The BRIL luminosity sub-detectors prepare CMS for a bright Run 3

After long months of preparation, the Beam Radiation, Instrumentation and Luminosity (BRIL) group has completed the installation of three instruments dedicated to the measurement of luminosity and beam conditions: the Beam Condition Monitor “Fast” (BCM1F), the Beam Condition Monitor for Losses (BCM1L) and the Pixel Luminosity Telescope (PLT). These three of the BRIL sub-detectors make up the BRIL sub-system which is segmented into 4 modules. They represent a new “generation” in their respective design history. Both PLT and BCM1F rely on silicon sensors, while BCM1L uses poly-crystalline diamond sensors.

Measuring the real-time rate of collisions at CMS is key to optimising both the trigger rates and the quality of the beams delivered by the Large Hadron Collider (LHC). Continuously assessing the beam conditions is also essential to the protection of the LHC machine and the sensitive CMS sub-detectors. Finally, the aggregated luminosity measurements need to be meticulously understood to determine the expected frequency of interactions in the analysis of data collected at CMS.

The design and production of new components, sensor characterisation, assembly, stress-testing under thermal cycles, troubleshooting, repairs and other tasks spanned a few years of challenging work, which ramped up as Long Shutdown 2 came to a close. The transport activities began before sunrise on 5 July 2021.

Each module of the sub-system was carefully loaded onto a special transport vehicle and dry air was circulated inside their transport boxes. Only days before, each module had been delicately readied for its journey, which included labelling them with their affectionately selected aliases: Calabrese, Capricciosa, Diavola and Margherita. After being lowered down the pit to the CMS experimental cavern, each module was craned up to the tracker sub-detector platform. The BRIL sub-detectors now lie at the heart of the CMS detector, about 1.8 m from the interaction point, just beside the forward pixel tracking sub-detector. 

One of the most significant changes in the design of the instruments has been the implementation of a new active cooling circuit for BCM1F, which is essential for a silicon-based detector. The PLT cooling loop has been modified to include a new section for BCM1F. The design of the BCM1F cooling circuit follows the approach implemented for the PLT during Run 2: the cooling structure has been 3D printed in a titanium alloy using the selective laser melting technique.

The silicon sensors used for BCM1F and three of those used in one of the PLT channels were sourced from a batch currently being developed for the CMS Phase II upgrade for the High-Luminosity LHC. “This is the first time that these prototype Phase II silicon pixel sensors will be installed in CMS, so the whole community is eager to see how this material behaves,” says Anne Dabrowski, CMS BRIL project manager.

The installation of the BRIL sub-detectors was closely followed by the sealing of the bulkhead, which encloses them with the CMS silicon pixel and strip tracker sub-detectors. The work is now focused on the full commissioning of all BRIL systems in anticipation of the first beams of Run 3 of the LHC.


This story was originally published on the CMS website

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