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A digital positron lifetime spectrometer for the measurements of 2 GBq level radioactive materials has been constructed and successfully tested. The equipment consists of a state-of-the-art digital oscilloscope and three movable scintillation detectors operating in triple-coincidence mode. The time resolution of 188 ps full-width at half-maximum (FWHM) is achieved for the positron lifetime measurements of radioactive specimens. Also by comparison of measurements of well-known material with and without high 60 Co background radiation the ability to measure radioactive specimens is proved. In this paper the performance, construction, installation, optimization and calibration of the setup is reported.

The hardening and embrittlement of Reactor Pressure Vessel (RPV) steels is of great concern in the actual nuclear power plant life assessment. This embrittlement is caused by irradiation-induced damage, like vacancies, interstitials, solutes and their clusters. Fe-Cu binary alloys are often used to mimic the behaviour of such steels. Their study allows identifying some of the defects responsible of the hardening, especially when compared to pure iron or C-micro-alloyed iron. Owing to their self-seeking and selective trapping, positrons are used to determine the nature of these defects. Recently, at SCK·CEN, a new Positron Annihilation Spectroscopy (PAS) setup has been built, calibrated and optimized to measure the Coincidence Doppler Broadening (CDB) and Lifetime (LT) of neutron irradiated materials. This set-up has been used to measure the CDB and LT of n-irradiated pure Fe and Fe-Cu alloys. It is found that the clustering of Cu take place at the very early stages of irradiation using the CDB while LT measurement are showing much more vacancy clustering for low Cu alloys than in the higher ones. Increasing the neutron dose up to 1.3 × 10 20 n/m 2 , allows the follow up of the kinetic of Cu and V-clustering especially in Fe-Cu alloys. It is found that both copper and carbon decrease the size of vacancy-cluster, when added to iron.

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