Annihilation of incompletely thermalized positronium studied by age-momentum correlation

The dependence of the Doppler broadening of the 511 keV photon annihilation line of positrons annihilating in a variety of solid and liquids is studied as a function of the positron age (= time spent by the positrons in the sample between injection and annihilation) by age-momentum correlation (AMOC) measurements. At small positron ages an extra broadening of the momentum distribution of the annihilating positrondashelectron pairs is observed in all positronium-forming materials investigated (fused quartz, boron nitride, methanol, ndashhexane) but not in materials in which there is no evidence for positronium formation (Al, diamond, Al2O3). The “young-age broadening” is attributed to delayed thermalization of positronium atoms with kinetic energies of a few electron volts.