There is an uncanny resemblance to last year. No sooner had the ion run finished, at 6pm on the 7th December, than the party was in full swing at point 2 (or so I’ve been told). As the last party goers crawled home at 6.30am the following morning a new team arrived to begin the annual winter shutdown. Once again the approximately 1250 tons of concrete shielding was removed from the cavern shaft and cavern space before the magnificent L3 magnet doors were pulled open to lay the ALICE experiment bare.
This shutdown involves the installation of several detector supermodules and on-going modifications and maintenance. Continuing from last year the two major projects are the installation of 3 supermodules of the Transition Radiation Detector (TRD) and two supermodules of the Electromagnetic Calorimeter (EmCal). These additions will give the two detectors more coverage of collision events during the 2012 run.
Werner Riegler, ALICE’s Technical Coordinator, explains the plan of action so far. “Removing the shielding, opening the doors and setting up the infrastructure to install the TRD supermodules took until 23rd December. We had a break for Christmas and started installation on the 2nd January, which took just 8 days.” Of the 18 supermodules in the TRD design there are now just 5 left to be installed in future shutdowns. Last week we changed the infrastructure to accommodate the EmCal installation which is taking place this week and next.”
Other important projects being carried out during this shutdown are as follows:
• The filters of the Silicon Pixel Detector (SPD) cooling system are clogged with dust and pollutants generated in the cavern. These filters are buried deep in the experiment and so are inaccessible. The nearest access point is 5 metres from the offending filters. A team has been testing solutions such as increasing pressure in the system or using an ultrasound device to clean the dust away. However, these have been unsuccessful. Rosario Turrisi who heads the team explains, “We have had to increase the violence of our intervention. Our solution is to insert a cable with a small milling tool at one end down the cooling pipe. We will drill a hole 1 mm in diameter to unblock the system.” They are still in the testing phase but will work on the SPD next week.
• High voltage capacitors will be removed from the Time Projection Chamber (TPC). In the high rate environment of the experiment the detectors sometimes discharge a high voltage. The presence of many particles can cause this discharge, which can damage electronics. Removing the capacitors will stabilise the detector and reduce the amount of energy that is discharged. “This is a tricky operation,” explains Werner, “because it has to be done on all the chambers of the TPC. It will take the entire 2 and a half months of the shutdown to complete the job. However, it is important to do this because some of the TPC front-end electronics broke from the discharges.”
• The Muon Tracking Chambers have problems with low voltage connections, resulting in some inefficiency in the detector system. “The problem is simple. Bad low voltage connections cause slight variations of the voltage and this can disrupt the electronics. However this tracking system has 1 million channels and so with all the cabling the problem becomes bigger. There is an ongoing consolidation effort to get this system back in shape,” explains Werner.
In addition to these large projects are many modifications and maintenance jobs such as for the electronics infrastructure. There are also safety tests such as for the emergency stop buttons.
In the middle of February the teams will start re-commissioning the experiment. It will take 3-4 weeks to replace the shielding around the experiment. However, the giant SX2 crane used to move the tons of concrete shielding is currently faulty, although there was an attempt to repair it last week. “Hopefully we can use it again for replacing the shielding,” says Werner. After this the teams will conduct tests to set up for the 2012 run.
A classic installation issue Werner relates is the installation of one TRD supermodule. “We only had a clearance of a few millimetres to the neighbouring detector. Nominally the clearance was about 1.5 cm but sometimes you can end up with very little space. It took a lot of time to convince ourselves it would fit. In the end it did. If there hadn’t we would have been in trouble and would have had to move the other detector and so on.” Although there are detailed, 3-dimensional drawings to ensure the teams know exactly where to place things there are always slight adjustments to be made. “However, the whole shutdown plan has been going quite smoothly up to now,” Werner concludes.