Fig 1. A schematic of the EOS detector (person shown for scale). A total of 240 PMTs will be deployed around a 4-tonne inner volume, made of UV transparent acrylic. The inner vessel and PMTs are contained within a 20-tonne outer stainless-steel vessel, which is filled with water. Radioactive calibration sources can be deployed through a port at the top of the detector.

EOS is a 20-tonne scintillation-based detector constructed at UC Berkeley. EOS provides an integrated test-bed for advanced R&D towards hybrid neutrino detectors, such as THEIA.

EOS consists of an acrylic vessel within a larger stainless steel tank (see Fig 1). The acrylic vessel is filled with water-based liquid scintillator (WbLS) and the steel tank is filled with water. Surrounding the acrylic vessel are 204 8-inch R14688-100 high quantum efficiency, fast-timing photomultiplier tubes (PMTs). Additionally, 24 R11780 12-inch PMTs are positioned at the top of the detector in order to further increase the coverage.

The lower array of PMTs and dichroicons connected to the rest of the detector and installed in the assembly stand.

EOS is currently designed and constructed at UC Berkeley. The data-taking plan consists of using deployed, radioactive sources. Using these deployed sources, we will verify a detailed optical model of the WbLS (which is used to extrapolate to ktonne scale detectors, such as THEIA), and test advanced reconstruction algorithms designed to use both the Cherenkov and scintillation photons to reconstruct event position, energy, and direction.

EOS provides a testbed for many emerging technologies. EOS can change the hybrid target material within the acrylic vessel to test different novel materials at a large scale. Additionally, EOS will test fast-timing and novel photodetectors, including a planned deployment of several dichroicons, likely positioned at the bottom of the detector, which provide spectral sorting of the short- and long-wavelength photons in order to separate the Cherenkov and scintillation components of the emitted light. Overall, EOS plans to enable broad, world-leading physics and nonproliferation programs.