Fast positronium formation and dissociation at surfaces

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Abstract
The origin of short‐lived components in the annihilation lifetime spectrum of positronium (Ps) is shown to be due to fast Ps that is collisionally dissociating at the surfaces of the surrounding confinement cavity. The results are consistent with a model of fast (10–100 eV) Ps production by backscattered positrons from the incident beam. It is found that the typical lifetime of dissociating Ps scales with the mean free path of the cavity, and the relative formation intensity depends inversely on the incident positron beam energy. This ubiquitous effect will be present in any Ps formation experiment involving a free surface and can only be eliminated at beam energies less than 10 eV. More practical methods of minimizing the undesirable systematic effects of fast Ps quenching in depth‐profiled positron lifetime spectroscopy and in precision Ps decay rate measurements will be discussed. © 1995 American Institute of Physics