Enhancement of Positron-Atom Annihilation near the Positronium Formation Threshold

Abstract

The behavior of the positron- 2 gamma annihilation rate on an atomic target near the positronium (Ps) formation threshold is determined. When the positron energy epsilon approaches the threshold epsilon(thr) from below, the effective number of electrons contributing to the annihilation, Z(eff), grows as Z(eff) approximately A/square root of [epsilon(thr)-epsilon], where A is related to the size of the Ps formation cross section, sigma(Ps) approximately B square root of [epsilon-epsilon(thr)], by A = B square root of [2 epsilon(thr)]/32 pi (in atomic units). Taking account of the finite Ps lifetime eliminates the singularity in Z(eff) and shows that close to threshold the positron annihilation cross section is identical to the para-Ps formation cross section.

Unified Treatment of Positron Annihilation and Positronium Formation

“…An application of the S-wave threshold law to Eq. (1) leads to the divergence as ÿ ÿ1=2 [7], where E ÿ E th 1s, instead of the more rapid divergence as ÿ ÿ1 expected by Laricchia and Wilkin. The recent time-dependent close-coupling calculation by Yamanaka and Kino [6] fails to reproduce this remarkable threshold increase.…”

“…[10,11]. Gribakin and Ludlow [7] showed that this modification leads to the absence of any threshold divergence of an or the threshold rise of Ps ; they connect smoothly from one to the other across E th 1s.…”

Virtual-State Formation in Positron Scattering from Vibrating Molecules: A Gateway to Annihilation Enhancement

“…The features of positron annihilation cross sections (PACSs) observed for very-low energy positrons in molecular gases, on the other hand, have received thus far less attention from theory and computations (for a general discussion, see [3][4][5] and references therein). The positron scattering from molecular targets also differs considerably from the case of electrons because of the possibility of positronium (Ps) formation and of the resulting annihilation by the emission of two or three quanta.…”

A Method for the Spectroscopic Diagnostic of the Peculiarities of Materials Using the Semi-Auger Effect

“…We use the many-particle PT, exactly the RPAE method [5,6,10]. In the lowest order in the electron interaction the amplitude of presented decay is determined by the diagrams in Fig.…”

“…The summation Π includes all the projections of orbital and spin momenta of initial and final vacancies, as well as the factor F is determined as [10][11][12]:…”