THE PERFORMANCE OF THE DELPHI-JADRON CALORIMETER AT LEP

The DELPHI Hadron Calorimeter was conceived more than ten years ago, a s an instrument to measure the energy of hadrons and hadronic jets fro m e(+)e(-) collisions at the CERN collider LEP. In addition it was exp ected to provide a certain degree of discrimination between pions and muons. The detector is a rather simple and relatively inexpensive devi ce consisting of around 20,000 limited streamer plastic tubes, with in ductive pad read-out, embedded in the iron yoke of the 1.2 T DELPHI ma gnet. Its depth is at minimum 6.6 nuclear interaction lengths. The ele ctronics necessary for the pad readout was designed to have an adequat e performance for a reasonable cost. This detector has proved over six years of operation to have an entirely satisfactory performance and g reat reliability; for example less than 1% of the streamer tubes have failed and electronic problems remain at the per mil level. During the past two years an improvement programme has been under way. It has be en found possible to use the streamer tubes as strips, hence giving be tter granularity and particle tracking, by reading out the cathode of individual tubes. The constraints on this were considerable because of the. inaccessibility of the detectors in the magnet yoke. However a c heap and feasible solution has been found. The cathode readout leads t o an improved energy resolution, better mu identification, a better pi /mu separation and to possibilities of neutral particle separation. Th e simultaneous anode read-out of several planes of the endcaps of the detector will provide a fast trigger in the forward/backward direction which is an important improvement for LEP200. On the barrel the syste m will provide a cosmic trigger which is very useful for calibration a s counting rates at LEP200 will be very low.