No abstract

We investigate direct positron annihilation during a collision in a gas; this is physically separable from indirect annihilation, i.e., via formation of positronium ͑Ps͒ and its decay. The cross section of direct annihilation is calculated from the evolution of a wave packet with a time width much shorter than the Ps lifetime, where the direct contribution is extracted through the time dependence of the survival probability. The cross section obtained for a hydrogen atom is found to be enhanced in energies around 10 eV well above the Ps formation threshold. This enhancement is shown to be due to temporal positron capture with electronic excitation in the atom.

The nuclear polarization ͑NP͒ correction is calculated for the ground states of the hydrogenlike 82 208 Pb and 92 238 U ions. We take into account the transverse interaction, ignored in the previous studies, as well as the Coulomb interaction. The NP correction is formulated in the bound-state QED formalism with the ladder and cross diagrams of two-photon exchange between an electron and a nucleus. We take into account nuclear excitations of low-lying states and giant resonances with multipolarities of ϭ0 ϩ , 1 Ϫ , 2 ϩ , and 3 Ϫ , and describe them in a collective model. The NP corrections are obtained to be Ϫ0.2 and Ϫ180.7 meV, respectively, for 82 208 Pb and 92 238 U. The effect of the transverse interaction is found to be essential to the NP correction for electric dipole giant resonances, strongly canceling a contribution of the Coulomb interaction. Dependence of the NP correction on different nuclear excitations is investigated.

No abstract

No abstract