The first steps towards Super-ATLAS

The ATLAS collaboration spent more than a decade designing and building the world’s largest particle detector. But there is more work still to do to – the detector will have to undergo a major overhaul as it is being prepared for use in the LHC upgrade. Steinar Stapnes at CERN is involved in coordinating the efforts to build Super-ATLAS.

The 44-metre-long ATLAS detector is divided into four concentric sections, like the layers of an onion. The bulky outer detector layers will mostly remain unchanged during upgrade work, although there are significant electronics changes foreseen, but the 2-metre-wide, 7-metre-long inner detector must be ripped out and replaced. It may be relatively small, but building a new inner detector is a mammoth task, says Stapnes.

“We need to have better granularity in the detector,” he says. “Powering that is a huge challenge – we can’t simply multiply the power cables feeding the detector in the same way that we want to multiply the detector elements, or else the entire detector will be filled with cables.”

Cross-collaboration on power and safety

The power issue is so much of a problem that there is an entire section of the super-LHC FP7 project devoted to it. “Work package 8 is specifically looking at how to distribute power with more efficiency,” says Stapnes. “We will be following their progress closely.”

It’s equally important for the ATLAS upgrade project to work closely with their colleagues in work package 5, who are studying the radiation simulation and measurement, including safety implications, of a tenfold increase in the radiation level within the LHC.

But as the twin issues of power and safety are becoming better understood, it will soon be time to reunite the members of the ATLAS collaboration scattered around the world and design and build the new inner detector.

“Most of the original research groups are back on board,” says Stapnes. “Although we have to build a new inner detector from scratch we already have a lot of expertise in developing and building parts, and then assembling the detector.”

New opportunities

A new inner detector provides the ATLAS collaboration with the opportunity to apply state-of-the-art technology that wasn’t available when the original detector was constructed. In particular, the new detector will be constructed entirely from silicon – the one that currently sits at the heart of ATLAS is part silicon and part gas-based.

“In the early 1990s it was thought very difficult and risky to build a huge silicon detector,” says Stapnes, and so the collaboration settled on a half-silicon alternative. “But electronics prices came down and access to new technology meant it was more feasible by 2000.” That change came in time for the CMS collaboration to redesign an all-silicon core for their detector. The ATLAS collaboration opted to stick with their original design but also profited significantly from improved availability and performance of new technologies.

“In the upgrade we will go for a fully silicon design,” Stapnes says. “The gas detector will work fine in the first LHC, but we think that technology will reach a limitation when we upgrade and go for higher collision rates – an all-silicon version will perform better, and we have now much more confidence in the scalability of silicon systems.”

The immediate future for the ATLAS upgrade project is to identify what work needs to be done to build the new components. “During this year our main goal is to write a Letter of Intent describing the changes to be made,” says Stapnes. “In parallel, and within the next two or three years, we will be forming more coherent projects for building each new part. As the project takes shape the individual research groups around the world will define their roles in the upgrade project, talk to their funding agencies and we will get started with the actual construction.”

Computer generated images of the Pixel, part of the ATLAS inner detector, which will be replaced in upgrade work