2
C. W. Fabjan and H Schopper.
collisions. Electron-positron colliders are characterised by events with relatively few
outgoing particles since two pointlike particles collide and the strong interaction is
negligible in such reactions. After the shutdown of LEP in 2000 the next electronpositron collider is far in the future and progress is now depending on proton-proton
collisions at the LHC at CERN or heavy ion colliders, e.g. GSI, Germany, RHIC at
BNL in the USA and also LHC. Protons are composite particles containing quarks
and gluons and hence proton collisions produce very complicated events with many
hundreds of particles. Consequently, detectors had to be developed which are able to
cope with extremely high data rates and have to resist high levels of irradiation. Such
developments were in particular motivated by the needs of the LHC experiments.
It seems plausible that accelerators and colliders have to grow in size with
increasing energy. But why have detectors to be so large? Their task is to determine
the direction of emitted particles, measure their momenta or energy and in some
cases their velocity which together with the momentum allows to determine their
mass and hence to identify the nature of the particle.
The most precise method to measure the momentum of charged particles is to
determine their deflection in a magnetic field which is proportional to B · l where B
is the magnetic field strength and l the length of the trajectory in the magnetic field.
Of course, it is also determined by the spatial resolution of the detector to determine
the track. To attain the highest possible precision superconducting coils are used in
most experiments to produce a large B. Great efforts have been made to construct
detectors with a spatial resolution down to the order of several microns. But even
then track lengths l of the order of several meters are needed to measure momenta
with a precision of about 1% of particles with momenta of several 100 GeV/c. This
is the main reason why experiments must have extensions of several meters and
weigh thousands of tons.
Another possibility to determine the energy of particles are so-called “calorimeters”. This name is misleading since calorimeters have nothing to do with calorific
measurements but this name became ubiquitous to indicate that the total energy of
a particle is measured. The measurement is done in the following way. A particle
hits the material of the detector, interacts with an atom, produces secondary particles
which, if sufficiently energetic, generate further particles, leading to a whole cascade
of particles of ever decreasing energies. The energy deposited in the detector material can be measured in various ways. If the material of the detector is a scintillator
(crystal, liquid or gas), the scintillating light is approximately proportional to the
deposited energy and it can be observed by, e.g., photomultipliers. Alternatively, the
ionisation produced by the particle cascade can be measured by electrical means.
In principle two kinds of calorimeters can be distinguished. Electrons and
photons produce a so-called electromagnetic cascade due to electromagnetic interactions. Such cascades are relatively small both in length and in lateral dimension.
Hence electromagnetic calorimeters can consist of a homogenous detector material
containing the whole cascade. Incident hadrons, however, produce in the cascade
also a large number of neutrons which can travel relatively long ways before losing
their energy and therefore hadronic cascades have large geometrical extensions even
C. W. Fabjan and H Schopper.
collisions. Electron-positron colliders are characterised by events with relatively few
outgoing particles since two pointlike particles collide and the strong interaction is
negligible in such reactions. After the shutdown of LEP in 2000 the next electronpositron collider is far in the future and progress is now depending on proton-proton
collisions at the LHC at CERN or heavy ion colliders, e.g. GSI, Germany, RHIC at
BNL in the USA and also LHC. Protons are composite particles containing quarks
and gluons and hence proton collisions produce very complicated events with many
hundreds of particles. Consequently, detectors had to be developed which are able to
cope with extremely high data rates and have to resist high levels of irradiation. Such
developments were in particular motivated by the needs of the LHC experiments.
It seems plausible that accelerators and colliders have to grow in size with
increasing energy. But why have detectors to be so large? Their task is to determine
the direction of emitted particles, measure their momenta or energy and in some
cases their velocity which together with the momentum allows to determine their
mass and hence to identify the nature of the particle.
The most precise method to measure the momentum of charged particles is to
determine their deflection in a magnetic field which is proportional to B · l where B
is the magnetic field strength and l the length of the trajectory in the magnetic field.
Of course, it is also determined by the spatial resolution of the detector to determine
the track. To attain the highest possible precision superconducting coils are used in
most experiments to produce a large B. Great efforts have been made to construct
detectors with a spatial resolution down to the order of several microns. But even
then track lengths l of the order of several meters are needed to measure momenta
with a precision of about 1% of particles with momenta of several 100 GeV/c. This
is the main reason why experiments must have extensions of several meters and
weigh thousands of tons.
Another possibility to determine the energy of particles are so-called “calorimeters”. This name is misleading since calorimeters have nothing to do with calorific
measurements but this name became ubiquitous to indicate that the total energy of
a particle is measured. The measurement is done in the following way. A particle
hits the material of the detector, interacts with an atom, produces secondary particles
which, if sufficiently energetic, generate further particles, leading to a whole cascade
of particles of ever decreasing energies. The energy deposited in the detector material can be measured in various ways. If the material of the detector is a scintillator
(crystal, liquid or gas), the scintillating light is approximately proportional to the
deposited energy and it can be observed by, e.g., photomultipliers. Alternatively, the
ionisation produced by the particle cascade can be measured by electrical means.
In principle two kinds of calorimeters can be distinguished. Electrons and
photons produce a so-called electromagnetic cascade due to electromagnetic interactions. Such cascades are relatively small both in length and in lateral dimension.
Hence electromagnetic calorimeters can consist of a homogenous detector material
containing the whole cascade. Incident hadrons, however, produce in the cascade
also a large number of neutrons which can travel relatively long ways before losing
their energy and therefore hadronic cascades have large geometrical extensions even