In order to detect the signatures of new physics processes, the CMS detector will need to identify and precisely measure the energies and location of electrons and photons. This is the task of a sub-detector called the Electromagnetic Calorimeter (ECAL) which is split into barrel and end-cap sections to surround the collision point of the interacting protons. The Rutherford Appleton Laboratory is responsible for designing both of the ECAL end-caps for CMS and will build one of them.
| The ECAL consists of around 118,000
tapered lead tungstate crystals with about 25,000 in the
end-caps. Due to the density of the lead tungstate (8.3
g/cm**3), electrons and photons entering the crystals
interact and produce ``showers'' of secondary particles.
Each shower itself consists of many electrons and
photons, but these possess less and less energy as they
travel through the calorimeter and are absorbed. By
measuring the scintillation light produced by all of the
secondary electrons, it is possible to determine the
energy and position of the primary electron or photon
which first hit the calorimeter. The scintillation light is collected within each crystal by total internal reflection and finally detected at the end of the crystal by a photodetector. For this reason, the crystals have to have good light transmission in the wavelengths concerned, but at the same time be dense enough to absorb the many hundreds of secondary particles that can be produced in each shower. |
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