An interesting question on nuclear astrophysics is the fate of X-ray burst ashes as they descend under gravity to the deep layers of an accreting neutron star crust. As the ashes sink, they are subject to a range of nuclear reactions which transmute the nuclei. The primary of these reaction mechanisms are electron capture reactions, which are activated for different isotopes depending on the electron Fermi energy. At sufficiently high mass densities (ρ = 2.1 x 109 g cm-3 for 12C + 12C and ρ = 3.2 x 1012 g cm-3 for 40Mg + 40Mg), density induced fusion reactions, known as pycnonuclear reactions, begin to occur.
By combining existing electron capture formalism and a phenomenological expression for pycnonuclear reactions (specifically developed for a multi component plasma environment) with a network solver, the fate of the X-ray burst ashes has been addressed. In this context it has been found that with increasing mass density, the rigid Coulomb lattice of the neutron star crust dissolves into the isotopes 40Mg and 46Si, with an integrates energy release of between ~ 1.8 MeV/u and ~ 2.4 MeV/u, depending on lattice models. This result is fairly insensitive to the initial abundance distribution. The production of 46Si is determined to be the result of a cyclical pycnonuclear fusion, electron capture process.