Positronium formation and diffusion in crystalline and amorphous ice using a variable-energy positron beam.

Abstract

The behavior of positrons in crystalline and amorphous ice has been studied with a beam of monoenergetic positrons with incident energies 0--4.5 keV. Positronium (Ps) is formed in the bulk ice and diffuses until it annihilates or escapes from the surface. Measurements were carried out on the fraction of ortho-Ps leaving the surface and of the Doppler broadening of the 511-keV \ensuremath{\gamma} annihilation line. For incident energies 0--60 eV the Ps formation probability shows large variations. These variations are associated with Ps formation in the so-called Ore gaps and reflect the electronic structure of ice as demonstrated by Monte Carlo simulations of the positron slowing-down process. At higher energies, up to about 1 keV, the total Ps yield increases from about 50 to 75 %, which is attributed to Ps formation via spur processes. A large difference is found between the Ps diffusion coefficient in crystalline ice (about 0.2 ${\mathrm{cm}}^{2}$/sec) and in amorphous ice (roughly ${10}^{\mathrm{\ensuremath{-}}3}$ ${\mathrm{cm}}^{2}$/sec). From the red shift of the 511-keV annihilation line the Ps work function (affinity) in the crystalline ice is estimated to be -2\ifmmode\pm\else\textpm\fi{}1 eV. Evidence for low-energy-positron diffraction in the crystalline ice is found with scattered intensities higher than 25%. Sputtering of the crystalline ice creates surface damage which strongly reduces the yield of Ps escaping the surface. Cavities of average diameter larger than about 17 A\r{} are found in the as-grown amorphous ice. They anneal out at about 100 K, which is below the crystallization temperature of about 135 K.