Abstract
Lifetime spectra for positrons annihilating in helium gas have been measured at room temperature for gas densities up to 60 amagat. The equilibrium annihilation rate for the free positron component is proportional to the gas density D, in amagats, given by lambda 1=(0.793+or-0.004 D mu s-1 which is equivalent to Zeff=3.94+or-0.02 and is in good agreement with recent calculations. The variation of the equilibrium decay rate with gas density for the ortho-positron component is also linear, given by lambda 2=7.24+(0.1006+or-0.0006) D mu s-, which is consistent with pick-off quenching. No other long-lived component has been observed. For neon at room temperature, a shoulder region of width 1700+or-200 ns amagat has been established. Some new measurements for argon, krypton and xenon are also reported. A comparison is made with other recent measurements.