Peter J. Schultz

Improved slow-positron yield using a single crystal tungsten moderator

A well-annealed W(110) single crystal was used as a fast-to-slow positron moderator. The measured moderator efficiency at room temperature using a58Co positron source in the backscattering geometry isɛ =(3.2±0.4)×10−3, roughly a factor of three better thanɛ for the best previously reported Cu(111)+S moderator. We find a stable positron moderation efficiency over a period of several weeks when maintained at pressures around 10−9 Torr and an energy spreadΔE = 0.7 eV of the emitted slow positrons. An initial attempt was made to fabricate a hybrid Cu on W(110) moderator, which yieldedɛ of about 1.2×10−3 after annealing.

Positron annihilation in Al single crystals from 85 mK to 300K

Doppler-broadening and positron lifetime data have been taken for well annealed ultra-pure Al(110) crystals from 85 mK to 300K. The Doppler-broadening data are independent of specimen temperature from 85 mK to 77K, while they show an increase between 77K and 300K consistent with the linear slope observed previously by Hood and Schultz (1979). Over the whole temperature range studied, the Doppler data scale to the static thermal expansion coefficient. The lifetime data also show no obvious dependence on temperature over the entire range studied, although they were statistically limited by the cryostat geometry. These results, which represent the lowest temperatures studied by any positron annihilation technique, provide no evidence for positron localisation at low temperatures in very well annealed single crystals.