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

Abstract

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.

The automatic measurement system for internal stress of acrylic

“…There are three fitting parameters -"a" stands for sin 2 (2𝛼), "b" stands for optical path difference 𝛿 and "s" stands for additional step when there is absorption effect for ordinary light and extraordinary light [13, pp. 147-150], [14][15][16][17][18]. R-square (𝑅 2 ) as goodness of fit is used to evaluate fitting.…”

“…Measuring from Y-axis and Z-axis can give the stress level in the bonding area if the stress direction in the bonding area is along X-axis. The results of optical path difference of three directions are listed in table 2, and the stress optical coefficient of (3.9 ± 0.1) × 10 −12 /Pa was used in stress calculation for bonding area [14].…”

The novel detection equipment of residual stress of PMMA

“…1. Mobile detection equipment has been applied in the JUNO experiment as an important tracking tool for on-site installation [5], which is shown in Figure 2. The two types of detection equipment have some common performance characteristics, such as the uncertainty can reach 5%, an upper limit of 10000 nm can be got for optical path difference measurement, repetition accuracy can reach 1 nm for optical path difference measurement, and can work in natural light environment.…”

The novel equipment for measuring internal stress in acrylic

“…The measurement of internal stress in acrylic includes two aspects -the calibration of stressoptical coefficient of acrylic and the measurements of BOPD in acrylic samples or panels [8]. Stress-optical coefficient is the intrinsic property of acrylic.…”