Research Objectives
To implement and develop nuclear structure calculations, including modern Monte Carlo techniques, for solving the shell model problem. To solve the core collapse supernova problem. To develop next-generation radiation transport and radiation hydrodynamics codes for computational astrophysics.
Computational Approach
During the past several years, an alternative method for the exact solution of the shell model, based on auxiliary field Monte Carlo techniques, has been developed. The resulting problem becomes one of many-dimensional quadrature, which is carried out by Metropolis sampling. These calculations are compute-intensive and are ideally suited to take advantage of developments in parallel computing (C90/J90/T3E).
Two sets of astrophysics simulations were conducted on the C90 and J90: (1) one-dimensional simulations of core collapse supernovae using a transport code, BOLTZTRAN, that solves the neutrino Boltzmann kinetic equations; and (2) two-dimensional simulations using one-dimensional multigroup flux-limited diffusion neutrino transport and two-dimensional piecewise parabolic method (PPM) hydrodynamics.
Accomplishments
We began a series of multi-major oscillator shell calculations (the largest ever attempted) to investigate the properties of extremely neutron-rich nuclei. Some of these systems play a role in r-process nucleosynthesis. We have also investigated nuclear transitions that determine the electron capture rates in iron-region nuclei. These nuclei play an important role in the precollapse phase of the supernova.
A detailed comparison was made between Boltzmann neutrino transport and multigroup flux-limited diffusion, focusing on key quantities that are central to the neutrino shock reheating mechanism about which all current supernova modeling revolves. The increased heating is promising and will be fully explored this year.
Significance
The accurate description of nuclear structure remains an important and challenging many-body problem. New experiments are studying nuclei at extreme temperatures, angular momentum, and/or neutron/proton balance; such data will increase dramatically with operation of radioactive beam facilities including the ORNL Holifield facility. These experimental investigations will allow us to explore nuclei that are weakly bound and have large spatial dimension. Clearly a substantial theoretical and numerical effort is required to develop and further our understanding of nuclei far from stability. The theory group at ORNL continues to maintain an active and leading role in this effort.
Publications
Mezzacappa, A., A. Calder, S. Bruenn, J. Blondin, M. Guidry, M. Strayer, and S. Umar. N. d. An investigation of neutrino-driven convection and the core-collapse supernova mechanism using multigroup neutrino transport. ApJ, in press.
Mezzacappa, A., A. Calder, S. Bruenn, J. Blondin, M. Guidry, M. Strayer, and S. Umar. N. d. The interplay between proto-neutron star convection and neutrino transport in core-collapse supernovae. ApJ, in press.
Radha, P. B., D. J. Dean, S. E. Koonin, K. Langanke, and P. Vogel. N. d. Gamow-Teller strength distributions in fp-shell nuclei. Phys. Rev. C, in press.
Two-dimensional entropy plots showing the evolution of neutrino-driven convection
beneath the supernova shock wave in our 15 solar mass model, at 137, 212, and 512 ms
after core bounce.