The Daya Bay Neutrino Experiment is an international collaboration involving scientists from the United States, China, and Europe. The experiment investigates how electron antineutrinos produced by nuclear reactors transform into other neutrino flavors as they travel through space. These measurements provide critical insight into the fundamental properties of neutrinos and help scientists better understand why matter dominates over antimatter in the universe.
Located near the Daya Bay Nuclear Power Complex in southern China, the experiment consists of three underground experimental halls equipped with sophisticated antineutrino detectors. These detectors are housed within large water pools that not only support the detector systems but also help shield them from background radiation. Additional instrumentation within the pools identifies and rejects signals caused by cosmic rays and natural radiation from the surrounding environment.
By comparing the number of antineutrinos detected near the reactor cores with those measured approximately two kilometers away, researchers can determine how many antineutrinos have changed flavor during their journey. This approach enabled the precise measurement of the neutrino mixing angle θ, one of the key parameters governing neutrino oscillations.
UW and PSL Contributions
The University of Wisconsin–Madison and the Physical Sciences Laboratory (PSL) played leading roles in the design, engineering, and fabrication of the experiment’s antineutrino detectors. Under the leadership of Assistant Professor Karsten Heeger, the UW team coordinated efforts among engineers and scientists from multiple institutions and collaborated closely with Lawrence Berkeley National Laboratory, Brookhaven National Laboratory, and the Institute of High Energy Physics in Beijing.
Leveraging PSL’s specialized engineering capabilities, UW made the largest university-based contribution to the project.
Detector Vessel Engineering
A critical component of each detector is a set of nested acrylic vessels designed to capture the faint flashes of light produced when antineutrinos interact with detector liquids. PSL engineers were responsible for designing and analyzing these thin-walled structures, which required exceptional dimensional accuracy and structural integrity.
Each detector contains:
- An inner acrylic vessel filled with gadolinium-doped liquid scintillator, serving as the primary antineutrino target.
- A surrounding acrylic vessel filled with undoped liquid scintillator to improve detection efficiency.
- An outer volume filled with mineral oil that provides optical and radiation shielding.
The successful fabrication and testing of prototype vessels demonstrated the feasibility of this highly specialized design, leading to the production of the full detector set.
Precision Filling and Calibration Systems
Because the nested acrylic vessels have delicate structural requirements, the detector liquids must be introduced carefully to maintain nearly equal liquid levels on both sides of the vessel walls. PSL engineers developed a precision filling system that controls this process while measuring detector target masses with better than 0.1% accuracy. This level of precision is essential for achieving the experiment’s scientific goals.
Instrumentation and Detector Integration
PSL also designed and manufactured numerous supporting components for the detector systems. These included specialized “dry boxes” that provide sealed pathways for high-voltage and signal cables connected to the photomultiplier tubes (PMTs) inside each detector. More than 1,600 custom feedthrough assemblies were produced and supplied for detector integration, along with interface hardware used to mount and seal detector subsystems within the outer stainless-steel vessels.
Through these contributions, PSL and UW helped deliver the advanced detector technology that enabled one of the most important neutrino physics measurements of the past decade.