Positron Annihilation in Nitric Oxide and Nitrogen Dioxide

Positron-annihilation lifetimes in nitrogen dioxide and nitric oxide gases are carefully determined. Nitrogen is used as a dilutant. The free-positron-annihilation rate in units of the effective number of electrons per molecule $Z_{\mathrm{eff}}$ for nitrogen dioxide as a mixture of dimers and monomers is determined to be 720 to 1090 and the free-positron-annihilation rate for nitric oxide is $Z_{\mathrm{eff}}=34\mathrm{}$. The positronium quenching rate in nitrogen dioxide gas is very high and the conversion reaction by N${\mathrm{O}}_2$ is found to be mainly responsible for the quenching. The quenching rate is determined to be $460D$ ${\mathrm{nsec}}^{-1\mathrm{}}$ ${\mathrm{amagat}}^{-1\mathrm{}}$, where $D$ is the partial density. The positronium quenching rate in nitric oxide is determined to be only about $0.15D$ ${\mathrm{nsec}}^{-1\mathrm{}}$ ${\mathrm{amagat}}^{-1\mathrm{}}$, and it changes with the concentration of nitrogen.