The installation of new detectors into the ALICE experiment, taking place across the winter shutdown, finished yesterday without any setbacks. Six new Electromagnetic Calorimeter (EMCal) supermodules and three Transition Radiation Detector (TRD) elements were installed during the technical stop.
“Last week, it was quite perfect - there were no problems - it was done in time, exactly as expected,” said Manoel Dialinas, the engineer in charge of the EMCal installation, adding: “The most important point is that there is co-ordination between all people. What is very important is to test beforehand every elementary task - to make sure we do not have any issues while working.”
Forward planning is the key, explains Diego Perini, one of ALICE’s main engineers. “Everything went fine because it was well prepared. We had the time to think about improving the insertion tools - there were many things that were changed since the first installation… and when you save one hour here and two hours there, finally you find that you don’t have to work the complete week of Christmas - you need just a few days.”
CERNALICE's L3 Doors opened for the installation of new modules
A special tool was used, in conjunction with the cranes built into the ALICE cavern, to maneuver new elements into their positions. The cylindrical device is designed to house and rotate modules into the correct orientation with respect to the ALICE’s spaceframe – the stainless steel structure which houses the central detectors within the experiment – before guiding the elements into position. The whole installation process, from the top of the ALICE pit to their final resting place inside the L3 Magnet, takes around one day for each new element being installed.
“It took less time than was anticipated,” said Robert Porret, a crane operator who has worked with CERN for 37 years. Alongside his seven colleagues, Porret, who is very passionate about ALICE, makes up the team involved in the installation operations.
Care has to be taken during the installation process; with the new modules weighing around eight tons each, plus the additional eight ton mass of the installation tool itself, the slightest shock to the apparatus could cause damage to both the new installation and ALICE’s pre-existing hardware.
Further difficulties arise as each new element is added – the added weight causing small amounts of deformation to occur within ALICE’s spaceframe (or the smaller, stainless steel Calframe, in the case of the EMCal.) The engineers make sure to monitor the deformation with each installation, checking the recorded values against predicted strains to ensure the integrity of the structure has not been compromised.
“Sometimes you get stick-slip,” says Perini. “You put one element in, and nothing is happening, and then you put the second one and you get the effect of the second one plus the first. So, one has to be careful when watching the results. You cannot install 50 tons of material without checking that everything is okay!”
Special methods of balancing the installation tool, as the modules were transferred to ALICE’s support structures, also had to be developed to counteract the shift in mass from the tool.
CERNAn EMCal supermodule is guided into ALICE
All that remains now are a few preparatory tasks – including the replacement of the many concrete blocks which form the shielding in front of the ALICE detector when the doors to the L3 magnet are closed. Currently residing in the building above the cavern, it took a full day to remove them at the start of the shutdown in December.
While these works are complete, there will still be modules to be added to ALICE in the future. There are eight vacant spaces - five at the top of ALICE and three at the bottom - which are to be occupied by additional TRD elements. Perini explains that when they are ready, they will be installed – but care always has to be taken to balance out the internal structure.
“You cannot load it in any possible configurations,” he says, adding: “We cannot simply put the five top elements in without the ones in the bottom. Perhaps we will have to prepare some counterweights - just to balance out the spaceframe.”
Alongside this, 6 new calorimeter supermodules – DCal – are being planned for 2014. These will provide the experiment with a greater angle of coverage – which will enhance ALICE’s ability to study jets.