A RICH detector using a gas radiator of C4F10 and a photosensitive MWPC with pad readout has been developed, tested in particle beams at CERN and used in the CAPRICE98 balloon borne experiment. The MWPC was operated with a TMAE and ethane mixture at atmospheric pressure and used a cathode pad plane to give an unambiguous image of the Cherenkov light. The induced signals in the pad plane were read out using the AMPLEX chip and CRAMS. The good efficiency of the Cherenkov light collection, the efficient detection of the weak signal from single UV photons together with a low noise level in the electronics of the RICH detector, resulted in a large number of detected photoelectrons per event. For ß~ 1 charge one particles, an average of 12 photoelectrons per event were detected. The reconstructed Cherenkov angle of 50 mrad for a ß ~ 1 particle had a resolution of 1.2 mrad (r.m.s). The RICH was flown with the CAPRICE98 magnetic spectrometer and was the first RICH detector ever used in a balloon borne experiment capable of identifying charge one particles at energies above 5 GeV. The RICH provided an identification of cosmic ray antiprotons up to the highest energies ever studied (about 50 GeV of total energy). The spectrometer was flown on 28–29 May 1998 from Ft Sumner, New Mexico, USA.
Top1. Introduction
A gas Ring Imaging Cherenkov detector (RICH) has been developed, tested in particle beams at CERN and flown in the CAPRICE98 balloon borne experiment (Ambriola, 1999). The detector was built to identify cosmic rays (Barbiellini, 1997; Bergström, 1999; Francke, 1999) in the rigidity range 2– 330 GV.
The CAPRICE98 spectrometer science objectives were to measure the antimatter component in the cosmic rays in the energy range 2–50 GeV and to study the cosmic ray composition in the atmosphere in the energy region between a few hundred MeV and 330 GeV. The main goal of the gas-RICH detector was to identify antiprotons in a large background of electrons, atmospheric and locally produced negative pions and muons. Since the first observation by Golden (Golden, 1984) there has been several experiments measuring the energy spectrum of antiprotons in the cosmic rays. The energy spectrum has recently been determined with good accuracy below 4 GeV kinetic energy (Orito, 2000 and references therein). The observed spectrum indicates that the cosmic ray antiprotons are mostly of secondary origin, produced by the collision of cosmic ray nuclei with the interstellar gas. Above 4 GeV only two measurements exist (Golden, 1984; Basini, 1999), both with low statistics, and which give very different flux values.
The detection of antiprotons at high energies is an experimental challenge due to the difficulty in detecting and identifying the antiprotons against a vast background of other particles. Sophisticated particle identification detectors are therefore needed to identify the rare antiprotons. The information from the gas-RICH combined with the information from the other detectors in the CAPRICE98 magnetic spectrometer provided excellent particle identification. The RICH detector described in this paper is capable of mass resolving protons/antiprotons between 18 and 50 GeV (Bergström, 2000). Below 18 GeV it acts as an efficient threshold Cherenkov detector for proton/antiproton identification.
The spectrometer was one of the most complex instruments ever flown in a balloon borne experiment for cosmic ray studies. The apparatus included a tracking system with three drift chambers (Hof, 1994), a time-of-flight system, a gas-RICH detector, a silicon-tungsten imaging calorimeter (Bocciolini, 1996; Ricci, 1999) for particle identification and a 4Tesla super-conducting magnet (Golden, 1978). It was launched with a balloon from Ft Sumner, New Mexico, USA, on 28th May 1998 and floated at an atmospheric depth of about 5.5 g/cm2 (corresponding to an altitude of about 36 km) for a period of 21 hours, at a mean vertical cutoff rigidity of about 4.5 GV. Information about more than 5 million cosmic ray particles was collected during the flight.