Positronium thermalization inSiO2powder

Slow positron pulses are implanted at variable energies into a ${\mathrm{SiO}}_2$ powder target to form positronium (Ps). By time-of-flight methods we estimate the energy spectrum of the Ps that diffuses out of the target into the vacuum. As expected, higher implantation energies and lower target temperatures result in a colder Ps energy spectrum. However, even with 19-keV positron implantation energies, the Ps escaping from a target at 4.2 K is only about 2% thermalized. No evidence was found for a low-energy cutoff due to Ps localization between the powder grains.