The study of acrylic bonding gap stress for the JUNO Central Detector

The Central Detector (CD) of Jiangmen Underground Neutrino Observatory (JUNO) has a huge acrylic spherical vessel, which will contain 20000 tons of liquid scintillator, being immersed in pure water, endure about 3 × 107 N of buoyancy. It is composed of 263 pieces of acrylic spherical panels bonded by the mass polymerization. Acrylic stress is critical to the structural safety of the JUNO CD, and it is required not to be greater than 3.5 MPa. The specialized measurement system for acrylic stress of the JUNO CD has been designed and realized, which supports non-destructive and quantitative measurement. The measurement system was applied to the main acrylic spherical panels without bonding gap first to verify stress level, which met JUNO's requirement. The bonding quality of acrylic spherical panels is critical, and the stress of bonding gap is concerned. The stresses of acrylic bonding gaps were measured using acrylic samples to help study the bonding techniques. The stress optical coefficient of acrylic bonding gap was calibrated and different bonding methods were studied and compared. We found that the objective function of numerical fitting for acrylic bonding gap needs modification when the measured optical path difference is large, which was analyzed and discussed in this article.