Study of<b><i>d</i></b><i><i>E</i></i>/<b><i>d</i></b><i><i>x</i></i>in a Drift Chamber with He-C_<b>2</b>H_<b>6</b>Gas Mixture

We studied d E / d x in a small-cell drift chamber filled with a gas mixture of 50% He and 50% C 2 H 6 . The d E / d x resolution, measured in terms of the 80% truncated mean of 41 layer samples, was 5.2% when the incident angle of the track with respect to the wire direction was 45°. The βγ dependence showed a smaller relativistic rise than in the case of P-10 gas. At an incident angle of 90°, we observed gas gain saturation due to the space charge effect. Although this makes the βγ dependence of d E / d x weaker, the d E / d x resolution was improved. As a result, a π- K separation of 2σ is achieved at all incident angles for 2 GeV/c tracks.