Resonant Cavity Characterization for the Project 8 Neutrino Mass Experiment*

The discovery of neutrino oscillations proved that neutrinos, once thought to be massless, have a non-zero mass. The Project 8 experiment aims to use the novel technique of Cyclotron Radiation Emission Spectroscopy (CRES) to measure the neutrino mass. The CRES technique reconstructs the beta decay spectrum of tritium by measuring the frequency of the radiation emitted by the decay electrons as they undergo cyclotron motion inside a resonant cavity. A small metal resonant cavity prototype was built by the Project 8 group at Penn State. The electromagnetic (EM) field inside the cavity can be mapped using the bead-pull method, which measures the change in resonant frequency when a small bead is moved throughout the cavity. Open-ended, concentric cylindrical structures at each end of the cavity are used to terminate the EM fields, defining the cavity resonant modes. This terminator design initially presented mechanical challenges, and we switched to a new design that lines the inner edges of the cavity.  In this work, we report on the design and testing of the new terminator configuration in the prototype cavity using the bead puller system. We found that the new terminator data matches simulations, creating electromagnetic field maps similar to those obtained with the original terminators.