Free-Radical Quenching of Positron Lifetimes

The annihilation lifetime of positrons stopping in benzene has been measured as a function of the percentage of added free radical, diphenylpicrylhydrazyl. The long component of the complex annihilation curve quenches from 2.67×${10}^{-9\mathrm{}}$ sec in pure benzene to 5.3×${10}^{-10\mathrm{}}$ sec at a 3% free radical concentration. An annihilation model following Bell and Graham's assumptions is discussed, postulating the partial formation of bound positron states prior to annihilation. The long lifetime is regarded then as a measure of the conversion rate from triplet to singlet states due to collisions of the positron system with the unpaired electrons of the free radical. The experimental points yield, using such a model, a conversion cross section of $\sigma =1.18\mathrm{}\times {10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$.