Effect of Impurities of Angular Correlation of Positron Annihilation Radiation

Abstract

Angular distribution measurements on two-quantum annihilation radiation from positrons stopping in liquid targets have been made as a function of concentration of certain impurities in the target materials. The impurities tested were ${\mathrm{Mn}}^{++}$ and ${\mathrm{Co}}^{++}$ ions (dissolved from their chloride salts) and N${\mathrm{O}}_2^{-}$ and N${\mathrm{O}}_3^{-}$ ions (dissolved from their sodium salts) in water, and chloroform and the free radical diphenyl-picryl-hydrazyl in benzene. The paramagnetic ions are found to produce a strong enhancement of the narrow-distribution component, ascribed to exchange conversion between triplet and singlet positronium. Conversion cross sections of a few times ${10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$ are obtained assuming a positronium velocity about 5 times thermal velocity at conversion. These are in fair agreement with recent determinations made from lifetime studies. The nitrate and nitrite ions may cause a further broadening of the broad component, which appears to be associated with the chemical binding of positrons to the ions. It is also found that positrons are chemically bound to the chloroform molecule. Some evidence is presented to show that the free radical (diphenyl-picryl-hydrazyl) may not, as previously supposed, act to convert triplet to singlet positronium, but may in fact give rise to chemical binding of positronium to the free radical.