Beam-foil production of fast positronium.

Abstract

Positronium (Ps) with kinetic energies from 10 to 500 eV is formed when keV-energy positrons are partially transmitted through a 50-\AA{} carbon film in vacuum. A time-of-flight measurement shows that the number of triplet Ps atoms formed per incident positron and moving in the forward direction (\ensuremath{\theta}=0) is approximately dN${=(10}^{\mathrm{\ensuremath{-}}3}$ eV${)}^{1/2}$${\mathrm{E}}^{\mathrm{\ensuremath{-}}3/2}$dE d\ensuremath{\Omega}, where d\ensuremath{\Omega} is the detector solid angle in steradians and the Ps with kinetic energy in eV is in the range (E, E+dE). The total amount of Ps with kinetic energy greater than 10 eV is approximately 1/2% of the incident positron flux times the detector solid angle. A measurement of the Doppler shift of the singlet-Ps-annihilation photons shows that the fast Ps with Eg20 eV has an angular distribution with ${\ensuremath{\theta}}_{\mathrm{rms}}$=[(15 eV)/E${]}^{5/8}$ approximately.