Carbon fiber composites enable critical physics discovery

This summer, physicists at the European Organization for Nuclear Research (CERN) announced a discovery that could answer fundamental questions about the universe, and they couldn’t have done it without composites.

The Higgs boson, a subatomic particle that could explain the origin of mass, was discovered at the Large Hadron Collider (LHC), the world’s largest particle accelerator, with help from the ATLAS Experiment, a particle detector used to measure the collisions made by the LHC. When protons slam into each other, they create hundreds of other types of particles. The ATLAS Experiment uses layers of sensors to measure properties such as the trajectory and momentum of the particles. As the particles pass through the sensors, which are generally made of silicon, they create a charge, allowing the sensors to take snapshots of what’s happening at a precision of about 10 microns, or about one-half of one-thousandth of an inch, at intervals of 25-billionths of a second.

“The data is filtered and then eventually recorded and analyzed by computers all over the world,” says Murdock Gilchriese, a senior scientist at the Lawrence Berkeley National Laboratory (LBNL), one of many labs around the world that collaborate with CERN. He is part of a team that works to create carbon fiber composite structures to support the sensors inside the ATLAS Experiment.

Carbon fiber composites were chosen for this important task in part because they have high strength but low mass and low weight, which helps reduce interference between particles and matter inside the detector.

“The particles that go through matter don’t just go through matter; they interact or scatter,” Gilchriese says. “The lighter the material, the less they do that.”

Carbon fiber also has high thermal conductivity, which comes in handy because the sensors inside the detector function best at low temperatures, around –10 to –15 C. The composite structures help to channel heat generated by the collisions away from the electronics to a thin-walled metal tube filled with coolant. New types of carbon fiber structures are needed for the next generation of upgraded tracking detectors planned for the CERN experiments.

The structures themselves are like sandwiches of carbon foam and carbon fiber to which sensors are glued. They begin with low-density glassy carbon foam, which has thermal conductivity that is not much better than that of air. Californiabased Allcomp Inc. then processes the foam with a chemical vapor deposition heat treatment to add layers of carbon to the native carbon foam. This process increases the foam’s density to around 0.2 grams per cubic centimeter, improving its thermal conductivity by more than 200 times. The foam is made in blocks that are then sliced into sheets and machined to the lab’s specifications.