High-Resolution Measurement of Internal Conversion Lines of 83m-Kr

Conversion electrons from the decay of Kr-83m are a standard calibration tool for tritium endpoint direct neutrino mass experiments. They were used as such in Phase II of the Project 8 experiment, where the first frequency-based neutrino mass limit was extracted with the Cyclotron Radiation Emission Spectroscopy (CRES) technique. While the tritium spectrum was only recorded in a high-statistics but low-resolution apparatus configuration, the Kr-83m conversion electron spectra were also recorded in several other configurations to explore the highest achievable resolution in the apparatus. In this talk I will present the analysis of those spectra to extract the 32151.6(5)-eV gamma energy and the binding energies of the shell electrons. These are the first physics measurements using CRES with ~1eV resolution. For these high-resolution spectra, the start frequency reconstruction error, a systematic effect that was minor for the Phase II tritium result, became important. I will show quantitative validation of the estimates of the related systematic bias and uncertainties. The extracted gamma energy and binding energies agree with literature and in some cases have higher precision. In future phases, we expect to be able to make an improved measurement of the gamma energy with NMR instrumentation under development for absolute magnetic field calibration.