The First Indication of Neutrino-Induced Nuclear Fission

Abstract

More than 50 years ago, it was predicted that a neutrino interaction could split an atom, but no dedicated experimental effort has been made to confirm this phenomenon. The existence of this process would inform nucleosynthesis models, expand the nuclear reactor monitoring toolkit, and give a vantage into a process that bridges both the weak and strong fundamental interactions. This would add the neutrino to the selective group of particles confirmed to induce nuclear fission. To that end, the NuThor Detector was built in 2022 as a dedicated neutrino-induced nuclear fission (hereafter referred to as "nuFission") detector on thorium. The NuThor Detector hermetically seals 52.0 kg of thorium metal inside a novel, custom-made neutron multiplicity meter built to efficiently capture and detect fission neutrons peeled off of the fissioned thorium nuclei. The neutron multiplicity meter is composed of gadolinium-doped water to moderate and capture the fission neutrons. Then an array of 7.7 kg NaI[Tl] scintillator crystals from the Homeland Security Advanced Portal Program are affixed all around the complex of thorium and Gd-Water to detect neutron-capture gamma rays. This entire apparatus is exposed to the intense neutrino flux of the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. The intense, pulsed neutrino source coupled with the NuThor apparatus presents a unique and promising opportunity to conclusively put this half century mystery of nuFission to rest. This work reports the findings from several thousand hours of data that were collected by the NuThor detector over the course of 3 Science Runs. This document reports a comprehensive account of the experiment from early concepts to a full-fledged analysis.