Construction and beam test results for the DELPHI two metre straw detector

A straw tube detector has been built as part of the 1995 upgrade for the DELPHI experiment at LEP. The detector is cylindrical in design, with inner wall and outer wall radii of 25 cm and 30 cm respectively. It consists of 960 straws in 5 layers, with a length of 2.1 m, running at atmospheric pressure with a 50:50 argon-ethane gas mixture. The efficiency and spatial resolution arising from the measurement of the drift time have been measured in a test beam. Preliminary results with a full scale prototype and a 2:1 argon-isobutane gas mixture at a gain of 2 × 104 give a coordinate resolution of 99 μm per straw, a resolution for a track element in the detector of 49 μm and an efficiency per straw of more than 99%.