Effect of Paramagnetic Impurity on the Long Lifetime of Positron in Liquid Ammonia

We have computed the paramagnetic quenching effect of lithium and sodium in liquid ammonia on the long-lifetime component of positron annihilation. The triplet-to-singlet conversion cross sections, calculated in the Born approximation, turn out to be 5.9×${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$ for Li and 7.2×${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$ for Na for thermalized positronium. Reasonable agreement with the meagre existing experimental data is obtained.