Ultra-light gas mixtures for drift chambers

We present results on the measurements of gas gain for gas mixtures at absolute pressure below the atmospheric pressure, down to 100 mbar, and their relative stability. Besides the obvious advantage of further limiting the contribution to the momentum measurement due to multiple scattering, the operation at low pressure allows for a fine tuning of the working parameters of a drift chamber like drift velocity, diffusion and specific ionization. Furthermore, such a possibility is of particular interest for experiments like the direct muon to electron conversion experiment Mu2e at Fermilab, where the tracking detector needs to operate in vacuum. Plans for extending the measurements to transport parameters, like drift velocity and diffusion, will also be presented.