PHY6040 Particle Detectors Dr C N Booth

Electromagnetic Calorimetry

Calorimeters or shower detectors work in a radically different way from the tracking detectors we have considered previously.  Tracking detectors cause minimal disturbance to charged particles which pass through them, sensing the small amount of ionisation or excitation along the path of the particle.  Shower detectors, on the other hand, degrade the energy of the particle, sharing it among a very large number of shower products, which are measured or sampled to determine information about the primary particle.

Shower detectors have two advantages over tracking detectors:

In an electromagnetic shower, electrons undergo bremsstrahlung, producing photons.  Photons in turn undergo pair production, producing electrons (and positrons).  The number of particles therefore rapidly increases until the average energy of the products drops below the critical energy, at which point energy loss is primarily by ionisation (by the charged particles) and the shower decays away.

The number of charged particles in the shower (or the track length of these particles) is proportional to the energy of the primary particle.

The length scale of the shower is set by the radiation length X0 of the material involved.  (The transverse spread is parametrised in terms of the Moliére unit, which is also derived from the radiation length.)

Practical Electromagnetic Calorimeters

A reasonable size for the shower detector implies that the radiation length of the material must be small.  This in turn requires a high-Z material.  There are two classes of such shower detectors:

Electromagnetic Calorimeters often form the first module of a combined electromagnetic and hadronic calorimeter.

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