UCRL-PROP-220751

**Nuclear Properties**

April 20, 2006

Nuclear reactions and nuclear structure play an essential role in the
science to be investigated in the Science-Based Stockpile Stewardship
Program. In addition, there are nuclear physics applications relying on
similar data in Threat Reduction, next-generation nuclear reactors,
nuclear astrophysics, and at the Rare Isotope Accelerator (RIA).

Our first challenge is to arrive at a comprehensive and unified
description of nuclei and their reactions that is grounded in the
interactions between the constituent nucleons. Current phenomenological
models of nuclear structure and reactions should be replaced with a
well-founded microscopic (e.g. ab-initio) theory that will deliver
maximum predictive power with minimal uncertainties that are well
quantified. To achieve this goal requires a national effort linking
theoretical physics and computational science together to develop
forefront software for state-of-the-art architectures. Our second
challenge applies to heavy nuclei, such as actinides, where truly
ab-initio methods are still beyond our reach: This challenge is to
develop more rigorous and predictive theories for fission, and radiative
capture, and use a rigorous validation methodology to demonstrate the
improved predictive capability.

A national capability to calculate nuclear structure and low-energy
nuclear cross sections (and to assess their uncertainties) will require
the development of a multi-pronged program of theoretical, algorithmic,
and computational improvements that will utilize leadership-class
computation to solve several long-standing fundamental problems in
nuclear physics, and will deliver nuclear cross section information
critical to DOE programs that is more accurate than is currently
available. DOE needs this capability because in many instances
experimental data are not available or obtainable even with the
construction of advanced exotic beam facilities. Thus, a reliance on
theory or extrapolations from known data is required in order to make
predictions. An expansion of the computational techniques and methods
currently employed, and developments of new treatments, will be
necessary to take advantage of petascale architectures and to
demonstrate the capability of the leadership class machines to deliver
new science heretofore impossible. The effort would entail both
improving the predictive power of nuclear calculations by connecting
microscopic effective interactions to the basic nuclear interactions,
and assessing quantitative error estimates of the results.

The need to develop better theory for the description of nuclear
structure and reaction cross sections for various applications has been
pointed out in several high-level documents including the Nuclear
Science Advisory Committee (NSAC) 2004 Report: *A Vision for Nuclear
Theory*; the NSAC 2005 Report: *Guidance for Implementing the 2002
Long-Range Plan*; and the DOE *Path to Sustainable Nuclear Energy*
(2005), which points to the need for cross sections in planning for the
advanced reactor technology. During the past 10 years, the nuclear
theory community has been pursuing developments of sophisticated
computational techniques to investigate properties of nuclei based only
on the underlying nuclear forces. These developments have led to a new
understanding of light nuclei using ab-initio many-body methods, and of
heavier nuclei using self-consistent mean-field techniques. Furthermore,
the first steps have been taken to understand nuclear reactions using
ab-initio techniques.

Nuclear reactions involving fission to date are modeled in a
semi-phenomenological way. In part this is because of the extreme
sensitivity of the calculated fission cross section to nuclear structure
properties such as fission barriers and nuclear level densities, at
various nuclear deformations. However many applications, including
advanced reactors and fuel cycles, need more accurate predictions of
fission cross sections for shorter lived unstable species where few
reliable measurements exist. We need more advanced theories for fission,
combined with nuclear reaction theories that utilize these advances
(including possible dynamical effects), to predict fission cross
sections for unstable actinide species. A deeper understanding is also
needed for post-scission fission physics, regarding our understanding of
the energy spectrum of emitted fission neutrons and gamma-rays, and
fission fragment distributions. The significant challenge for the
community here is to demonstrate increased predictive power for a new
fission theory, i.e. not just to develop more sophisticated fission
theories, but to demonstrate through a more rigorous validation process
that such a new theory indeed has an increased predictive capability for
fission cross sections.

We also need to have more precise predictions of radiative neutron
capture cross sections in the keV-MeV region. Capture is notoriously
difficult to predict theoretically. However, a wide range of
applications (s and r-process nucleosynthesis, advanced reactors and
transmutation technologies, as well as NNSA applications) need more
accurate theoretical methods to extrapolate from measured on-stability
regions to off-stability regions that are important in these extreme
environments. We should emphasize, though, that improved capture cross
sections are also needed for stable targets, because of historical
difficulties in precise measurements. Thus, in the higher energy region
(400 keV-1 MeV, say) few reliable measurements exist, and methods --
both theoretical, and phenomenological methods tied to measurements --
are needed to more accurately determine these capture cross sections.
Advance reactor technologies need such data for the actinides.

We need to understand neutron cross sections, nuclear decay, and fission
on many nuclei ranging from light to heavy systems. Calculation of
neutron cross sections requires a detailed knowledge of nuclear
properties. Theoretical error estimates for cross sections will be
relevant to covariance studies for various evaluations.

Examples of specific research areas include:

-   calculate light nuclear structure and reaction information using
    realistic nuclear potentials,

-   describe medium mass and heavy nuclei with nuclear density
    functional theory and its extensions,

-   incorporate new density functional information into reaction cross
    section calculations including the building of theoretically based
    optical potentials,

-   develop a robust computational description of nuclear level
    densities,

-   develop improved theories of nuclear fission and capture

-   integrate mathematical algorithms with science applications for
    optimal performance, and

-   deliver to the community open-source software for the calculation of
    nuclear cross sections relevant to the various DOE missions.

Contacts:

Bob Little, LANL, rcl@lanl.gov

Dennis McNabb, LLNL, mcnabb3@llnl.gov

Tom Mehlhorn, SNL, <tamehlh@sandia.gov>

Mark Chadwick, LANL, <mbchadwick@lanl.gov>

Joe Carlson, LANL, <carlson@lanl.gov>

**Acknowledgement**

This work was, in part, performed by the Lawrence Livermore National
Laboratory, Los Alamos National Laboratory, and Sandia National
Laboratories under auspices of the U.S. Department of Energy.

**This document was prepared as an account of work sponsored by an
agency of the United States Government. Neither the United States
Government nor the University of California nor any of their employees,
makes any warranty, express or implied, or assumes any legal liability
or responsibility for the accuracy, completeness, or usefulness of any
information, apparatus, product, or process disclosed, or represents
that its use would not infringe privately owned rights. Reference herein
to any specific commercial product, process, or service by trade name,
trademark, manufacturer, or otherwise, does not necessarily constitute
or imply its endorsement, recommendation, or favoring by the United
States Government or the University of California. The views and
opinions of authors expressed herein do not necessarily state or reflect
those of the United States Government or the University of California,
and shall not be used for advertising or product endorsement purposes.**

**This work was performed under the auspices of the U.S. Department of
Energy by University of California, Lawrence Livermore National
Laboratory under Contract W-7405-Eng-48.**