Radiation damage in the LHCb vertex locator

Abstract

The LHCb Vertex Locator (VELO) is a silicon strip detector designed to reconstruct charged particle trajectories and vertices produced at the LHCb interaction region. During the first two years of data collection, the 84 VELO sensors have been exposed to a range of fluences up to a maximum value of approximately 45 × 10 12 1 MeV neutron equivalent (1 MeV n eq ). At the operational sensor temperature of approximately −7 • C, the average rate of sensor current increase is 18 µA per fb −1 , in excellent agreement with predictions. The silicon effective bandgap has been determined using current versus temperature scan data after irradiation, with an average value of E g = 1.16 ± 0.03 ± 0.04 eV obtained. The first observation of n + -on-n sensor type inversion at the LHC has been made, occurring at a fluence of around 15 × 10 12 of 1 MeV n eq . The only n + -on-p sensors in use at the LHC have also been studied. With an initial fluence of approximately 3 × 10 12 1 MeV n eq , a decrease in the Effective Depletion Voltage (EDV) of around 25 V is observed. Following this initial decrease, the EDV increases at a comparable rate to the type inverted n + -on-n type sensors, with rates of (1.43 ± 0.16) × 10 −12 V/ 1 MeV n eq and (1.35 ± 0.25) × 10 −12 V/ 1 MeV n eq measured for n + -on-p and n + -on-n type sensors, respectively. A reduction in the charge collection efficiency due to an unexpected effect involving the second metal layer readout lines is observed.

Radiation Damage Effects and Operation of the LHCb Vertex Locator

“…Charge carriers created by a traversing particle may also be trapped in the trapping centers created by defects, causing a delay in the signals collected by the electrodes. In general, the degrading influence of the particle radiation manifests itself in three important ways: 1) increasing leakage current; 2) changing the effective doping concentration in silicon (which has an impact on the operation voltage needed for total depletion); and 3) losing charge collection efficiency (CEE) [3].…”

Erratum to: Measurements of prompt charm production cross-sections in pp collisions at s = 13 $$ \sqrt{s}=13 $$ TeV

“…The charge collected in the LHCb VELO sensors is affected by radiation damage. One such effect, which is more pronounced in the outer regions of downstream sensors, arises from charge induction on second metal layer routing lines [2]. Prior to the start of Run 2, modifications were made to the digitization step in the LHCb simulation framework to model this effect.…”

Erratum to: Measurement of forward J/ψ production cross-sections in pp collisions at s = 13 $$ \sqrt{s}=13 $$ TeV

“…One such effect, which is more pronounced in the outer regions of downstream sensors, arises from charge induction on second metal layer routing lines [2]. Prior to the start of Run 2, modifications were made to the digitization step in the LHCb simulation framework to model this effect.…”