A precise electron beam energy calibration for use in nuclear physics

An accurate calibration of the MIT-Bates electron beam energy has been achieved. The electron beam was injected into the MIT-Bates South Hall Ring and the spin-precession frequency, Ωs, was measured. From the spin-precession frequency the equilibrium energy of the ring was determined to be 370.3115±0.0021±0.0004 MeV, where the first error is statistical and the second is systematic. The energy of the beam delivered by the linac was determined to be 369.954±0.023±0.300 MeV. This provides a rigorous check on the accuracy of the facility's energy analyzing magnets. The first moment of the energy distribution was also measured. By observing the time constant of the spin diffusion with zero power in the South Hall Ring RF cavity, we obtained a value of σγγ = (3.7±0.5) × 10−4. We provide a lower limit to the polarization lifetime. At a beam energy of 370 MeV no measurable decrease in the polarization was observed up to 2 ms after injection.