Back underground

20 October 2008

The schedule presented by Marzio in last week's plenary session. Click to enlarge

With no more beam coming this year, ATLAS is getting ready to open the detector back up and do as much repairing and refurbishing on those parts that need a little extra work. The operation starts October 20th and comes to a close by the end of March.

But opening the detector has a big impact on the quality of the cosmic ray data coming from ATLAS. When the detector is closed, we can have the magnetic field on and track curved paths of charged particles through the ATLAS detector rather than just the straight lines created while the field is off.

On the other hand, detector teams have a few small problems to fix, and the sooner they can get a start, the better. Moreover, dignitaries from CERN member and observer states will be present for the October 21st inauguration, and they can see ATLAS in all its splendour if the magnets are off.

From then, detector groups will work their way inward starting with fixing gas leaks in a few RPC and MDT barrel chambers. The muon group will also look for damage in the TGC chambers of the small wheels and EIL4 (which are fixed to the barrel but act as extensions to the small wheels), exchanging two or three chambers. They plan on adding a few more elements to the barrel muon spectrometer (EE chambers, which extend the reach of the big wheels).

Technical Coordinator Marzio Nessi compares the gas systems to a tire with a slow leak. You can still run the car, but you need to add air every couple of months. Of course, this mode of running is inefficient. “You prefer a perfect tire,” he explains.

Efforts to repair any damage are scheduled from December to the end of March.

Fixing the calorimeters is one of the main tasks. “There are problematic Front End Boards in the Liquid Argon readout,” said Marzio, in his “Point 1 Operation” talk in the October ATLAS Plenary sessions. The problems are not well-understood yet, but repairing these boards will be critical to ATLAS performance in its first long run next year.

Also, technicians will refurbish the electronics of between 10 and 40 drawers on TileCal. Work on the calorimeters is slated to begin in mid-November and end in late February.

Because the Inner Detector had just a few months to run and debug thus far, the ID teams need to use the last months of 2008 to collect cosmic data, calibrate, and align. Just before the winter break, the Inner Detector end plates will be opened up to double-check the services layout since the closing took place under tough time constraints.

After that, the ATLAS detector will receive its first annual winter tune-up. Teams will make general checks to ensure that services like compressors and cooling loops are running to specification and that no safety hazards have emerged.

Furthermore, the optical fibres that send data out of the MDTs placed on the big wheels can’t withstand as much radiation as the LHC will produce at design luminosity, so they must all be removed and replaced with radiation-hard fibres. This requires opening each individual wheel and giving access to both faces of the MDT panels. “Not a simple task!” says Marzio.

At the end of March, the magnets will be powered up once more for final debugging, planning to be ready for beam by the end of April.

The people on hardware have their work cut out of them this autumn and winter, making the thousand-and-one tiny fixes that will make ATLAS run even better. All in all, Marzio estimates that the proportion of working channels on ATLAS will be one per cent higher than when the cavern closed for first beam, approaching the ideal 100%. 

These small problems weren’t enough to justify opening up the cavern, but Marzio says: “While you have the chance to do it, you fix the problems.” When recording missing energy is as important as recording particle tracks, minimizing the number of dead channels is crucial.


Katie McAlpine

ATLAS e-News