[Paper Review] ATLAS Pixel Detector: Operational Experience and Run-1 to Run-2 Transition

A. La Rosa|arXiv (Cornell University)|Oct 23, 2014

Particle Detector Development and Performance3 citations

TL;DR

This paper details the operational experience of the ATLAS Pixel Detector during LHC Run-1 and the comprehensive upgrade during the long shutdown, including the installation of the Insertable B-Layer (IBL). The IBL, featuring radiation-hard 3D pixel modules and improved cooling, enhanced vertex resolution and tracking performance, achieving 99% hit efficiency and stable operation at -12°C with 99% module reliability post-commissioning.

ABSTRACT

The Pixel Detector of the ATLAS experiment has shown excellent performance during the whole Run-1 of LHC. Taking advantage of the long shutdown, the detector was extracted from the experiment and brought to surface, to equip it with new service quarter panels, to repair modules and to ease installation of a new innermost layer, the Insertable B-Layer (IBL). An overview of the operational experience, the refurbishing of the Pixel Detector and of the IBL project as well as the experience in its construction, integration and commissioning are described.

Motivation & Objective

• To summarize the operational performance and challenges of the ATLAS Pixel Detector during Run-1 of the LHC.
• To describe the refurbishment and upgrade activities performed during the long shutdown, including replacement of service components and repair of failed modules.
• To present the design, production, and integration of the Insertable B-Layer (IBL) as a new innermost pixel layer.
• To report on the qualification, commissioning, and performance validation of the upgraded IBL detector system.

Proposed method

• The Pixel Detector was extracted from ATLAS and refurbished with new service quarter panels and repaired modules during the long shutdown.
• The IBL was constructed using 710 radiation-hard 3D pixel modules, with front-end chips thinned to 150 µm and flip-chip bonded using Sn-Ag solder bumps.
• Module production involved rigorous quality assurance: electrical, functional, and thermal stress testing at -40°C to +40°C, and calibration at -15°C.
• The IBL staves were assembled from 18 produced staves, with 14 selected based on performance; all underwent cold calibration and radioactive source testing.
• The IBL was installed in May 2014 and commissioned using cosmic ray runs, achieving 120 kHz readout capability.
• Performance was validated by comparing QA results with integration test data, confirming stable operation at -12°C and consistent threshold/noise tuning.

Experimental results

Research questions

• RQ1 How did the ATLAS Pixel Detector perform during Run-1 in terms of efficiency, noise, and radiation damage effects?
• RQ2 What were the main failure mechanisms in the original Pixel Detector, and how were they mitigated during the upgrade?
• RQ3 How was the Insertable B-Layer (IBL) designed to withstand high radiation levels and improve tracking resolution?
• RQ4 What were the key challenges in the production and integration of the IBL modules, and how were they resolved?
• RQ5 To what extent did the IBL upgrade improve the overall performance of the ATLAS tracking system?

Key findings

• The ATLAS Pixel Detector achieved 99% hit efficiency across all layers, with only 0.1% of pixels masked due to noise, and maintained low noise levels (180 e⁻ for normal pixels).
• Leakage current increased with integrated luminosity in line with predictions, with steps observed during thermal annealing, confirming radiation damage effects were within expectations.
• The IBL was successfully installed and commissioned, achieving stable operation at -12°C with 120 kHz readout capability during cosmic ray runs.
• Module production yield reached 75% for two-chip modules and 62–63% for single-chip 3D modules (FBK and CNM), after resolving initial bump-bonding failures due to excessive flux.
• After tuning to 1500 e⁻ threshold at -12°C, noise levels were 180 e⁻ for normal pixels, 300 e⁻ for ganged pixels, and 200–300 e⁻ for long pixels, consistent with design expectations.
• The IBL upgrade enabled a 4-layer pixel system with improved vertex resolution, enhanced secondary vertex detection, and better b-tagging performance for Run-2 physics analyses.