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Semiconductor Tracker Leaves UK
for CERN
Aug 24, 2005: After years of painstaking effort, the last of
the 4 barrels that make up the central part of the Semiconductor Tracker (SCT),
the heart of the biggest physics collaboration in the world has today (August 24th)
left Oxford for its new home at the European Particle Physics Laboratory, CERN,
near Geneva.
At CERN, physicists from around the world are assembling the
Large Hadron Collider (LHC) which will send two counter-rotating beams of
particles round an underground ring at 99.999999 per cent of the speed of light.
When the beams are brought into collision, a shower of new particles will be
produced reproducing conditions similar to those immediately after the Big Bang.
These will be studied at four detectors around the ring. The largest of these
detectors is called ATLAS and at its heart lies the SCT tracking the movements
of the charged particles produced in the high-energy collision.
The SCT detector consists of a central region and two end
caps. The central region is formed of four concentric barrels, covered with 2112
silicon modules (30 square metres worth!). The modules were produced by
collaborators in different countries and sent to the University of Oxford for
precision assembly. To be assembled, each of the fragile silicon modules had to
be mounted on an ultra-lightweight carbon fibre cylinder using a specialised
robot.
Each silicon module has 1536 channels, where each channel is
a separate detector element which is connected by a wire bond to an individual
channel of an ASIC (application specific integrated circuit) which contains all
the read out electronics. Dr Richard Nickerson, who led assembly of the SCT at
Oxford, said “the assembly of the SCT barrels has proved to be a very
challenging project and so we are all extremely pleased to see it reach a
successful conclusion with 99.5% of the 3 million channels working - even more
than we hoped for as we had built in an element of redundancy.”
Dr Tony Weidberg, also University of Oxford, said, “It has
been both a privilege and a challenge to play our part in the world’s largest
physics collaboration. We now eagerly await the full assembly of the Atlas
detector and the start of the LHC in two years time which will allow us to
better study the nature of the Universe.”
The SCT will track the positions of charged particles passing
through the detector with an accuracy of better than 20 microns (less than the
diameter of a human hair) over one metre. A superconducting solenoid will
provide a magnetic field of 2 Tesla so from these measurements it will be
possible to calculate the momentum of the particles, an important clue in
sorting through the debris of a high energy particle collision.
This information forms a crucial part of the data gathered by the full
ATLAS Detector system and will be essential in the task of unravelling the
physics in these complex events.
Other sections of the Atlas detector will pick up other
particle properties. Between them, the different sections should allow a
complete picture to be formed.
“The LHC will be the world’s primary experiment to probe
the structure of matter and forces of nature when it becomes operational in
2007,” said Professor Keith Mason, CEO of the Particle Physics and Astronomy
Research Council (PPARC) which funds UK involvement in CERN. Professor Mason added “UK research groups are involved in
each of the LHC’s four detectors and the SCT is a clear example of our
technical expertise.”
The ATLAS experiment is the world’s largest collaboration
in physical sciences, involving more than 1800 scientists from around the world.
The detector is a leviathan, measuring 44m long and 22m high, as large as a
five-story building, and weighing 7000 tons. Yet at its heart, where the SCT
will operate, narrow beams of particles will be focused to collide in an area
much less than 1 square mm. Out of nearly 1000 million collisions a second, only
a few will have the special characteristics that might lead to new discoveries.
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