Positron Dynamics in Rare-Gas Solids

Abstract

Using a beam of slow positrons in ultrahigh vacuum, we estimate the positron mean free path and energy-loss rate and measure the inelastic thresholds due to exciton, electron-hole pair, and positronium formation in solid rare-gas targets. The measurements are used to explain the large emission energies of positrons from the wide-band-gap solids in terms of a hot-positron model. The large diffusion length of the hot positrons in the solid rare gases makes them very efficient moderators for producing slow positrons. PACS numbers: 71.60. + Z, 78.70.Bj, 79.20.Hx There have been many studies of positrons in ionic crystals. 1 Measurements have revealed the positronium binding energies, wave function, effective mass, and formation mechanisms. Powdered insulators provided the first sources of positronium in vacuum 2,3 and the first practical efficient moderator for producing slow positrons. 4 Recently there have been studies of the positronium formation mechanism at the surface of ice, 5 of positronium emission from quartz, 6 and of positron reemission by various ionic solids. 7 "" 9 Unfortunately, this large body of work has not led to a precise picture of how a low-energy positron interacts with an insulator. The present study attempts to rectify this situation by careful measurements on the ideal wide-band-gap insulators, the rare-gas solids.When positrons are implanted into an ionic solid at kiloelectronvolt energies, one observes the reemission of positrons with a spread of kinetic energies comparable to the band-gap energy. In Ref. 9 this reemission was interpreted in terms of a modified version of the positronium breakup mechanism originally proposed by Canter et ai 4 In this model the reemitted positrons result from positronium formed in the bulk which diffuses to the surface and breaks up, emitting the positron via an Auger-iike process. We now find that positrons are copiously emitted with several electronvolts of kinetic energy from solid Ne, Ar, Kr, and Xe. However, because of the simplicity of these materials we have also been able to examine the emission process in detail by measuring the mean free path and inelastic thresholds for few-electronvolt positrons. The measurements are not consistent with positronium breakup and suggest that the emitted positrons are simply those that have not thermalized and happen to scatter back to the surface. Our new results also show that the rare-gas solids are a new class of very efficient slow-positron moderators.We propose to explain the observed positron reemission as follows. In any material, energetic positrons lose energy rapidly by making inelastic collisions involving electronic transitions. However, in an insulator there can be no more of these events once a positron has less energy than is needed to make an electron-hole pair, an exciton, or positronium. The positron continues to lose energy by creating phonons, but since the maximum phonon energy (E m&x ) is small, the diffusion length of the hot positron is large. Positrons reaching the surface before their...

Theory of positrons in solids and on solid surfaces

“…Gne-phonon processes degrade the energy inefFiciently, and the probability of multiphonon scattering is very small. Positrons in insulators may therefore not have enough time to reach thermal equilibrium before annihilation, trapping into defects, or reemission into vacuum (Mills and Crane, 1985;Gullikson and Mills, 1986;. In a wide-gap insulator a positron with kinetic energy less than the band gap can also lose energy in positronium (Ps) formation.…”

High-intensity positron microprobe at the Thomas Jefferson National Accelerator Facility

“…Below a certain threshold the only possible energy loss mechanism is through phonon interaction. 37 After thermalization, positrons close to the surface with energies larger than the work function (φ + ) may be reemitted to the surface. The positron implantation profile in solids is well-described by a Makhovian distribution, which is given as:…”

Characteristics of condensed gas moderators for the generation of very slow polarized muons

“…Such large escape depths have been previously reported for slow positrons and x-ray induced secondary electrons emitted from rare gas solids. 5,26 Although the interaction mechanisms responsible for the slowing down and escape are different, the peculiar properties of rare-gas solids ͑wide band gap, weak van der Waals bonding, closed atomic shell͒ lead to favorable conditions for so many different particles. In this respect it would be interesting to perform similar detailed investigations on the moderation of protons in rare-gas solids.…”