Image credit: CERN-PHOTO-201409-176 – 4.
With the end in sight for CERN’s Long Shutdown (LS1), the accelerator chain has been gradually restarting. Since early October, the Super Proton Synchrotron (SPS) has been delivering beams of protons to experiments, including NA62, which has now begun a three-year data-taking run.
NA62’s main aim is to study rare kaon decays, following on from its predecessors NA31 and NA48, which made important contributions to the study of CP violations in the kaon system. To make beams rich in kaons, protons from the SPS strike a beryllium target. The collisions create a beam that transmits almost one billion particles per second, about 6% of which are kaons.
After almost eight years of design and construction, NA62 was ready for the beam by start-up in October. In early September, the last of the four straw-tracker chambers had been lowered into position in the experiment. The straw tracker is the first of its scale to be placed directly into the vacuum tank of an experiment, allowing NA62 to measure the direction and momentum of charged particles with high precision. From the first design to the final plug-in and testing, teams at CERN worked in close collaboration with the Joint Institute for Nuclear Research in Dubna, who helped to develop the straw-tracker technology and who will participate in the running of the detector now that construction and installation has been completed.
Each straw-tracker chamber weighs close to 5000 kg and is made up of 16 layers of state-of-the-art, highly fragile straw tubes. Although heavy, the four chambers had to be delicately transported to the SPS North Area at CERN’s Prévessin site, lowered into the experiment cavern and installed to a precision of 0.3 mm. The chambers were then equipped with the necessary gas connections, pipes, cables and dedicated read-out boards, before beam commissioning began in early October to tune the tracker prior to integrating it with the other sub-detectors for data taking.
This unique tracker, placed directly inside the experiment’s vacuum tank, sits alongside a silicon-pixel detector and a detector called CEDAR that determines the types of particles from their Cherenkov radiation. A magnetic spectrometer measures charged tracks from kaon decays, and a ring-imaging Cherenkov detector indicates the identity of each decay particle. A large system of photon and muon detectors rejects unwanted decays. In total, the experiment extends across a length of 270 m, of which 85 m are in a vacuum.
• For more about the installation and construction of NA62, see the CERN Bulletin http://cds.cern.ch/record/1951890.
