T-2, Nuclear and Particle Physics, Astrophysics and Cosmology

Opportunities in astrophysics for new reaction-rate calculations

Sanjib Gupta
T-16, LANL

I will describe astrophysics modeling work being done here at T-16 that benefits greatly from large databases of important reaction rates. Specifically I will describe;

(1) How neutron star crust heating is determined from weak interactions, specifically Electron Captures, and exciting new directions in Electron-capture-delayed neutron-emission currently being pursued with the calculations from a Hauser Feshbach code (collaboration with T. Kawano, T-16) Recently Electron Captures (henceforth EC) into excited states of neutron-rich nuclei were shown by the LANL-MichiganState-Mainz collaboration to result in Neutron Star (henceforth NS) Crust heating which was 5 times that of previous calculations. We are now exploring the nucleosynthesis and heating from neutron processes deeper in the NS Crust beyond about 1011 g/cc. Electron captures into excited states of neutron-rich nuclei above neutron separation energies requires a Hauser-Feshbach code to calculate the branchings between 1-, 2-,3-,...neutron emission rates in the stellar environment. Since the evolving composition has a free neutron fraction at a very density, the equilibrium composition at a given depth requires readjustments with respect to both the electron chemical potential and the neutron chemical potential, thus the emitted neutrons can be captured into other mass chains with a net release of heat. From a nucleosynthesis perspective, we have a very interesting and hitherto unexplored pattern of weak interactions and neutron processes similar to the r-process, with the exception that the weak processes are primarily density-driven in the rather cold crust ($T_9=0.4-0.6)and in the $\beta^{+}$ direction, that is, toward increasing neutron richness.

(2) We have an ongoing collaboration with an s-process group interested in branching points and competition between beta-decay and neutron captures on isomeric states of branch-point nuclei. We have an interesting formalism that allows us to take into account communication between ground state and isomers of these nuclei (through higher-lying states) which essentially splits each branching nucleus into two separate ensembles which can then be put into a reaction network. However, the beta-decay rates and neutron-capture (and charged particle also!) rates from each state of the original branch-point nucleus are required. In the case where the level densities are high enough, this is amenable to a statistical approach. The division of the branching nuclei into two equivalent ensembles without losing any physics has the potential for answering some very important questions regarding s-process abundances in the Sr-Y-Zr region and also in the Eu-Gd-Sm region.

(3) Importance of weak interactions for astrophysics in general, current state of the art and opportunities for significantly improved input for X-ray bursts, and Type 1 and 2 supernovae. We use the QRPA calculations of Peter Moller to generate a global rate set without the model-space limitations which large-scale shell model rate compilations encounter. This will become increasingly important as 3D supernova simulations become fast enough to do nucleosynthesis concurrently rather than in a post-processed fashion and as modern dynamical r-process calculations require stellar weak-interaction rates of exotic nuclei.