Measurement of implantation profiles and median penetration depths for 5–10 keV positrons in aluminium

Abstract

The implantation profile P(x) for 7 and 10 keV positrons in aluminium, and the median penetration depths for positrons in the energy range 5–10 keV, have been measured by monitoring the mean S parameter for a 0–800 nm thick wedge of aluminium deposited on a glass substrate. Because of the short positron diffusion lengths in both the aluminium and glass, and the uniform density of the sample, P(x) is essentially equal to the annihilation profile A(x), and is obtained by differentiating the S vs wedge thickness data. The results are distinctly non‐Makhovian, and are close in shape to the recent Monte Carlo results of Jensen and Walker. The measured median penetration depths vary as (42.3±3.6)En Å (n=2.07±0.04; E=positron energy in keV), in marked contrast with the widely used form. The effect on these last measurements of positron diffusion to the Al‐glass interface is discussed, as are the consequences for the analyses of prositronium fraction ad S parameter profiling data.