Positron annihilation spectroscopy of vacancy aggregates in neutron-irradiated mgo crystals

Abstract Positron annihilation measurements in neutron-irradiated MgO crystals show that the positron lifetime is shorter than in as-grown crystals, suggesting that most of the defects produced by neutron irradiations are positively charged. The concentration of the neutral anion vacancy (possibly also the neutral anion divacancy) is estimated to be no more than ∼ 1016 cm−3 for samples irradiated to a dose of 1017 to 1019 n cm−2. Annealing experiments on the neutron-irradiated crystals show a significant increase in the positron lifetime after anneals at 900 K. The increase is attributed to positron trapping by anion—vacancy aggregates. A lifetime of (284±15)ps is tentatively assigned to positrons trapped in these aggregates.