Annihilation of Positrons in Argon at High Densities

The annihilation of positrons in argon as a function of argon density at densities from 10 to 65 amagats has been investigated at room temperature. The direct annihilation rate, the reciprocal of shoulder width, and the quenching rate of $o$-positronium are found to be linearly dependent on argon density in the density range investigated within the range of the experimental error. The value of $Z_{\mathrm{eff}}$ of the direct annihilation rate is found to be 26.2 for an argon density up to 50 amagats.