Geneva, 12 November 2007. One of the most fragile detectors for the Large Hadron Collider beauty (LHCb) experiment has been successfully 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 has been a challenging task.
This milestone marks the fruition of the construction phase of the VELO project. “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 behaviour of matter and antimatter.
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 only 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 round 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.
The VELO project has been ongoing for the past 10 years, involving several institutes of the LHCb collaboration, including Nikhef, EPFL Lausanne, Liverpool University, Glasgow University, CERN, Syracuse University and MPI Heidelberg.