Annihilation of positrons in xenon gas

The annihilation rate of room-temperature positrons in low-density gaseous xenon-nitrogen mixtures (nXe5*10-9 Amagat and nN21*10-8 Amagat) has been measured. The annihilation rate corresponds to a value of Zeff=40120 for xenon. This is in agreement with measurements in xenon-gas mixtures made at densities nine orders of magnitude greater, but differs significantly from values measured in pure high-density xenon gas. These results indicate that the positrons were not thermalized in the latter case.