New CMS detectors under construction at CERN

While the LHC will play the starring role in the 2013/2014 Long Shutdown (LS1), the break will also be a chance for its experiments to upgrade their detectors. CMS will be expanding its current muon detection systems, fitting 72 new cathode strip chambers (CSC) and 144 new resistive plate chambers (RPC) to the endcaps of the detector. These new chambers are currently under construction in Building 904.

CMS engineers install side panels on a CSC detector in Building 904.

"The original RPC and CSC detectors were constructed in bits and pieces around the world," says Armando Lanaro, CSC construction co-ordinator. "But for the construction of these additional chambers, we decided to unify the assembly and testing into a single facility at CERN. There, CMS technicians, engineers and physicists are taking raw materials and transforming them into installation-ready detectors.”

This new facility can be found in Building 904. Once the assembly site for the straight magnet sections of the LHC, the building underwent two years of renovations to become the detector construction facility it is today. "For starters, there were severe structural issues that had to be addressed,” says David Hay (EN-MEF), who co-ordinated the Building 904 construction work. "Before the building could receive CMS engineers, we had to refurbish the roof and resurface the floor, install fire detection systems and a storage platform, and modify the existing electrical and IT systems."

And after completing basic renovations, there were still a number of project-specific installations to construct. "The chambers have to be constructed and stored at a specific humidity and temperature, so we built four new climate-controlled semi-clean rooms," explains David. "Furthermore, as the chambers use gas to track the muons, we equipped the building with supplies of argon, nitrogen, carbon dioxide, freon and isobutane."

View of the main construction hall in Building 904.

Specific tools were also needed in constructing and testing the new chambers. But instead of building new machines, CMS engineers decided to recycle. "Fermilab refurbished some of the original machines used to construct the CSCs in the late 1990s," says Armando. "These were shipped to CERN, where they are now doing exactly the same job they did over 10 years ago."

Engineers and technicians from Europe, the US, China and Russia are currently at work on the new detectors, and should have them finished in time for LS1. This June, CERN access-card holders will have the opportunity to see these experts at work on the detectors. For more information, see the box below.

How do the Cathode Strip Chambers (CSC) work? In CMS, CSCs are used to detect muons. CSCs consist of arrays of positively charged “anode” wires wound around negatively charged copper “cathode” strips within a gas volume. The chambers are then assembled in “modules”, each one containing six wire and strip layers (see picture). The chamber layers contain a gas that amplifies the signals from charged particles coming from collision events. When muons pass through the gas, they knock electrons off the gas atoms (ionization), thus creating positively charged ions. Because of the electric field in the chamber gaps, electrons flock to the anode wires creating an avalanche, via a multiplication process, which is the signal collected by the read-out electronics. The avalanche around an anode wire from an ionization event creates an induced charge distribution on the cathode strip. Positive ions move away from the wire and towards the cathode, also inducing a charge pulse in the strips. This signal is also recorded by the read-out electronics. Because the strips and the wires are perpendicular, physicists get two position coordinates for each passing particle, which allows them to infer their trajectory.

by Katarina Anthony