Estimates of stellar $e^{-}$ captures and $\beta$-decay in $LaBr_{3}$ crystals from first-principles simulations.

Simonucci S. Morresi T., Taioli S.
  Giovedì 14/09   16:00 - 19:00   Aula A208   I - Fisica nucleare e subnucleare
The evolution of Li content in the Universe is still unexplained, presenting various puzzles to astrophysics. One of the open issues is the determination of reliable electron capture rates, notably in Be, for density and temperature conditions different from solar; this input is of crucial importance to model the galactic nucleosynthesis of Li. As a contribution to the solution of this problem, we discuss a new theoretical method for calculating electron capture rates in conditions typical of evolved stars. This method goes beyond previous approaches by adopting a mean-field adiabatic approximation to the scattering process. Indeed, "traditional" estimates of the electronic density at the nucleus, to which the $e^{-}$ capture decay rates for Be are proportional, are based on the widely used Debye-H\"uckel (DH) model for the electron screening; however, it is found that, already for solar conditions, where the DH approximation should hold, we obtain sizeably different results with respect to our new approach. Furthermore, we apply our method to a rather broad range of $T$ and $\rho$ values, embracing those typical of red giant stars, where both bound and continuum states contribute to the capture. In this more general case, the DH or Thomas-Fermi approximations do not stand, so that the more general $ab initio$ methods that we suggest should be preferred. Furthermore, a benchmark of the previous results, obtained via solution of the Hartree-Fock equations, with a Path-Integral Monte Carlo technique to include electron-electron dynamic correlations will be discussed. Finally, a relativistic extension of this method to the $ab initio$ calculation of the $\beta$-decay in heavy nuclei will be shorty reviewed. In particular, we will show some theoretical and experimental results on the $\beta$-decay of lanthanum in cerium-activated lanthanum bromide crystals, which have been recently proposed as radiation detectors due their unique scintillation properties.