GENEVA, Nov. 12, 2007 -- One of the most fragile detectors for the Large Hadron Collider beauty (LHCb) experiment has been installed in its final position. LHCb is one of four large experiments at CERN1’s Large Hadron Collider (LHC), expected to start up in 2008. For the LHCb collaboration, installing the vertex locator (VELO) detector into its final location in the underground experimental cavern at CERN (European Organization for Nuclear Research) particle physics center has been a challenging task.
“It was a very delicate operation”, said Paula Collins, LHCb-VELO project leader, “With its successful completion, the VELO is now in place and ready for physics.”
The VELO is a precise particle-tracking detector that surrounds the proton-proton collision point inside the LHCb experiment. At its heart are 84 half-moon shaped silicon sensors, each one connected to its electronics via a delicate system of more than 5000 bond wires. These sensors will be located very close to the collision point, where they will play a crucial role in detecting b quarks, to help in understanding tiny but crucial differences in the behavio, CEr of matter and antimatter, CERN said in a statement.
Right: The LHCb team installs the VELO detector in its final position (below). (Photos courty CERN; copyright CERN DSU-Communication)
The sensors are grouped in pairs to make a total of 42 modules, arranged in two halves around the beam line in the VELO vacuum tank. An aluminum sheet just 0.3-mm thick provides a shield between the silicon modules and the primary beam vacuum, with no more than 1 mm of leeway to the silicon modules. Custom-made bellows enable the VELO to retract from its normal position of just 5 mm from the beam line, to a distance of 35 mm. This flexibility is crucial during the commissioning of the beam as it travels around the 27-km ring of the LHC.
“The installation was very tricky, because we were sliding the VELO blindly in the detector,” said Eddy Jans, VELO installation coordinator. “As these modules are so fragile, we could have damaged them all and not realized it straight away.” However, the verification procedures carried out on the silicon modules after installation indicated that no damage had occurred, he said.
The VELO project has been in progress for the past 10 years, involving several institutes of the LHCb collaboration, including the Netherlands' National Institute for Nuclear Physics and High Energy Physics in Amsterdam (Nikhef); Ecole Polytechnique Federale de Lausanne, Suisse; the University of Liverpool; Glasgow University, CERN, Syracuse University and Max Planck Institute, Heidelberg, Germany.
At a brief ceremony deep under the French countryside last week, CERN1 Director General Robert Aymar sealed the last interconnect between the main magnet systems in the LHC. Its superconducting main magnets will operate at just 1.9 degrees above absolute zero (–271.3°C) -- colder than outer space. To cool the magnets, over 10.000 tonnes of liquid nitrogen and 130 tonnes of liquid helium (1 tonne = 1000 kg) will be deployed through a cryogenic system including more than 40,000 leak-tight welds. The ceremony marked the end of a two-year project to connect all the main dipole and quadrupole magnets in the LHC, incluidng both electrical and fluid connections.
The LHC is a circular machine, 27 km in circumference and divided into eight sectors, each of which can be cooled to its operating temperature of 1.9 degrees above absolute zero and powered individually. One sector was cooled, powered and warmed up in the first half of 2007. This was an important learning process, allowing subsequent sectors to be tested more quickly, CERN said.
“Over the coming months, we’ll be cooling down the remaining sectors,” said Lyn Evans, LHC project leader. “Five sectors will be cooling by the end of 2007, with the remaining three joining them early next year.”
If all goes as planned, the first beams could be injected into the LHC in May 2008, and circulating beams established by June or July. With a project of this scale and complexity, however, the transition from construction to operation is a lengthy process.
“There is no big red button, and there are inevitably hurdles to be overcome as we bring the LHC into operation,” sai Aymar. “Every part of the system has to be brought on stream carefully, with each subsystem and component tested and repaired if necessary.”
“There have been no show-stoppers so far,” added Evans. “For a machine of this complexity, things are going remarkably smoothly, and we’re all looking forward to doing physics with the LHC next summer. If for any reason we have to warm up a sector, though,” he cautioned, “we’ll be looking at the end of summer rather than the beginning.”
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