A RICH detector capable of detecting unit charged particles, e.g. antiprotons and positrons, was in 1994 used successfully for the first time in a balloon borne magnet spectrometer. The thin and compact CAPRICE94 RICH detector uses a NaF solid radiator, TMAE vapor as photo-converter and cathode pad readout in the photosensitive MWPC operated at low gain. 15 photoelectrons are detected per ring for ß = 1, perpendicular incidence particles giving a resolution on the Cherenkov angle of 8 mrad, increasing to 14 mrad at 20oC incidence angle. Besides particle identification on an event-by-event basis it efficiently rejects multiparticle events and albedo particles (Barbiellini, 1996).
Top1. Introduction
The Cosmic AntiParticle Ring Imaging Cherenkov Experiment (CAPRICE) has for the first time used a RICH detector in a balloon borne magnet spectrometer that is sensitive to unit charged particles. Previously, the Chicago University group (Buckley, 1992) has flown a gas-Rich detector sensitive to helium and higher charges.
The aim of the experiment is to measure the flux of antimatter, mainly antiprotons and positrons, and light isotopes in the cosmic radiation. Sophisticated particle identification detectors are needed to identify the rare antiprotons in a large background of electrons, muons and pions, and the positrons in an even larger background of protons.
The NMSU/WiZard CAPRICE94 spectrometer (Golden, 1991) is shown in Figure 1 and includes from top to bottom: a solid radiator RICH detector, scintillators for time-of-flight and dE/dx measurements, a 4 Tesla super conducting magnet with a tracking system of multiwire proportional chambers (MWPC) and drift chambers (Hof, 1994), and an electromagnetic Si-W calorimeter (seven radiation lengths deep) (Barbiellini, 1993).
Figure 1.
The CAPRICE94 magnet spectrometer; it consists of a superconducting magnet giving an up to 4 T strong magnetic field in which the deflection of the cosmic particles is measured by a set of drift chambers. The particle identification is done by a solid radiator RICH detector, a W-Si electromagnetic calorimeter and a time-of-flight system.
The flight took place from Lynn Lake, Manitoba, Canada, on August 8-9, 1994. Data was successfully collected during the 3.5 hour long ascent and for 23 hours at a float altitude above 36 km (less than 5 g/cm2 residual atmosphere). Totally, data from more than 6 million cosmic rays were recorded. All detectors worked well during the flight.
Top2. The Caprice94 Rich Detector
The CAPRICE94 RICH detector is shown in Figure 2. Cherenkov light is emitted in a 10 mm thick solid radiator of NaF when a charged particle transverses the detector if β > 1/n (β is the particle velocity relative to the speed of light, n is the refractive index of NaF). The cone of light refracts out of the crystal and is expanded in a nitrogen volume before entering into a photosensitive MWPC. The Cherenkov photons are converted into photoelectrons by the photosensitive compound tetrakis-(dimethylamino)-ethylene (TMAE) (Anderson, 1980), and are amplified in a MWPC. The induced pulse in the cathode pad plane is detected and gives an unambiguous image of the Cherenkov light. The MWPC is operated at low gain (a few times 104) with pure ethane as amplification gas. The size of the cathode pads are 8 by 8 mm2.
Figure 2. A schematic view of the CAPRICE94 RICH detector; a 1.7 GeV/c antiproton event is indicated. The Cherenkov light emitted in the NaF crystal is detected in a photosensitive MWPC with pad readout after expansion in nitrogen. A quartz window transparent to the ultra violet Cherenkov photons separates the nitrogen gas from the TMAE and ethane gas mixture in the MWPC. In the pad plane at the bottom is illustrated the two types of signals that occurs. At the edges are the signals from the Cherenkov photons. In the center are the signals from the ionization of the amplification gas by the charged particle.