Inner Detector round-up

22 September 2008

The TRT team glimpse the first beam events of September 10th

Pixel Detector

Sitting as close as 5 centimetres away from the beam interaction point, the Pixel Detector, with its 80 million individual miniscule silicon pixels, is extremely sensitive. If a beam were to scrape the edge of a collimator (the tungsten blocks placed just outside the detector and used to narrow or entirely cut off a beam), it could produce showers of secondary particles which would inundate the sub-detector with such high charge deposition that the front-end electronics would be left in tatters. Erring on the side of caution then, the Pixel team chose not to turn their detector on for the initial LHC injection on September 10th.

Nevertheless, the first couple of days of beam were still important, as the group used the opportunity to study the detector and prepare for the weekend, when, in the absence of beam, it was finally integrated into ATLAS data-taking. The first overnight cosmic track was spotted around 2 p.m. on the Sunday afternoon, and the Pixel team’s patience was justly rewarded when, during several long runs, tracks were observed in coincidence with the SCT. “This is a significant milestone,” says the Pixel Run Coordinator, Charles Young. “Most of us are cautious by nature, but we were pleasantly surprised at the rapid progress from initial integration to detecting tracks.”

Now Charles and his team are focusing on making the Pixel Detector “truly physics ready”. This will include repairing and tuning channels which aren’t performing as expected, and making improvements to the software and operational efficiency. “As in any project, getting to the 90 per cent point … is time consuming but relatively straight forward. The next five per cent is harder, and is often not something that can be dealt with effectively by a large team. It requires more expertise. The last five per cent can be difficult even for the experts,” says Charles of the task ahead.


Hugging the Pixel Detector, the SCT ran at reduced voltage to avoid electronic damage while the first low-intensity beam bunches were steered into a closed collimator just outside of ATLAS. The detector was showered with muons, whose tracks merged into one another on the event displays in ‘splashes’ of up to 20,000 hits.

“We saw beautiful beam splashes in the SCT Endcap,” says the SCT Run Coordinator Heinz Pernegger. “The response that we got from the detector in those events is approximately what we expected, though the exact rate of tracks was of course a bit of an unknown before,” he explains, after recounting that none of the SCT modules tripped or showed any otherwise unstable behaviour during the splash events.

“Fascinated, enthusiastic, and overjoyed,” are the words Heinz uses to describe what the mood of the SCT team was like immediately after the first event, “particularly all the people who have been with ATLAS for more than a decade.” The next steps, which are already underway, involve calibrating the detector and adjusting the precise readout time of the SCT with respect to the ATLAS Trigger.


There was a bit of a hairy moment for the TRT team on the morning of the first beam, explains the TRT Run Coordinator Anatoli Romaniouk: “The TRT went ‘BUSY’ just a few minutes before the injection moment!” Luckily, on-hand experts were able to cure the problem within seconds, and the detector went on to register the beam splash events and the beam halo, which was more than they had dared to hope for, according to Anatoli:

“For the very first event we were not expecting to see many tracks because our readout window is only 75 nanoseconds and there was little chance that we would be on time with a beam trigger which was never used before.”

But on time they were, and Anatoli has nothing but praise for the team which he says showed super-human levels of discipline under the pressurised conditions of September 10th: “We were all thrilled to catch the first beam, [but] were working hard and in sync in a very crowded and open to media environment,” he says, with only one word left to utter when asked about what’s next for the TRT: “COLLISIONS!”

  • To see images of the first event displays from each of the detectors, head to this week’s Gallery

Ceri Perkins

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