A Time-Of-Flight measurement system for the HADES wide-aperture dielectron spectrometer

The HADES spectrometer and a time-of-ilight detector based on long-sized (50-250 cm) scintillation counters are described. The detector is designed for identification of electrons, pions, and protons with momenta of 0.1-1.5 GeV/c at a flight distance of 220-250 cm. The operating conditions are analyzed, and the requirements to the detector and counter design are formulated. The design of the counters and their arrangement in the spectrometer are considered. The results of experimental studies of such characteristics of long counters as the light attenuation length in a scintillator and temporal and spatial resolutions, depending on the location of the point of the particle's hit on the counter, are presented. Analysis of the data has shown that the main factor determining the spatial resolution of long counters is the quality of the polishing of the side faces of the scintillators, which determines the effective (averaged over the scintillator side surfaces) coefficient of total internal reflection R. It is shown that, for R ≥ 0.99, it is quite possible to achieve temporal and spatial resolutions of 100 ns and 2-3 cm, respectively, for scintillators with a length of up to 250 cm and a cross section of 1-20 cm2. Foreign and Russian photomultipliers were used in the counters.