ZDC comes online

29 June 2009

A ceiling robot allows remote handling of the ZDC when things heat up with LHC collisions



As of May 4th this year, ATLAS has had a new sub-detector alive in the shape of the Zero Degree Calorimeter (ZDC). The diminutive forward detector, whose two units measure just 10-12 centimetres wide, 60 centimetres high, and 60 centimetres along the beam pipe, will be used to study high-energy neutrons as they fly out of the detector along the beam axis.

The units, one in LHC Sector 8-1 and the other in Sector 1-2, sit 100 metres outside of the ATLAS cavern. They share housing with LHCf, 140 metres from the interaction point. This is where the proton beams, having passed through the centre of the detector in one single beam pipe, enter two separate pipes. As all the charged particles are bent away, only neutrons are left tracking their original course.

The ZDC owes its name to the fact that it measures particles within a range of just five centimetres from the beam axis. Over a distance of 140 metres, this represents an extremely small angle – just a third of a milliradian.

The new sub-detector was originally proposed seven years ago to study LHC lead ion collisions. The nuclear forces holding protons and neutrons together are so weak that when heavy ion nuclei collide, the remnants of the nuclei pretty much fall apart. “The effective force in a nucleus is just a few MeV, which is miniscule on the scale of the TeV that we’ll be dealing with in the LHC, so [uncharged nuclear fragments] really just go forward,” explains ZDC project manager Sebastian White.

The number of neutrons going forward varies depending on how much overlap there is between colliding nuclei. Studying what gets thrown forward allows the details of a collision – was it head-on, or just peripheral? – to be inferred. “It’s what we call ‘event characterisation’,” Sebastian explains. “All interesting heavy ion physics depends on the initial conditions.”

The ZDC is also able to pick out when nuclei have passed close together but not actually collided, since the strong electromagnetic interactions between the nuclei can also cause them to break up.

Although the detector is primarily geared towards heavy ion physics, those working on it were quick to realise that its scope is actually much wider. Forward measurements can also be used to reveal details about the nature of protons’ interactions within the detector, although the process of arriving at these results is somewhat more complex.

“This detector is basically really useful for learning about the global properties of events,” says Sebastian.

The shutdown period since September 19th has been typified by adjustments, improvements, practice, and detectors generally making the most of the extra time afforded to them to prepare for beam. But the ZDC would never have been installed in time for first data if it weren’t for that fateful bus-bar splice. The critical hardware was installed in just one week, from Monday May 4th this year, and the following week, the detector jumped feet-first into a ‘slice’ run with the other forward detector, LUCID.

As well as the detector itself, a robotic system for remote handling of the detector has been installed. “We’re the hottest detector in the LHC. No-one gets more radiation than us!” says Sebastian. To avoid the hardware getting damaged when the beam intensity is very high, the ZDC can be lifted out and replaced with an absorber. “But if you lift that out and stand next to it for an hour, you’ve had your [radiation] fill for the year,” warns Sebastian.

To get around the problem, a robotic lifting system, designed by the CERN EN/HE group, was mounted on high-precision aluminium tracks on the LHC tunnel ceiling. Experts manipulate the robot from the safe distance of 100 metres down the tunnel, all the time monitoring their progress via cameras and television screens.

Joining at the last moment doesn’t seem to have worked against the ZDC: “We did pretty well in the slice tests, thanks, in part, to help from L1Calo,” reports Sebastian. “We hold the record for the least errors – there were just one or two – we’re proud of that!”

 

 

 

Ceri Perkins

ATLAS e-News