Beam-foil production of fast positronium

Positronium (Ps) with kinetic energies from 10 to 500 eV is formed when keV-energy positrons are partially transmitted through a 50-{} 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 ({θ}=0) is approximately dN=(10^-3 eV)1/2{E}^{{{-}}3/2}$dE d{Ω}, where d{Ω} 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 E>20 eV has an angular distribution with θᵣₘₛ=[(15 eV)/E]5/8 approximately.