First Observation of Cyclotron Radiation from MeV-Scale e following Nuclear Decay

We present an apparatus for detection of cyclotron radiation yielding a frequency-based fith kinetic energy determination in the 5 keV to 2.1 MeV range, characteristic of nuclear fi decays. The cyclotron frequency of the radiating fi particles in a magnetic field is used to determine the fi energy precisely. Our work establishes the foundation to apply the cyclotron radiation emission spectroscopy (CRES) technique, developed by the Project 8 Collaboration, far beyond the 18-keV tritium endpoint region. We report initial measurements of fi-'s from 6He and fi thorn 's from 19Ne decays to demonstrate the broadband response of our detection system and assess potential systematic uncertainties for fi spectroscopy over the full (MeV) energy range. To our knowledge, this is the first direct observation of cyclotron radiation from individual highly relativistic fi's in a waveguide. This work establishes the application of CRES to a variety of nuclei, opening its reach to searches for new physics beyond the TeV scale via precision fi-decay measurements.