Operational experience with the ATLAS Pixel detector at the LHC

C Deluca

doi:10.1088/1748-0221/6/12/c12042

Abstract

The ATLAS Pixel Detector is the innermost detector of the ATLAS experiment at the Large Hadron Collider at CERN, providing high-resolution measurements of charged particle tracks in the high radiation environment close to the collision region. This capability is vital for the identification and measurement of proper decay times of pico-seconds lifetime particles such as b-hadrons, and thus vital for the ATLAS physics program. The detector provides hermetic coverage with three cylindrical layers and three layers of forward and backward pixel detectors. It consists of approximately 80 million pixels that are individually read out via chips bump-bonded to 1744 n-in-n silicon substrates. In this paper, results from the successful operation of the Pixel Detector at the LHC will be presented, including monitoring, calibration procedures, timing optimization and detector performance. The detector performance is excellent: 97.5% of the pixels are operational, noise occupancy and hit efficiency exceed the design specification, and a good alignment allows high quality track resolution.

Highlights

250 μm thick n-in-n Si sensor 47232 (328x144) pixels, 46080 readout channels Typical pixel size of 50x400 μm Bias Voltage 150 - 600 V
Data with the correct timestampLE is retrieved by an external trigger to build the event
the length of the discriminator pulse (ToT), timestampLE and hit position are stored in the buffer for every hit Threshold is set to 3500e, with a typical dispersion of ~40e after tuning Noise for normal pixels is ~170e (~300e for ganged -higher capacitance- pixels) Threshold / noise ~ 25 (~12) for normal pixels

Summary

The ATLAS Pixel module

250 μm thick n-in-n Si sensor 47232 (328x144) pixels, 46080 readout channels Typical pixel size of 50x400 μm Bias Voltage 150 - 600 V. Zero suppression in the FE chips, Module Chip Controller (MCC) builds the module event. May 2007 – Installation in ATLAS Sept 2008 – First cosmic events Oct 2008 – LHC incident Nov 2009 – First beam 450 GeV Dec 2009 – 0.9 TeV and 2.36 TeV collisions March 2010 – 7 TeV Collisions End 2010 – Heavy Ions: great period! May 2011 – Luminosity 1033 cm-2 s-1 May 2007 – Installation in ATLAS Sept 2008 – First cosmic events Oct 2008 – LHC incident Nov 2009 – First beam 450 GeV Dec 2009 – 0.9 TeV and 2.36 TeV collisions March 2010 – 7 TeV Collisions End 2010 – Heavy Ions: great period! May 2011 – Luminosity 1033 cm-2 s-1

Calibration Cosmics

ToT tuning and calibration

Radiation damage effects

Module leakage current measurements

Very stable detector operations