THE QUENCHING OF TRIPLET POSITRONIUM BY IONS IN AQUEOUS SOLUTIONS

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Abstract
Measurements of positron lifetimes in aqueous solutions have been performed using a fast time-to-amplitude converter with a resolving time of ~10−9 sec. The lifetime, τ2, of the long-lived component of the complex decay observed in water has been found to decrease when paramagnetic salts are added. The experimental results are interpreted in terms of "pickoff" annihilation to explain the observed τ2 in pure water, and a rapid triplet ↔ singlet conversion process to account for the quenching of the τ2 component. The conversion is caused by the presence of unpaired electrons in the structure of the paramagnetic atoms. Triplet → singlet conversion cross sections have been calculated on the basis of this interpretation and they range from 4.0 × 10−20 cm2 for NdCl3 to 3.8 × 10−18 cm2 for FeCl3.