available in the literature offer the possibility of
resolving some existing questions about the interplay between surface effects,
the collective excitation model, and the relative normalizations of the
mean-square effective matrix elements for the n-n, p-p and n-p residual
interactions.
Code Availability
PRECO-2006 was formally released in July 2007
and is currently available as Fortran-77 source code plus a users manual from
the National Nuclear
Data Center at Brookhaven National Laboratory and the Radiation Safety Information Computational
Center at Oak Ridge National Laboratory (see the code description). Sample input and output files and a Windows
executable file are also included in the code package.
Last modified: 2 August 2007