Annihilation of Positrons in Argon I. Experimental

The annihilation of positrons in argon has been investigated as a function both of argon density and applied electric field. The direct annihilation rate decreases with increasing field as observed in earlier work. These electric field results are compared with simple theoretical models of the positron-argon atom interaction in the following paper. A small nonlinear dependence on density of the direct annihilation rate became apparent at densities larger than about 10 amagats. The linear portion of the direct rate was characterized by $Z_{\mathrm{eff}}=27.3\mathrm{}\pm 1.3$. From the density dependence of the orthopositronium lifetime, the free orthopositronium annihilation rate (${\lambda }_1$) and linear quenching rate in argon (${\lambda }_q$) were found to be