Positron annihilation in liquid helium and liquid argon under an electric field

Abstract

We have made new measurements of the lifetimes at 4.29 K and 2.92 K in He I, and at 2.14 K and 1.67 K in He II, without and with an electric field. At 4.19 K we have a new value of the lifetime, 2.01+or-0.002 ns, leading to a Zeff of 3.53 at zero field. In contrast to earlier reports, Zeff decreases very slightly as the temperature is lowered and the density increases. The orthopositronium lifetime at 4.19 K is 98.1+or-0.2 ns, at zero field, independent of field up to 18 kV cm-1. At 4.19 K the free positron decay rate falls exponentially with increasing field to a value 5% below its zero-field value, the drop-off rate being 0.36+or-0.04 cm amagat V-1. From 28 through 50 V cm-1 amagat-1 the decay rate of the free component rises slowly as expected because of the increasing positronium fraction over the same range of electric field. In liquid argon there is positronium enhancement beginning at or below 3 V cm-1 amagat-1, which contrasts with 160 V cm-1 amagat-1 for argon gas. At least two lifetimes make up the long triplet component, suggesting different degrees of thermalization, or different states of positronium within the liquid. The effect of the electric field is to enhance the longer-lived, or more thermal component, in strong contrast with the findings of Charlton et al. (1992) for the gas. The argon results suggest positron behaviour similar to that of electrons in the liquid phase, namely, such particles move in a conduction band subject to small effective scattering and diffusion cross sections. An analytic model in which the scattering length is an adjustable parameter yields the onset of positronium enhancement just below 3 V cm-1 amagat-1 when as=-0.043 a0, corresponding to an elastic scattering and diffusion cross section of 0.034 pi a02. Consequences for positron thermalization are discussed. A model of positronium losing energy slowly in the liquid without bubble formation is suggested.

Low Energy Cross-Sections for Positron Interactions with Cyclic Hydrocarbons

“…In liquids, in general, positron lifetimes depend on cross-sections, but detailed analysis requires knowledge of partial cross-sections down to zero energy. To our knowledge, such an analysis was done in He and Ar solely [25]. We note, see Table, for the three hydrocarbons studied here similar values of τ 2 lifetime component (i.e.…”

Multiterm solution of a generalized Boltzmann kinetic equation for electron and positron transport in structured and soft condensed matter

“…We are particularly interested in the effects of the material structure, and find interesting phenomena, including structure-induced negative differential conductivity and structure-induced anisotropic diffusion. Another theoretical study of positrons in liquid Ar [20] confined their attention to thermalization times, using a two-term solution of the spatially homogeneous Boltzmann equation. Transport coefficients such as drift velocity and diffusion coefficients were not considered by [20] and, furthermore, Ps formation was treated as a weak perturbation, something which appears of questionable validity given the very large Ps formation cross section [12].…”

“…Another theoretical study of positrons in liquid Ar [20] confined their attention to thermalization times, using a two-term solution of the spatially homogeneous Boltzmann equation. Transport coefficients such as drift velocity and diffusion coefficients were not considered by [20] and, furthermore, Ps formation was treated as a weak perturbation, something which appears of questionable validity given the very large Ps formation cross section [12]. In this paper, we calculate a full set of measurable transport coefficients, and treat positronium formation in a rigorous, nonperturbative way.…”

Positron Kinetics in Soft Condensed Matter

“…Comments on the work of Cohen and Lekner and others.-The seminal work of [9] for electron transport in structured media and subsequent applications for electrons (see review [8]) and positrons [14] all make the two-term approximation, thereby losing accuracy in the representation of the velocity dependence of f and reducing information on the structure of the medium. This Letter shows how these theories can be generalized by lifting these restrictions.…”