The measurement system of acrylic transmittance for the JUNO central detector

Abstract

The Jiangmen Underground Neutrino Observatory (JUNO) has 20 kilo-ton liquid scintillator as target mass contained by a huge acrylic sphere to carry out the research on neutrino physics. Acrylic transmittance is considered as an important optical performance of the JUNO Central Detector as it has a direct relationship to the number of photons detected by the JUNO Central Detector and further affects the energy resolution of JUNO. The transmittance requirement of acrylic spherical panel for the JUNO Central Detector is exceeding 96% at the wavelength of 420 nm in ultrapure water and a specific measurement system was built and studied in Institute of High Energy Physics for this purpose, as the existing equipment cannot perform the sample test in the liquid and support the long sample length of 120 mm. After improving the technology of thermoforming, the transmittances of the acrylic spherical panels meet the requirement of JUNO, which guarantees the batch production of acrylic panels on time. The design of the measurement system, the constitution of hardware, the development of control and data acquisition software are introduced, and the measurement uncertainty of the system and the transmittance results of the acrylic samples from the spherical panel are discussed in this article.