Development of low energy positron beams and their application to the study of the surface region of metals

Abstract

Two low energy positron beams have been designed, built and their performance characteristics evaluated. Both are magnetically guided systems using a combination of a solenoid and Helmholtz coils. Slow positrons are produced by the moderation of fast positrons, from a 22Na source, in annealed polycrystalline tungsten mesh with efficiencies greater than 2 x 10-4. The original beam, built to high vacuum specifications, has been incorporated into a fully automated microcomputer controlled Doppler-broadening spectrometer system. The newer beam line is built, to UHV specifications, into a liquid helium cryostat. It also has a vertical geometry making the whole system far more versatile and allowing the study of liquid surfaces. Doppler-broadening analysis has been applied to measurements taken using both beams applied to pure metals (Mo and Ga). A two-state model incorporating diffusion of thermal positrons back to the surface has been found to be inadequate at low incident positron energies. In this regime epithermal positron and positronium emission is significant. A model of simple back-scattering of epithermal positrons was found to be successful in fitting the experimental lineshape parameters. Both fast and slow positron techniques have been applied to the study of inert gas precipitates in metals. Using conventional methods a detailed Doppler-broadening study has been carried out on the anealing of bulk Cu samples containing 3 atomic % Kr in the form of a high concentration of solid precipitates at 300K. Melting of the Kr and bubble growth are clearly seen. Deconvolution of the annihilation lineshape indicates that positrons are trapped at the Cu-Kr interface.Slow positrons have been used to profile the defect distribution of Mo implanted with a high dose of Kr ions. Using the simplest model of a step-function distribution there is reasonable agreement with the expected Kr profile.