Ocean Bottom Detector: frontier of technology for understanding the mantle by geoneutrinos

Anti-neutrinos emitted from radioactive isotopes inside the Earth, geoneutrinos, bring unique and direct information on the Earth’s composition and yield insights into its heat balance and thermal evolution. Neutrino experiments, KamLAND (Japan) and Borexino (Italy), show that geoneutrino measurements can be translated into variable geoscientific insights, although questions remain on the contribution of the mantle to the global geoneutrino flax. Distinguishing the mantle flux by existing detectors, which are all located on the continents, is challenging, since 70 % of total flax is from the crust. Given the oceanic crust is thin, simple, and has low radioactive isotope abundances, remotely placing a neutrino detector on the seafloor provides the ideal location for identifying those geoneutrinos originating from Earth’s mantle. Since 2019, a working group on Ocean Bottom Detector, involving physics, geoscience and ocean engineering, has been developing technologies for an ocean-based liquid scintillator detector. This paper addresses the status and the future prospects of the OBD project.