Test beam study of a short drift GEM tracking detector

A short drift GEM tracking detector was studied in the H6 test beam at CERN as part of an effort to develop new tracking detectors for future experiments at RHIC. The detector consists of a triple GEM stack with a 16 mm drift gap and a COMPASS style readout plane with XY readout strips that is used in a mini TPC type configuration. Charge produced by tracks passing through the drift gap is collected over ~ 700 ns and sampled at a rate of 25 ns which allows a measurement of the drift time of the charge clusters from the initial ionization. This enables a determination of the angle of the track passing through the detector as well as its position. The resulting vector can be used to improve the position resolution compared to simple charge centroid determination for tracks passing through the detector at large angles, and can also reduce the number of detectors required to measure tracks with a given precision. We have studied some basic characteristics of the short drift GEM detector in the lab using a beta source and cosmic rays, and have also studied it more extensively in a test beam at CERN. This paper will report on the results of these studies.