Slow positron beam facility for investigations of plastically deformed metals and surface crystallization of silica

The simple slow positron facility at Bonn university and two recent experiments are presented. The following data briefly summarises the technical specifications of the instrument: overall size: 150=150=80 cm3 positron source: 22Na 10 mCi. moderator: Kr solid. energy filter: magnetic solenoid at 150 eV transport energy vacuum: high vacuum 10y6 hPa. spot size: 3 mm count rate: 3000 sy1 in 511 keV photopeak with BGO detector in coincidence for background suppression energy range: 0.15 to 12 keV with the sample at ground potential A sapphire plate which has, at 40 K close to the moderator temperature of 37 K., a thermal conductivity comparable to that of copper !. ensures both the electrical isolation and the thermal contact between the positron source and the He cryostat. With the moderator directly frozen onto the 22Na source the instrument reaches an efficiency better than 10y4. Slow positrons are extremely sensitive probes for investigations on microstructure and on the onset of surface crystallization of anorganic glasses. The formation of crystallization nuclei on the surface and the growth of the nuclei into the bulk material was investigated on amorphous SiO2. To this end specimens of amorphous silica were isothermally tempered at a temperature of 1773 K. In another experiment the back diffusion of positrons as a function of penetration depth was studied on weakly tensile deformed aluminium polycrystals. The role of dislocations and their effect on the mobility of positrons is in the center of this investigation. q1999 Elsevier Science B.V. All rights reserved.