Speaker
Description
The Jiangmen Underground Neutrino Observatory (JUNO) is a large-scale
neutrino experiment designed to address multiple physics goals such as determining
neutrino mass hierarchy, precisely measuring oscillation parameters, neutrino
detection from supernova, sun, and earth, etc. with its central detector (CD) requiring
a 20 kt liquid scintillator (LS) volume, ultra-low radioactive backgrounds (e.g., U/Th
below 10⁻¹⁵ g/g), and over 75% photomultiplier tube (PMT) coverage for a 20-year
operational lifespan submerged in pure water for shielding. The final design selected
for the CD consists of a spherical acrylic vessel with an inner diameter of 35.4 m and
120 mm thickness, supported by a stainless steel (SS) structure, chosen for its optimal
balance of physics performance, engineering feasibility, and risk mitigation. Key
technological advancements include the development of a custom acrylic formulation
excluding plasticizers and UV inhibitors to achieve high transmittance over 96% at 420
nm and low radioactivity levels (U-238 < 0.03 ppt), coupled with innovations in acrylic
node design using embedded steel pads to limit stress below 3.5 MPa, thermoforming
for spherical panels, and bulk polymerization techniques ensuring long-term durability
against creep and aging. The SS structure, a 40.1 m diameter spherical latticed shell,
was engineered with low-background materials (U-238 < 1 ppb) and precision welding
and bolting methods to control deformations within 3 mm for accurate PMT alignment,
incorporating high-strength lock bolts and surface treatments for reliable connections
under buoyancy loads. A Filling, Overflow, and Circulation (FOC) system was designed
to manage LS operations with radon mitigation strategies, such as surface roughness
control and nitrogen blanketing, using water-LS exchange for safe commissioning and
passive/active overflow tanks for thermal expansion management. Underground
assembly involved sequential erection of the SS structure, layer-by-layer acrylic vessel
construction on a lifting platform with strict cleanliness (class 10000-100000) and
temperature controls (21±1°C), and PMT installation, ensuring detector integrity. This
integrated approach sets a benchmark for large-scale neutrino detectors, combining
material science, structural engineering, and systems design to achieve JUNO's physics
objectives with reliability and low background.