Detector recycling leads NA62 to new standards
The NA62 collaboration has just installed the first eight (out of 12) Large Angle Veto detectors for the accurate identification of photons. These subdetectors will re-use 3000 lead glass crystals with attached photomultipliers from the OPAL experiment at LEP – CERN’s former accelerator. This will give a second life to the crystals and is a great opportunity for the collaboration to save money while ensuring the required performance.
One of the LAV stations is lowered into the NA62 decay volume.
The accurate identification of photons is a vital requirement of the whole NA62 physics programme. A failure in the system could lead to signals that mimic the very rare processes that the experiment is looking for, jeopardising the whole measurement. While it has not compromised on the overall performance of its detector, the NA62 collaboration has welcomed the challenge of re-using some existing equipment rather than developing new, more expensive solutions.
The thousands of lead crystals and their photomultipliers were stored in the UA1 cavern along the path of the SPS accelerator. “Although new technologies for photon identification have been developed since the crystals were used in the OPAL calorimeter, our tests confirmed that the hardware was still up to the job,” says Antonella Antonelli from Frascati Laboratories in Italy and Large Angle Veto (LAV) project leader for the NA62 collaboration.
The 3000 modules (i.e. a crystal plus a photomultiplier) were tested, some mechanical parts were cleaned using an ultrasonic technique, and new front-end electronics were designed and implemented. Once ready (again) to provide precise information on photons interacting along the decay volume of the experiment, the modules were installed in rings in cylindrical “stations”. In total, the design of the NA62 detector comprises 12 LAV stations distributed along the decay volume, spaced 6 m apart in the upstream region and up to 20 m apart in the station furthest downstream. “So far we have installed eight stations. A ninth station is ready but we will wait for the remaining three before installing all of them in one go next year,” says Antonella Antonelli.
The former OPAL crystals suffered two serious accidents – a flood at CERN and a heavy snowfall during transportation from Italy to Geneva – that seriously damaged part of the modules. But the whole project survived and the rescued crystals will live again in cutting-edge experimental research. Sustainability… yes, physicists need it too!
by Antonella Del Rosso