Analysis of the antineutrino capture on protons

Corrections to the zeroth-approximation cross-section formula for antineutrino capture on protons at nuclear reactors are evaluated. They include recoil corrections 1/M of three kinds. The true recoil correction to the cross section is negligible. The weak-magnetism—axial-vector interference is sizable, decreasing the cross section by ∼2% for 5-MeV positrons. One also has to include the recoil-neutron energy in relating the positron and antineutrino energies in order to evaluate the antineutrino flux. That effect decreases the positron yield by as much as 6% for 5-MeV positrons. The radiative corrections of order α have been also evaluated; they increase the cross section by ∼1.5% near threshold and by ∼0.7% at 5MeV. Formulas and a table of numerical values of all corrections are presented

2D Multi-Angle, Multi-Group Neutrino Radiation-Hydrodynamic Simulations of Postbounce Supernova Cores

We perform axisymmetric (2D) multi-angle, multi-group neutrino radiation-hydrodynamic calculations of the postbounce phase of core-collapse supernovae using a genuinely 2D discrete-ordinate (S_n) method. We follow the long-term postbounce evolution of the cores of one nonrotating and one rapidly-rotating 20-solar-mass stellar model for ~400 milliseconds from 160 ms to ~550 ms after bounce. We present a multi-D analysis of the multi-angle neutrino radiation fields and compare in detail with counterpart simulations carried out in the 2D multi-group flux-limited diffusion (MGFLD) approximation to neutrino transport. We find that 2D multi-angle transport is superior in capturing the global and local radiation-field variations associated with rotation-induced and SASI-induced aspherical hydrodynamic configurations. In the rotating model, multi-angle transport predicts much larger asymptotic neutrino flux asymmetries with pole to equator ratios of up to ~2.5, while MGFLD tends to sphericize the radiation fields already in the optically semi-transparent postshock regions. Along the poles, the multi-angle calculation predicts a dramatic enhancement of the neutrino heating by up to a factor of 3, which alters the postbounce evolution and results in greater polar shock radii and an earlier onset of the initially rotationally weakened SASI. In the nonrotating model, differences between multi-angle and MGFLD calculations remain small at early times when the postshock region does not depart significantly from spherical symmetry. At later times, however, the growing SASI leads to large-scale asymmetries and the multi-angle calculation predicts up to 30% higher average integral neutrino energy deposition rates than MGFLD.Comment: 20 pages, 21 figures. Minor revisions. Accepted for publication in ApJ. A version with high-resolution figures may be obtained from http://www.stellarcollapse.org/papers/Ott_et_al2008_multi_angle.pd