A large area transition radiation detector to measure the energy of muons in the Gran Sasso underground laboratory

We have designed and built a transition radiation detector of 36 m2 area in order to measure the residual energy of muons penetrating in the Gran Sasso cosmic ray underground laboratory up to the TeV region. It consists of three adjacent modules, each of 2x6 m2 area. Polystyrene square tubes, filled with a argon-carbon dioxide gas mixture, and polyethylene foam layers are used as proportional detectors and radiators respectively. We cover such a large surface with only 960 channels that provide adequate energy resolution and particle tracking for the astroparticle physics items to investigate. The detector has been calibrated using a reduced size prototype in a test beam. Results from one module exposed to cosmic rays at sea level are shown. (orig.)