Positron Lifetimes in Pure and Doped Ice and in Water

Abstract

Positron lifetime spectra were measured in mono‐ and polycrystalline light ice,polycrystalline heavy ice,doped light ice, as well as in light and heavy water. All spectra were resolved into three components. At temperatures between −196° and −100°C the lifetimes and relative intensities are virtually constant, being 0.12 nsec and 20%, 0.44 nsec and 28%, 0.68 nsec and 52%, and attributed to annihilation of para‐Ps, free positrons, and ortho‐Ps, respectively. Above −100°C the longest lifetime increases with temperature to 1.1 nsec at 0°C. Its relative intensity is unchanged, while the parameters for the other components show a complex behavior. The spectra for mono‐ and polycrystalline light ice and for polycrystalline heavy ice are identical. For water long lifetime components attributed to ortho‐Ps are 1.86 nsec, 27% for H2O and 2.01 nsec, 22% for D2O. Theoretical explanations are suggested. Fast frozen solutions of HF with concentrations larger than approximately 10−5 mole fraction have a lifetime component at −160°C of 1.23 nsec, 52% attributed to trapping of ortho‐Ps in amorphous regions. Very good agreement is found between the experiments and a trapping model. Irreversible changes of the spectra are found by heating above approximately −120°C. Measurements on a number of fast frozen aqueous solutions of acids, bases, and salts are reported, none of them showing as strong influence on the ortho‐Ps lifetime as HF.