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II.
The Calorimeters
Surrounding the ID concentrically are the Electromagnetic Calorimeters and the
Tile Calorimeter, hadronic end-cap and forward calorimeters. The Calorimeters
are based on “Sampling Calorimeter” technologies
whereby particles interact
with an absorber medium to generate particle showers which then deposit energy
into an active sampling medium. The calorimeters measure the energy of both
charged and neutral particles. They are designed to contain all el ectromagnetic
and hadronic
showers developing within them, and only neutrinos and muons
manage to exit from these layers.
a.
The Electromagnetic calorimeter
The Electromagnetic (EM) calorimeter measures mainly the energy of electrons
and photons. It consists of a barrel
region housed in a cryostat, flanked by two
end-caps (EMEC). The barrel component has an
accordion shaped structure
containing layers of lead and stainless steel that act as the particle absorbers.
Between these layers are copper grids immers ed in liquid Argon (LAr) cooled to
-183°C. The EMEC has a similar structure, but consists
of a parallel plate
geometry instead.
As a particle, for example an electron, passes through several layers of the EM
calorimeter, it generates a large
shower of low energy electrons, positrons and
photons. The shower particles then ionise atoms in the LAr, thus creating more
electrons and positive ions. The copper grid then functions as an electrode which
the negative charges migrate to and the current is measured.
The EM calorimeter is designed such that electrons and photons stop within it.
Therefore, by measuring the total charge deposited on the copper electrodes by a
particular
particle shower, the total energy of the original particle as it entered
the EM calorimeter can be reconstructed.
