The KLOE tracking chamber

Measuring ϵ′ϵ in the kaon complex with the KLOE detector at DAΦNE, the Frascati φ-factory, poses non-trivial problems to the tracking system. The solution adopted by the KLOE collaboration is a single cell drift chamber, non-conventional for both its huge dimensions and for the gas mixture. He-based gas mixtures are characterized by a slow, non saturated drift velocity and by a low electron yield. The R & D activity reported here shows however that binary He mixtures possess the necessary characteristics in terms of stability of operation, cell efficiency, spatial resolution.