Diamonds at the golden point

Alongside the CMS Pixel Luminosity Telescope (PLT) – installed last month (see here) – lie diamond detectors. No ordinary gems, these lab-grown diamonds will be playing a vital role in Run 2: differentiating signals from collision products with those from the beam background.


The BCM detector's green "c-shaped" printed circuit board is mounted on the PLT/BCM carbon-fibre carriage ready for installation.

Earlier this year, the CMS BRIL project installed beam condition monitors (BCM) at the heart of the CMS detector. Designed to measure the online luminosity and beam background as close as possible to the LHC beam pipe, the BCMs use radiation-hard diamonds to differentiate between background and collision signals. The BCM also protects the CMS silicon trackers from damaging beam losses, by aborting the beam if the signal currents measured are above an acceptable threshold.

These new BCMs are designed with Run 2 bunches in mind. “The new system has improved radiation hardness, faster front-end electronics, a higher granularity and a larger surface area - making the BCM more sensitive to the beam background in the higher particle-rate conditions expected in Run 2,” says Anne Dabrowski, BRIL deputy project leader and technical coordinator. "The upgraded triggerless back-end electronics ensure that every passing charged particle will be counted." 

Six metalized diamond sensors are located on the outer part of the rigid BCM "c-shape" printed circuit board.

To distinguish collision products from background, the detector has been placed at a very special location. "The BCM needs to be in a golden location for Run 2, some 6.25 ns away from the interaction point," says David Stickland, BRIL project leader. "Imagine a background particle comes along - typically at the same time as the beam itself - and they both pass the BCM. When the beams collide at the interaction point, that collision can result in particles returning to give signals in the BCM. Given its golden location - a return path of 12.5 ns from the interaction point - the BCM has the optimal window before the next bunch comes along the beam pipe."

At the core of a faster BCM detector is a new micro-electronic chip designed by the PH-ESE department and the University of Science and Technology AGH Krakow. The novel design was the brainchild of a Polish PhD student, Dominik Przyborowski, working side-by-side with Jan Kaplon and Vladimir Ryjov from CERN. These chips have an excellent time resolution, allowing the team to differentiate multiple hits in a 25-ns bunch crossing.

Finding space for all the BCM elements and connections was a challenge. "PH-DT came up with a clever design that brought all the elements into one piece," says Anne. "We placed the diamond sensors, signal converters and connections on a single circuit board, using a flexible circuit board for the wires between these elements and the optical converters that send the signals to the readout."

Engineers from PH-ESE and PH-DT involved in the design and production of the integrated BCM printed circuit board mounted on a carbon-fibre carriage (from left to right: Robert Loos, William Billereau, Rui De Oliveira, Vladimir Ryjov and Bertrand Mehl).

"The circuit board serves two masters, providing both mechanical structure and data connections,” concludes David. “Its design was just one of many accomplishments that led to the successful construction of the BCMs. An essential ingredient was the dedication of physicist Wolfgang Lange and the DESY Zeuthen team in the meticulous art of detector assembly and qualification. Now that installation is complete, we are looking forward to seeing the BCMs in action in less than two months’ time.”

by Katarina Anthony