The HUNTER Sterile Neutrino Search: Progress Towards a 131-Cs Decay Source

The HUNTER experiment (Heavy Unseen Neutrinos by Total Energy-Momentum Reconstruction) is a search for sterile neutrinos with keV-scale mass. We will measure on 131-Cs radioactive decays to reconstruct their electron neutrino missing mass, with a probability of measuring a keV sterile neutrino proportional to the coupling sin²(θ_e4). Charged decay products will be measured by reaction-microscope spectrometers with high solid angle, and the trigger x-ray will be detected using scintillator and silicon photomultiplier arrays, all with sufficient resolution to reconstruct a keV missing mass. The 131-Cs decay source will consist of an actively controlled MOT. The 9.7 d half-life of 131-Cs, along with the goal of running the experiment continuously for one year, present unique challenges for the MOT. We will present progress on the MOT system being developed at UCLA, including demonstration of an efficient thermionic emission-based atomic source.