| 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:
-
They are sensitive to both charged and neutral particles
-
When used to measure the energy of the particle, their accuracy increases
with the energy of the particle. This is because the fluctuations
in the shower process decrease as the number of particles in the
shower increases, i.e. as the energy increases. (In contrast,
the use of curvature in a magnetic field to
determine momentum gives a fractional error proportional to the
momentum of the particle. At high enough energy, the shower detector
must always therefore have a better energy
resolution.)
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:
-
Homogeneous Detectors In this case, the same medium is used
both to cause the shower development and to detect the produced particles.
-
One means of detection is through Cherenkov
light produced in a transparent medium such as lead glass (55% PbO,
45% SiO2), which has a radiation length of 2.36 cm.
The resolution
σ/E
is about 5%/√E
(E being measured in GeV).
-
Scintillation light can also be used to
detect the electrons in the shower, for example with sodium iodide
crystals. This has a radiation length of 2.6 cm, and a resolution
σ/E
of about 1.5%/E−1/4 (E again being
measured in GeV). Another possible scintillator used in
crystal
calorimeters is Bismuth Germanate, known as
BGO.
-
Heterogeneous or Sampling Calorimeters Here, the medium responsible
for the showering is separate from that used in detection. Typical
construction is that of a sandwich counter, with alternate high-Z
passive layer of converter, such as lead (with X0 of
0.56 cm), and active detecting layers. Commonly used detectors are
-
sheets of plastic scintillator. These
may be read out from the back, using wavelength-shifter sheets between
separate calorimeter modules in a "projective tower" geometry.
-
liquid ionisation chambers (employing
liquid argon, TMP (tetramethyl pentane), TMS (trimethyl silane) etc.).
-
planes of proportional wire chambers,
which provide spatial localisation of the shower, as well as energy information.
The energy resolution for sampling calorimeters depends on the thickness
of the absorber, as well as systematic effects such as uniformity and ageing
of the active material.
σ/E
is typically (5 - 15%)/√E
(E again being measured in GeV).
Electromagnetic
Calorimeters often form the first module of a combined electromagnetic
and hadronic calorimeter.
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