Effect of paramagnetic particles on the lifetime of o-Ps in positron spur

The effect of paramagnetic molecule O2 on the lifetime of o-Ps in different kinds of organic liquids (benzene, cyclohexane and hexafluorobenzene) was studied. The spur processes are quite different in these three liquids. In pure benzene, the triplet excited molecule (3M*) in the positron spur can quench o-Ps. In cyclohexane the active radicals are the main particles which quench o-Ps. In hexafluorobenzene both the radicals and triplet excited molecules can quench o-Ps, but the last is more important. The rate constants of the reaction between O2 and o-Ps measured in benzene, cyclohexane and hexafluorobenzene are 3.41 x 1010 M-1 s-1, 2.93 x 1010 M-1 s-1 and 2.97 x 1010 M-1 s-1,respectively. By analysing the experimental results, a more realistic value of pick-off annihilation lifetime of o-Ps in these three liquids was obtained. (author)