Speaker
Paula Collins
(CERN)
Description
LHCb is a dedicated experiment to study new physics in the decays of beauty and charm hadrons at the Large Hadron Collider (LHC) at CERN. The beauty and charm hadrons are identified through their flight distance in the Vertex Locator (VELO), and hence the detector is critical for both the trigger and offline physics analyses. The VELO is the highest resolution vertex detector at the LHC.
The VELO is the silicon detector surrounding the LHCb interaction point. The sensors have an inner radius of 7mm from the LHC beam and an outer radius of 42 mm. Consequently the sensors receive a large and non-uniform radiation dose. A dose of 0.7 x 1013 1 MeV neutron equivalents /cm2 per fb-1 of data is predicted at the tip of the sensors. The sensors are fabricated from oxygenated n-on-n silicon with one module made from n-on-p silicon (as proposed for several of the sLHC upgrades).
The radiation damage is monitored by three studies: 1) the currents drawn as a function of temperature and voltage 2) studying the noise versus voltage behaviour and 3) charge collection efficiency, studied with tracks from proton-proton collisions, as a function of voltage. The results of all three studies are presented, clear signs of radiation damage being observed in each technique. Clear differences in behaviour, as expected, are observed between n-on-n and n-on-p sensors. Type inversion is observed in the tips of the n-on-n sensors.
The detector so far shows no significant performance degradation, however many interesting effects have been observed in the sensors. Effects that will be reported include a coupling of charge to the second metal routing line layer after irradiation, and the evolution of the depletion voltage of n-in-p sensors is compared to that in the n-on-n sensors.
Author
Paula Collins
(CERN)
