Members of the CMS collaboration have spent much of this year preparing for crucial tests before starting to lower the gigantic detector into the underground cavern in the autumn.
After many years of hard work and long hours, the team building the CMS detector at CERN will get the chance to test the giant magnet in the final stage of commissioning, together with pieces of all the sub-detectors, in the magnet test and cosmic challenge (MTCC). Over the past year one set of end-cap disks has been completely equipped with its muon detection system, the end-cap hadron calorimeters have been put in place and most of the barrel muon detectors have been installed and commissioned. In mid-July the CMS superconducting coil, the barrel rings containing pieces of the inner detectors and the semi-equipped endcaps, were pushed together to be tested for the first time. CMS is unique among the experiments for the Large Hadron Collider (LHC) as it is being assembled on the surface at intersection point 5 in Cessy, France.
Meanwhile, a great opportunity exists for the collaboration to understand parts of the detector that have already been installed, including the hadronic calorimeter (HCAL – all of which is installed although only one section is being operated during the test), two supermodules of the electronic calorimeter (ECAL) and some pieces of the prototype tracker as well as the muon chambers. These sub-detectors will be read out using the real data-acquisition system when cosmic muons are detected during the cosmic challenge. This “slice testing” will continue for two months to ensure the correct alignment and synchronization of the detectors, as well as to confirm that the event builder works as expected and that the software is flexible enough to make any changes needed.
Step one: cooling the solenoid
The gigantic CMS solenoid has already been cooled to 4.5 K, the operating temperature reached on 25 February after cooling began 23 days earlier. The magnet will be operated at this temperature throughout the summer, while the magnet team commission it and test all the systems. The huge coil consists of 14.5 tonnes of niobium-titanium superconducting cables embedded in 74 tonnes of aluminium, with 126 tonnes of high-mechanical-strength aluminum alloy and 9 tonnes of insulation. The temperature is extremely well contained inside the outer covering, enabling engineers to stand within the solenoid during cooldown (figure 1).
After cooling the magnet, the next step was to turn on the current. However, before that could happen, the yoke had to be closed to channel the return flux. The CMS Magnet and Infrastructure Group tested the first low currents to check the control and safety systems before the yoke was closed. Final tests were then made on all auxiliary systems, including cryogenics, electronics and fine-tuning the control system and the power supply. During commissioning the current will rise from 1 kA to 19.5 kA before reaching the nominal magnetic field.
Step two: the hadronic calorimeter
At the beginning of March the first half of the barrel HCAL was tested using a radioactive source before insertion into the solenoid in early April (figure 2). The second HCAL half-barrel was inserted a month later. Both operations involved moving the HCAL pieces from their storage alcoves on an air-pad system and then sliding the halves onto rails welded to the inside of the solenoid. The HCAL comprises layers of brass interleaved with plastic scintillators embedded with wavelength-shifting optical fibres. The light is read out via hybrid photodiodes.
Step three: the ECAL supermodules
Two out of 36 supermodules that make up the barrel ECAL have been installed specifically for the MTCC using a rotatable insertion device known as the “squirrel cage”. This is no easy task, as each section weighs more than 3 tonnes and is very delicate (figure 3). Inside each of these boxes lie 1700 lead-tungstate crystals inserted into glass-fibre “alveolar” structures. Scintillation light produced in the crystals by incident electrons and photons is detected by avalanche photodiodes glued to the back of the crystals. The supermodules also contain the associated front-end electronics, laser monitoring and cooling systems.
Step four: the prototype tracker
During the early hours of 19 May, a special climate- and humidity-controlled truck transported a prototype particle tracker to the CMS site. To avoid shocks to the delicate parts, the truck travelled at a maximum speed of 10 km an hour. Once it arrived, crews hoisted the 2 tonne apparatus up 10 m to the opening of the solenoid (figure 4). Surveyors then aligned the pieces, using the two supermodules of the ECAL for reference. While the prototype tracker is equipped with 2 m2 of silicon sensors, the real tracker will comprise 16,000 strip sensors and about 900 pixel sensors – around 200 m2 of sensors.
Once the MTCC is complete, CMS will be pulled apart in preparation for lowering the sections into the cavern (due to start in October/November). However, before the tracker is removed, an important test will be made of procedures to remove and replace one of the supermodules while the tracker remains inside. CMS plans to install all 36 supermodules into the HCAL on the surface before the real tracker is inserted, but if the schedule slips some supermodules may need to be installed underground with the tracker in situ. This is a demanding task, as neither piece can touch each other and there is only about a centimetre of clearance between them.
After the MTCC is complete, the team will remove the tracker and the ECAL, close CMS and conduct a magnetic-field map test. This will show how uniform the field is and where there are discrepancies. With this information, the differences can be incorporated into the operating software of the experiment.
The MTCC provides a unique opportunity for the CMS collaboration to test installation procedures and to commission a large fraction of the detector, including the data-acquisition and online-monitoring system. The lessons learned will be invaluable for the final push – the full installation of all elements for start-up in late summer 2007.
