LHC

The Large Hadron Collidor (LHC) accelerator to be build at CERN will provide
particle beams with an energy up to 7 TeV per particle to conduct experiments.
In head-on collisions, the particles will "explode" and their kinetic energy
will be converted to sub-atomic, short living particles. These secondary particles
provide inside information about the core of the atom.
Detectors are used for measuring these physical events. One of them, ATLAS , will
be one of four major detectors to be installed at CERN. It combines several sub-
detectors including liquid Argon calorimetry within one toroidal apparatus.


ATLAS

ATLAS, which is an abbreviation for A Toroidal LHC apparatus, contains super-
conducting magnets which will bend the path of the particles. By measuring the
amount of bending, the momentum can be determined. Measuring the path will be
done by inner and outer muon detectors. The inner detectors consist of silicon
layers, with conducting tracks on the surface. The outer detectors are built
together with modules, containing large amounts of tubes filled with gas and
wires through the centre. These wires are set to high voltage. When a particle
passes through a tube, the gas will be ionised and a small current flows from
the wire to the tube. These currents are amplified and sent to a central computer,
which can reconstruct the path of the particle and show it graphically.

To measure the amount of energy of a hadron, a calorimeter is used. When a hadron
passes through a material with a high atomic number, it looses energy since it
interacts with the Coulomb field around a nucleus and emits an energetic photon
(Bremsstrahlung). This photon will interact predominately via pair production, and
the particles generated will also interact with other nuclei hence producing a
"shower" of interacting particles, losing some energy at each event. A higher energy
of the incoming particle means a higher amount of produced low-energy particles.
These low-energy particles can be detected by interleaved inert absorbers, sensitive
to particles passing through. By counting the number of particles, the energy-level
of the responsible hadron can be determined.
In ATLAS, this material with a high atomic number will be liquid argon (LAr). To
calibrate the calorimetry in ATLAS, a test beam is provided at CERN. The energy-
amount of the particles in this beam is known precisely. When a calorimeter is placed
in the path of this beam, the readings can be adjusted to meet the exact value.
The LAr is contained in three large (5,5 to 10 m³) cryostats, and fed via a network of
valves and pumps into the calorimetric test facility.
Each time a particle interacts with the liquid argon, a slight amount of heat is
generated. Without proper precautions, the LAr starts to boil and gas will be generated.
Apart from this heat generation by particle interactions, the LAr will be heated due to
thermal losses of the containment.
To correct for these losses, a heat exchanger is placed in the cryostat. Liquid nitrogen
is fed through the exchanger, and gaseous argon will condense again and drip back in the
vessel.


LIQUID ARGON VALVE BOX

One important part of the distribution network, is the Liquid Argon valve box. This valve box controls the Argon flow to the cryostats, and can also be used to empty the cryostats and pump the Argon back into the dewar. In addition, the Argon can be fed through a purification module. A centrifugal pump is used for these operations. To avoid cavitation in the pump, no
gaseous Argon may enter the inlet.
Inside the LAr valve box, the argon is cooled down with Liquid Nitrogen. The valve box containing the
vessel filled with LAr is placed about 1 meter above the pump. Twelf valves, located
on top of the vessel control the Argon flow. The cryo vessel is operated at 5 bar.