A b + g coincidence positron lifetime spectrometer equipped with two electromagnetic lenses was constructed. The electromagnetic lens was used to have energy selected positron beam. It accomplished a time resolution of 385 ps (FWHM) for the total system with a Pilot U plastic scintillator. By using a silicon avalanche diode instead of Pilot U plastic scintillator to initiate start timing signals, the time resolution was improved to 345 ps (FWHM). With this method, in situ positron lifetime measurements at high temperatures or under high stresses can be attained easily.
The formation of thermal vacancies in Cu 3 Sn has been investigated by using positron annihilation lifetime spectroscopy. Positron lifetime measurement was performed in-situ between 293 K and 673 K. The positron lifetime below 433 K was found to be about 140 ps, which indicates that there are no detectable vacancies below 433 K. A sharp increase of the mean positron lifetime τ m , which is due to thermal vacancy formation, starts around 453 K and the saturation trapping of positrons by thermal vacancies are observed above 633 K. The Arrhenius plot of the positron trapping rate, which is proportional to the vacancy concentration, represents a linear behavior and the vacancy formation enthalpy was determined to be 0.63±0.02 eV. This value is relatively low comparing with that expected from the melting temperature (950 K) of this alloy.
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