Final results of an experimental study of a large-area planar drift chamber filled with a He-based mixture

This paper reports the final results of an accurate and complete experimental study of the performance of a large-area (1150 × 566 mm2) planar drift chamber filled with a HeiC4H10 (70−30%) mixture. Sixteen chambers of this type will constitute the elements of the central part of the tracking device of FINUDA, one of the two spectrometers approved for running at DAΦNE. The measured spatial resolution across the full drift cell ranges from 80 to 120 μm for particles at normal incidence angle, and the measured spatial resolution along the sense wire, as measured by the charge division method, was better than 1% of the wire length. Efficiency, space-time relationship, long term stability and Lorentz angle were measured as well. Particular care was devoted to the study of the spatial resolution as a function of the incidence angle. The conclusion of this study is that the chamber performances are the same or better than those obtained with Ar-based mixtures.