Field Assisted Moderators

Abstract

Conventional positron moderators rely on diffusion to transport a small proportion of e⁺ implanted from a radioactive source to a surface where they may be emitted into vacuum with an energy distribution peaked at a value characteristic of the negative workfunction of that material. The energy spread of the distribution will depend on the e⁺ properties in the bulk and at the surface of the moderating material. Since the initial conception of a positron moderator by Madanski and Resetti^l and the first practical demonstration by Cherry² in 1958 there has been considerable improvement in conversion efficiencies. Of particular significance was the work of Mills who showed that many clean metal surfaces emit slow positrons and was thus able to develop higher efficiency moderators⁴. The factors which govern the suitability of a particular material for use as a positron moderator are given in the following empirical relationship for the moderator efficiency, p1,5,

$$P = {y_o} \alpha \sqrt {D\tau }$$

(1)

where y_o is the probability of slow positron emission from the surface (the branching ratio), α is the β⁺ absorption coefficient and $\sqrt {D\tau }$ the e⁺ diffusion coefficient with D and τ the positron diffusion constant and lifetime respectively. To maximise these factors, recently work has largely concentrated on dense single crystal materials especially W(110) for which an effi-ciency of 3.2 × 10⁻⁶ has been reported⁶.