Helium-3 Magnetometry for Project 8 Magnetic Field Calibration*

Precision measurement techniques enable β-decay spectroscopy experiments with great potential for exploring physics beyond the Standard Model. In particular, Project 8 will use Cyclotron Radiation Emission Spectroscopy (CRES) to observe the β-decay of tritium in order to constrain the neutrino mass with high sensitivity and low systematics. CRES converts the energy of β-decay electrons into a microwave-frequency signal via cyclotron motion of electrons in a trapping magnetic field, so an absolute energy calibration requires knowledge of the absolute magnetic field. At the tritium endpoint, an absolute energy calibration with an accuracy of a hundred meV requires a magnetic field calibration better than 0.2 parts per million. Such an energy calibration allows the tritium endpoint energy to be fixed, simplifying spectrum analysis and reducing sources of systematic uncertainty. Helium-3 magnetometry is an excellent candidate technique for mapping the Project 8 magnetic field, capable of accuracy and precision at the parts per billion level. In this talk, we describe progress towards a high-sensitivity helium-3 NMR magnetometer operating near 1 Tesla that can characterize the trapping field in Project 8 with the required accuracy and precision.