lecture_6_11 - Phys 433 Lecture 6 Detectors such as drift...

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Phys 433 – Lecture 6 Detectors such as drift chambers usually consist of an enclosed gas volume, a small diameter anode wire that has high tensile strength (why) and a cathode plane Typical anode diameters range from 10-100 m with gold plated Tungsten wire being the popular choice (why gold plated?) Charged particles traversing the gas volume of these detectors leave free electrons along their trajectory The electric field causes the ions to drift towards the anode wire and as they approach the wire the electric field strength increase as 1/r, which results in an avalanche of secondary ionization as the energetic electrons collide with gas atoms. Many photoelectrons are produced as the avalanche develops; these can spread throughout the volume of the chamber causing undesirable ionization, including photoemission from the cathode surface that results in more electrons drifting towards the anode wire. A quenching gas (usually organic molecules that have a large photo- absorption cross section) added to the gas will absorb most of these photons. Gas gains of 10 4 to 10 5 and occasionally higher are typical November 10, 2011 Henry Lubatti 1
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Physics 433 – Lecture 6 The avalanche near the wire develops with the electrons at the head (nearer to the wire) and the positive ions trailing behind because they move more slowly and drift in the opposite direction A spread of the electrons in the direction of the wire results from diffusion and electrostatic repulsion of the electrons Avalanche multiplication is characterized by first Townsand coefficient given by = 1/ which is the number of an ionizing collisions per unit path length. If we have n electrons, after a distance dx there will be dn = n dx new electrons generated In a distance x we have n = n 0 exp( x) electrons where n 0 is original number of electrons November 10, 2011 Henry Lubatti 2
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Physics 433 – Lecture 6 Cylindrical Drift Chamber – a simple and effective device Consists of a hollow conducting cylinder with a coaxial, tensioned wire of radius r = a Because of the drift field is proportional to 1/r the drift velocity will not be constant as the electron drifts towards the wire Isochrones, equal time surfaces, are concentric with wire but unequally
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lecture_6_11 - Phys 433 Lecture 6 Detectors such as drift...

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