Moller Polarimetry for the Neutron Skin Experiments at Jefferson Lab

The two sister experiments PREX-2 and CREX have recently performed measurements of the neutron skin on Pb-208 and Ca-48 respectively via parity-violating electron scattering. The measured scattering asymmetries are normalized to the beam polarization making electron beam polarimetry central for precision determinations of the neutron skins. Furthermore, polarimetry was essential for minimizing and interpreting key systematics effects such as the transverse and null asymmetries. M\o ller polarimetry has been utilized in Hall A for two decades. However, the system has been upgraded since PREX-1 to utilize a foil target of pure iron magnetized out of plane along the direction of the beam, allowing for sub-1% precision on the beam polarization. Modifications were also made to the spectrometer to allow for the higher energies of the 12 GeV era at Jefferson Lab. During PREX-2 and CREX, the upgraded Hall A M\o ller polarimeter was first commissioned at low energy (1-2 GeV) and indeed delivered precision results. I will describe the M\o ller polarimetry data taken for these experiments, including some unexpected findings.