by Virginia Greco and Arturo Tauro. Published: 09 January 2017

During the 2016-17 winter technical stop ALICE will perform various activities of maintenance and fixing to be ready for the run to come.

The ALICE detector with the magnet doors open.

 

On December 5 the LHC stopped delivering particle beams and collisions, thus marking the start of the 2016 end of year technical stop. Since it will be an extended stop, 21-week long instead of 13 as usual, it will allow the experiments to perform many upgrade and maintenance interventions.

Such a long period is particularly appreciated by the ALICE collaboration, which will have enough time to open the doors of the L3 magnet and, as a consequence, obtain a confortable and safe access to the detectors located inside. This is a delicate and time-consuming operation, due to the remarkable weight of the doors (400 tons) and the presence of shielding made of concrete blocks. Maintenance operations can be performed when the doors are closed as well, but in that case the area is considered a confined working space, which implies that there are limitations due to safety issues and to difficulty of rescue in case of accident.

One of the doors was opened at the end of December, the other has been opened in these days. During the CERN Christmas break, scheduled for the period 22 December-4 January, there were no activities in the cavern. After completing the opening of the doors, operations will start in consecutive stages on all the parts of the detector. No structural changes to the detectors themselves or to the readout chains are foreseen, but many activities of maintenance will be performed both on the electronic equipment and on the services (e.g. gas, cooling and electricity).

Before opening the doors of the magnet it is necessary to remove the concrete blocks that form a shielding.

Particularly tricky is the work to be done on the Time-of-Flight (TOF). In order to replace some of its electronic boards, 12 detector modules have to be displaced by a few cm to allow the access to their rear end, where the boards sit.

A similar intervention will be done on the electronic equipment of the Photon Spectrometer (PHOS). Even if in this case it is possible to reach the boards without moving the modules, the access is not particularly easy either. The lead-tungstate crystals of the PHOS, which during operation are kept at a temperature of -25°C, will be warmed up to room temperature, nevertheless the modules will not be taken out from their location.

The Time Projection Chamber (TPC) will go through standard maintenance of the electronics and then, towards the end of the technical stop, the gas will be changed. The present mixture of Ar-CO2 (88-12) will be replaced by Neon-CO2-N2 (90-10-5), in the attempt to reduce the distortions that are currently affecting the TPC data and that require complex correction algorithms. The Ne-CO2-N2 mixture was originally chosen for the TPC in run1, but was later decided to switch to Ar after the appearance of instabilities in the chambers. Further studies performed in the meanwhile have shown that the presence of neon does not play a determinant role on the instabilities, while it could contribute to reduce the distortion effect.

An intervention on the gas is foreseen for the Transition Radiation Detector (TDR) as well; the mixture in this case is composed of Xenon and CO2. Both neon and xenon are quite expensive gases, thus it is important not to waste but recover them. This is possible by cooling down the gas mix with the help of a big cryostat that employs liquid argon to reduce the temperature. Since different gases have different cryogenic points, the components of a mixture can be separated and retrieved.

Some minor interventions will be performed on the electronics and mechanics of the two calorimeters too.

Another important activity, which has already started in these days before Christmas, is the replacement of the lift descending into the underground cavern. ALICE will have to change its lift, as it dates back to the LEP times. The replacement operations in ALICE will last about ten weeks and will be carried out by the company providing the equipment, under the management of the CERN engineering department. A careful planning of the activities has been necessary to limit the interference with the other activities to be performed, since there is only one lift that leads directly to the cavern (PX24). The second one leads to the accelerator tunnel (PM25 area), from which it is possible to get to the cavern -of course-, but the safety constrains and the training required to access the experimental area from that side are different.

Last, but not least, maintenance activities of the various services - including cooling systems of electronics and detectors, ventilation, electricity and safety chains - will take place as well.

All these activities are fundamental to have the best possible performance of all the parts of the ALICE detector in next year’s run. But the collaboration is also looking towards the upgrade that will be implemented during the long shut-down 2 of the LHC, which will start at the end of 2018 and will last for two years.

Thus the occasion of the present extended technical stop will be taken to install different prototypes of optical fibers necessary for the upgrade, which will be tested during the next run. Given the mechanical constraints, installing these fibers and the trays where they lay is not a trivial operation and it requires precise studies of design and integration.

A general inspection will also allow identifying precisely what is needed for the future integration and to validate the design of some components.

The shutdown will last until end of April. It is foreseen to restart the commissioning of the different detectors already one month before, in order to be ready for the restart of the run in May.