Nuclear Instruments and Methods in Physics Research A 367 (1995) 240-243 zyxwvutsrqponmlkjihgfedcbaZYXWV NUCLEAR INSTRUMENTS &MElwooS IN PHYSICS ELSEVIER =zzF The TIC - a multi-particle threshold imaging Cherenkov detector C.W. Fabjan”, A. Franz”, F. Piuz”, J.C. Santiard”, M. Spegel”‘“, T.D. Williams”, M. Cherneyb, S. Esumi”, T. Sugitate”, J. Schmidt-Sorensend, B. Kubica”, M. LeGuay”, G. Paic”“, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR “CERN, Geneva, Switzerland bCreighron University, Nebraska, USA ‘Hiroshima University, Japan dNiels-Bohr-lnstitute, Copenhagen, Denmark ‘SUBATECH, Nantes. France zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM Abstract The Threshold Imaging Cherenkov (TIC) was built as part of an upgrade programme for the NA44 spectrometer in view of 160 GeV. A lead-ion beams from the CEFW SPS. The detector is used to distinguish pion tracks from those of heavier particles (K, p, d) in the momentum range of 4-8 GeV/c. Cherenkov photons produced by pions in a 1 m long isobutane radiator are converted to electrons in TMAE at 25°C and detected in a h4WPC with 2-dimensional cathode pad readout. A threshold is set on the number of pads hit within a fiducial zone on the cathode plane around a projected external track. 1. Introduction 2. Detector geometry In lead-lead collisions, the focussing spectrometer NA44 views on average 3-4 charged particles within its acceptance. To preserve the particle identification capa- bility of the experiment, which was originally designed for one or two particles, a position sensitive Cherenkov counter was developed to distinguish pion tracks from those of heavier particles in the momentum range of 4-8 GeV/c2. Due to the magnet optics of the spectrometer-the tracks do not have a common geometrical origin - and the particle momentum range, conventional ring imaging methods [2,3] are not applicable. The alternative idea of a position sensitive threshold Cherenkov detector is based on the fact that in the interesting momentum range only pions emit, under a small angle of about 3”, Cherenkov light in the chosen radiator gas3. Fig. 1 shows a vertical cut through the detector. Chere- nkov light is produced in the 130 X 90 X 40 cm3 radiator tank filled with isobutane at atmospheric pressure. Two flat mirrors at 45” reflect the UV-photons through 80 X 25 cm* quartz windows into two photon detection chambers with two-dimensional cathode-pad readout (see Section 4) at the top and bottom of the tank. At these positions, the chambers are outside the particle acceptance of the TIC (25 X 67 cm’). The V-shaped mirror geometry and the resulting division of the photo-sensitive area in two parts minimizes the variation of the photon yield due to the different path lengths of particles and photons through the radiator gas. * Corresponding author. E-mail spegel@afsmaiI.cem.ch ’ On leave of absence from Ruder Boskovic Institute. 2 A description of the NA44 spectrometer in the classical setup as well as after tbe upgrade for lead ion beams can be found in Ref. [l], where also the Threshold Imaging Cherenkov is treated in some more detail. ‘Isobutane at atmospheric pressure has an index of refraction n = 1.0017, so the Cherenkov threshold is xh = 24 and the corresponding threshold momenta are pz = 2.4 GeVlc and p,: = 8.5 GeVlc for pions and kaons respectively. Fig. 1. Schematic side view of the Threshold Imaging Cherenkov. The detector is surrounded by a temperature-stabilized box to allow efficient photon absorption in TMAE. 0168-9002/95/$09,50 0 1995 Elsevier Science B.V All rights reserved SSDI 0168-9002(95)00569-2