Positron annihilation in gaseous nitrogen and nitrogen-neon mixtures at 77 K

Abstract

Annihilation lifetime spectra have been measured for positrons in ${\mathrm{N}}_{2}$ gas at 77 K at densities in the range 1.9-3.6 amagat, and in ${\mathrm{N}}_{2}$-Ne gas mixtures at 77 K having ${\mathrm{N}}_{2}$ partial densities in the range 0.5-3.7 amagat and with Ne concentrations in the range 0.34-0.98. The annihilation rate of positrons with ${\mathrm{N}}_{2}$ was found to be independent of the Ne concentration in the gas mixtures. ${({Z}_{\mathrm{eff}})}_{{\mathrm{N}}_{2}}$ remains constant at 40.0\ifmmode\pm\else\textpm\fi{}0.9 at ${\mathrm{N}}_{2}$ densities below about 2.3 amagat, but increases to about 108 at 3.7 amagat. Shoulder regions in the lifetime spectra of positrons in ${\mathrm{N}}_{2}$ and in ${\mathrm{N}}_{2}$-Ne mixtures have been observed which have a constant width of about 14 nsec amagat at ${\mathrm{N}}_{2}$ densities below about 2 amagat, but which increase to about 24 nsec amagat at 3.7 amagat of ${\mathrm{N}}_{2}$. For ${\mathrm{N}}_{2}$ densities in the range 2-3.7 amagat, the measured shoulder widths correspond to a constant time of about 7 nsec. The slow-positron annihilation rates and the shoulder widths are discussed in terms of the self-trapping of positrons within clusters of ${\mathrm{N}}_{2}$ molecules.