Development of a spin-polarised positron beam

The development stages of a system for variable-energy spin-polarised positron beam spectroscopy are described. Methods for achieving maximum practical beam polarisation include a Na-22 source configuration with a low-Z backing to reduce positron backscattering, and positioning a 250m beryllium foil in front of the source to absorb the slower and decelerate the faster beta positrons, the latter suffering significantly less depolarisation during moderation than the former. To switch the direction of spin polarisation of electrons in the target sample the direction of the positron guiding field was reversed, and strong rare earth magnets were placed behind the sample. Systematic problems associated with low beam intensity and electronic drifts are discussed. Results for single-crystal iron compare well with those expected from theory, but suggest a beam/target polarisation of 5% of that expected. Prospects for future beam development and application are outlined.