5.5.3.2  Overview

Cerenkov detectors are used for particle identification in physics experiments. They are based on the Cerenkov effect. The velocity of the light in transparent materials (glass, altuglass, gas,....) is smaller than in vacuum (their index of refraction is less than one). When a high energy charged particle travels through these materials its velocity, still limited to the velocity of light in the vacuum, can thus be larger than the velocity of light in the material. When this condition occurs a light is emitted, called the Cerenkov light. By detecting if a given particle emits the Cerenkov light, one can detect if its velocity is larger than a threshold velocity depending on the material used. In this manual a gas Cerenkov for electron identification is described.

Two similar threshold gas Cerenkov counters have been constructed as a part of the particle identification equipment to be included in the focal plane detectors of the High Resolution Spectrometers (HRS) of the TJNAF experimental Hall A. Each counter housing is made in steel with thin entry and exit windows made of tedlar. Light weight spherical mirrors have also been built resulting in a very thin total thikness traversed by particles. The counter is operated at atmospheric pressure with CO2.

This two counters have identical sections but different thicknesses, 1000mm for the hadron arm and 1500mm for the electron arm. These are gas Cerenkov which are use as threshold counters. The refraction indice of the gas is choose in order to give the maximum light impulsion for electrons and to stay inefficient to other particles, for instance pions until they get 4.8GeV/c of impulsion. With CO2 at normal pressure, the refraction indice is n=1.00041 which give a threshold of

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p_min = 17 MeV/c for electrons

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p_min = 4.8 GeV/c for pions

Each Cerenkov is made of 10 multiplicator tubes (PMT) and 10 mirrors.

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The PMT (type Burle 8854) have a spherical entrance window of 129mm of diameter which only a spherical part of 110mm of diameter is efficient to collect the light obtained after reflection on the mirrors. The photocathod is made of Bialkali with a quantum efficiency of 22.5% at 385nm and a extended response in the UV until 220 nm.

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Each mirror has a rectangular profile built in a empty sphere of interior radius (reflective face) of 900mm and thickness of 10mm. The very light structure is built like hony comb and is constituted as the following maner:

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vernis which protect aluminim

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aluminium which assure the reflectivity

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plexiglas which assure a good surface

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a sandwich (carbon-epoxi, phenolic honey comb, carbon epoxi) which assure the rigidity of the system.

The 10 mirrors are placed just before the output window and are grouped in two colomns of 5 mirrors. Each mirror reflect the light on a PMT placed at the side of the box (figure ). The mirrors of the same column are identicals and the two columns are almost symetrical. Positions ans angles of the PMT are not placed regulary like for the mirrors but were ajusted by a optical study in order to maximise the collection of light coming from the particular envelope of particle which have to be detected with the High Resolution Spectrometer (HRS) of the Hall A. PMT are fixed and mirrors can be ajsuted by hand.

The number of photoelectrons is not very high so that the distribution is a possonien distribution,

where represent the number of photoelectrons collected after the parcours in the radiator.
The inefficiency is defined by the probability to detect zero photoelectron, if we decide to put a threshold below one photoelectron, and then is defined by the quantity   This number of photoelectrons produced by Cerenkov effect by unity of lenght crossed in the radiator is given by the following formula:

with

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is fine structure constant

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is the wave lenght

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is the Cerenkov angle

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is the quantum efficiency of the photocathod

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=efficicency of transmission of the radiator (we take 100% )

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=efficiency of reflectivity of the mirrors (a typical curve is given in paragraph ).

If we make the integral with the caracterictics of the Cerenkov counters, we expect to have 15 photoelectrons per meter of gas. With our apparatus the lenght of gas crossed is 1.5 meters so we expect to have 23 photoelectrons. This theoretical value will be verified experimentaly with beam and will permit to have an efficiency greater to 99.99%.