CMS: Aerospace goes underground

Gleaming aluminium on the outside and carbon black on the inside, as long as a sedan and taller than a man, a strange hollow cylinder arrived at CERN by truck August 26.

Over several hours, crews carefully unloaded the structure, known to its builders as "the Tube", and delivered it into a huge, white clean tent inside Building 186. Though this structure is reminiscent of a satellite, rather than soaring into space, it will journey deep underground.
This tube will house the heart of the CMS detector, the CMS Tracker, where its payload of roughly 16 000 silicon sensors will track the paths of particles spraying from collisions between protons accelerated by the LHC. Although the CMS Tracker is for particle physics rather than astrophysics, the Tube has a lot in common with satellites: it must be very light, stiff, and keep its shape over its lifetime.
The CMS detector will be in a spacious cavern underground, but the Tracker will be buried under other detectors that form the rest of the experiment. On top of that, the streams of particles passing through the detector will leave it highly radioactive. For repairs or maintenance, it's nearly as difficult to access as the Hubble Space Telescope. Thus the Tracker has to be very reliable and, if it does sustain damage, to keep working regardless.
The engineers responsible for the CMS Tracker all have solid backgrounds in aerospace, so they were able to draw on their experience to design this equipment to be much like a satellite. Hans Postema previously collaborated on the AMS project, a particle physics experiment that went into orbit on board a NASA space shuttle and will be part of the International Space Station. Postema also flies his acrobatic plane in his spare time. Marco Oriunno and Paolo Petagna both did years of aerospace engineering and aeronautics research before arriving at CERN.
These engineers designed "the Tube" to be very light for its size and strength. After particles race through the Tracker, they reach other layers of the detector, preferably with little interference. "For a particle Tracker, you want it as transparent as possible. Any material there is just noise for the experiment," Petagna says. "Physicists would like to have a magical way of keeping their detectors in place without any mass." Short of such a wondrous device, they've designed a strong but light and thin honeycomb of carbon fibres that wraps around the Tracker for support and insulation.
Like a satellite buffeted by radiation and pummelled with small rocks, the CMS Tracker will take a pounding - but in this case, from the very particles it is looking for. "But in designing a device to withstand this punishment," Petagna says, "it's always a trade-off between the expected life of the experiment and what you can physically do." For this reason, the atmosphere inside the Tube will stay at a chilly -20° C to minimize radiation damage to the sensors. Eventually they may wear out, but everything has been designed to last at least 10 years at the LHC's expected luminosity.
How do they ensure reliability? Their philosophy, Petagna says, is test, test, test. "You accept the minimal amount of risk you can." In moving from the drawing board to the production floor, it made a big difference to have a company they trusted with the manufacturing: Plyform, of Varallo Pombia, a village outside Milan, specializes in making highly technical composite structures. In addition to the main support structure, the company is also making several other parts of the Tracker. The engineers hope that all these measures will give the CMS Tracker a long, productive life underground.