Ask more Info

Slow positrons for deep defect detection

Positron Annihilation Spectroscopy (PAS) is a unique method of measuring point defects [3]. It is sensitive to defects as small as mono-vacancies (sub-nano scale), in concentration as low as 10-6 appm. Moreover, depending on the apparatus used, it can probe both the surface and bulk regions of the material, allowing depth profiling of defects.  A slow positron beam is essentially a collimated, variable, low energy, positron source for PAS measurements. The collimation of the beam and energy control allows for non distractive studding of different layers of the material [4].
A typical slow positron beam is composed of three main components: (1) Positron source, (2) positron moderator, (3) positron transport beam line. Positrons are emitted from the source isotropically with a wide energy distribution, with a typical mean of 200 keV (for 22Na). In order to produce a mono-energetic beam, they are passed through a moderator, in which they lose most of their initial energy (up to few eV). Then they are shaped into a beam and accelerated using electro-magnetic fields to the sample. By varying the fields, one can control the positron energy.