Annihilation of Positrons in Nitrogen

The positron annihilation lifetime spectra in nitrogen have been measured by a new apparatus consisting of two time-to-amplitude converters. The annihilation of positrons in nitrogen as a function of nitrogen densities from 2 to 45 amagats has been investigated. The free annihilation rate is found to be 5.80 ± 0.07 μ${\sec }^{-1\mathrm{}}$ ${\mathrm{amagat}}^{-1\mathrm{}}$, and the $o-\mathrm{P}\mathrm{s}$ quenching rate, 0.22 ± 0.06 μ${\sec }^{-1\mathrm{}}$ ${\mathrm{amagat}}^{-1\mathrm{}}$. Both rates are found to be linear for densities up to 45 amagats. A distinctive shoulder has been observed in the positron annihilation lifetime spectra for nitrogen with densities less than 10 amagats. The shoulder width is estimated to be 14 ± 3 nsec amagat. The value of $Z_{\mathrm{eff}}$ for free-positron annihilation at thermal energy is found to be 30. The value of $Z_{\mathrm{eff}}$ for free-positron annihilation at energy about 0.5 eV or higher is estimated to be only about 23.