A NaF RICH Counter

A NaF RICH Counter

DOI: 10.4018/978-1-5225-0242-5.ch011
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Abstract

RICH detectors are an important type of photosensitive detectors often used in particle and astroparticle physics for particle identification. They are quite complicated to construct and operate. The following chapters will describe in more detail examples of such detectors and their performance.
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1. The History Of The Naf Rich Project: The Cplear Spectrometer

The initial impulse for the NaF RICH detector came originally from the CPLEAR experiment at CERN. The CPLEAR collaboration intended to measure the CP violating parameters in the neutral kaon system with initially-pure K0 and K0beams from the annihilation reaction p p-> p+- K-+ K0 (K0) at rest. To do so, a good separation of pions and kaons is necessary. The strangeness of the neutral kaon is identified by the sign of the charged kaon. This is determined by measuring the deflection in a 0.5 T magnetic field, while the separation of pions and kaons is performed by a Cherenkov counter. The Low Energy Antiproton Ring (LEAR) at CERN can deliver 2.106 antiprotons per second, and the branching ratio for the interesting reaction is 0.4%. Therefore, the particle identification device has to respond fast to reduce the rates at the first trigger level. It also has to be compact since an old magnet is used and the space allocated for the particle identification is small. Figure 1 shows a cross-section of the detector with a typical event.

Figure 1.

A cross-section of the CPLEAR detector with atypical event; an antiproton annihilates with a proton in the hydrogen target in the center producing a charged pion, a charged kaon and a neutral kaon. The neutral kaon decays into a π+π pair on its way out of the detector. The overall diameter is 2 m and its length is 5 m. The spectrometer consists (from the center and outwards) of multiwire proportional chambers, streamer chambers, scintillators, liquid Cherenkov detectors and a lead electromagnetic calorimeter.

From Jonsson, 1988.

The detector consists, moving from the center outwards, of the target, two proportional chambers, six drift chambers, a streamer tube system, scintillators (S1) used for time of flight, the Cherenkov counter, a second layer of scintillators (S2), an electromagnetic calorimeter and the coil of the magnet. The beam axis is perpendicular to the plane of the figure.

The requirements of the particle identifier are the following:

  • 1.

    Highly efficient K-π separation (1:104) up to 1 GeV/c

  • 2.

    Fast: A trigger signal has to be obtained in 250 ns.

  • 3.

    Compact: Only 8 cm are totally available.

  • 4.

    Low density material, not to convert γ-rays from π0 decays.

  • 5.

    Insensitive to magnetic field (0.5 Tesla).

Two independent projects started to try to develop a detector which fulfils the requirements: a conventional threshold Cherenkov counter and a RICH counter. The idea was to have a sandwich consisting of a 4 cm conventional threshold Cherenkov and a 4 cm RICH counter.

An 8 cm conventional threshold Cherenkov counter turned out to be the cheapest and simplest solution which gives satisfactory results and was later used in the experiment.

The development of the RICH counter continued anyway as a pure detector development project, and resulted in a fully tested prototype which performs better than ever expected in the beginning.

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2. The Conventional Cherenkov Counter

The conventional Cherenkov counter is a threshold Cherenkov counter and consists of 32 extruded tubes of UV transparent polymethyl methacrylate (PMMA) with a wall thickness of 2 mm (Rickenbach 1989). The ends of each tube are closed directly by light guides of the same material, and equipped with two PM tubes at each end. The Cherenkovs are filled with a liquid radiator C6F14 (FC72, Fluorinert) with a refractive index of 1.26 in the visible region of the spectrum and a very low dispersion down to a wavelength of 200 nm. The whole tube is wrapped in aluminum foil and made light tight. The Cherenkov light produced in the liquid travels to the PM tubes at the ends through total reflection in the detector walls. One detector module is shown in Figure 2.

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