Fused Quartz, PMMA, and the Challenge of Radiation Hardness: Delivering Reliable Emergency Lighting Across CERN's Underground Network
Goodfellow provided materials to help support the European Organization for Nuclear Research (CERN) in progressing towards the radiation hardening of LED emergency luminaires for evacuation and emergency response within accelerator tunnels. CERN is home to the world’s largest and most complex scientific instruments – particle accelerators and detectors – used to study the basic constituents of matter, fundamental particles. Particle accelerators boost beams of particles to high energies before the beams are made to collide with each other or with stationary targets. Detectors observe and record the results of the collisions.
Challenge
Within the CERN complex, there are around 45 km of underground tunnels that contain particle accelerators, beam transfer lines, and access tunnels at depths of up to 100m below the surface. During shutdown periods, extensive access is required for operatives to carry out maintenance tasks, and so a robust lighting system containing more than 3,000 luminaires is in place to ensure the safety of the workers. The current systems, a mixture of fluorescent and low-pressure sodium luminaires, date back to the original installation of the underground accelerators prior to the LHC. These are at the end of their working lifetime, necessitating the development of a replacement system. The materials required for the replacement luminaire system needed to be based on current lighting technology and radiation resistance. LED technology was the obvious choice for the transition, due to its compact dimensions, instant response, and high luminous efficacy. Consequently, a new generation of LED luminaires has been developed for CERN underground facilities.
The individual LED is generally equipped with a plastic lens to improve the light output distribution. This lens is usually made of Polymethylmethacrylate (PMMA) polymer due to its high light transmission; however, PMMA can degrade when exposed to radiation. Depending upon the luminaire design, the LED may also be protected with glass windows to allow light transmission. Borosilicate glass has been proven to retain its optical transmission properties relatively well when irradiated. However, the overall light output of the system may be further improved by careful selection of the glass material. [1]
Solution
Goodfellow discussed the challenge with CERN’s James Devine, and his colleague Alessandro Floriduz. Several products were suggested and samples of Borosilicate glass (BS), Fused Quartz (FQ), Polymethylmethacrylate (PMMA), and Polycarbonate (PC) were provided by Goodfellow. These materials are commonly used in commercial luminaires; glass materials such as Borosilicate glass and Fused Quartz for protective windows, and polymers such as PMMA or PC for secondary optics.
To qualify their use as optical materials in the radiation-hard LED-based luminaires used in CERN accelerator tunnels, the Goodfellow samples were tested under γ-ray irradiation up to doses of 100 kGy and their degradation mechanism was examined. The results demonstrated that Fused Quartz is the most appropriate material for windows in rad-hard LED luminaires, while Borosilicate glass should be avoided; moreover, PMMA polymer should be preferred over PC for secondary optics components. [2]
References
[1] J. D. Devine and A. Floriduz, "Radiation hardening of LED luminaires for accelerator tunnels," 2016 16th European Conference on Radiation and Its Effects on Components and Systems (RADECS), 2016, pp. 1-6, doi: 10.1109/RADECS.2016.8093210.
[2] A. Floriduz and J. D. Devine, "Radiation Testing of Optical and Semiconductor Components for Radiation-Tolerant LED Luminaires," 2018 18th European Conference on Radiation and Its Effects on Components and Systems (RADECS), 2018, pp. 1-8, doi: 10.1109/RADECS45761.2018.9328680.
