A problem to account for the direct electron-positron annihilation in positron-hydrogen scattering above the positronium formation threshold has been resolved within the time independent formalism. The generalization of the optical theorem is derived for the case when an absorption potential is present in the Hamiltonian. With this theorem the annihilation cross section is fully determined by s...

In this article‎, ‎a new scheme inspired from collocation method is‎ ‎presented for numerical solution of stiff initial-value problems and Fredholm integral equations of the first kind based on the derivatives of residual function‎. ‎Then‎, ‎the error analysis‎ ‎of this method is investigated by presenting an error bound‎. ‎Numerical comparisons indicate that the‎ ‎presented method yields accur...

Measurements of positron-on-molecule annihilation have established that positrons attach to a variety of molecules via vibrational Feshbach resonances VFRs . These resonances, in turn, result in greatly enhanced positron annihilation rates beyond those expected in simple collisions. A recent paper J. A. Young and C. M. Surko, Phys. Rev. Lett. 99, 133201 2007 described experimental studies and a...

We study the effects of dark matter annihilation during and after the cosmic recombination epoch on the cosmic microwave background anisotropy, taking into account the detailed energy deposition of the annihilation products. It is found that a fairly stringent constraint on the annihilation cross section is imposed for TeV scale dark matter masses from WMAP observations.

This article presents an overview of current understanding of the interaction of low-energy positrons with molecules with emphasis on resonances, positron attachment, and annihilation. Measurements of annihilation rates resolved as a function of positron energy reveal the presence of vibrational Feshbach resonances VFRs for many polyatomic molecules. These resonances lead to strong enhancement ...

Assuming that the enhanced MeV emission from the MeV blazar PKS 0208{ 512 observed by COMPTEL on occasions is due to annihilation in a pair plasma, we estimate parameters of the annihilation region: its size, the number density of particles, and the variability timescale. The values we nd are in accord with our present-dayknowledge of emission mechanisms operating in AGN. The constructed model ...

The electron-positron annihilation spectrum observed by SPI/INTEGRAL during deep Galactic Center region exposure is reported. The line energy (510.954±0.075 keV) is consistent with the unshifted annihilation line. The width of the annihilation line is 2.37±0.25 keV (FWHM), while the strength of the ortho-positronium continuum suggests that the dominant fraction of positrons (94±6%) form positro...

Positron annihilation spectroscopy, when combined with highquality supporting modeling of positron states and annihilation in matter, is a powerful tool for detailed defect identification of vacancy-type defects in semiconductors and oxides. Here we demonstrate that the Doppler broadening of the positron annihilation radiation is a very sensitive means for observing the oxygen environment aroun...

Electron and positron densities for a number of positron binding atoms and ions are used to benchmark methods used to compute positron annihilation rates in solids. The electron and positron densities from ab initio calculations are multiplied by some commonly used enhancement factors and integrated to give the annihilation rate. These are then compared with the close to exact annihilation rate...