Proposal for VEPP-4M beam energy measurement using magnetic spectrometer with Compton calibration and photon detector

A method for circular e$^-$/e$^+$ accelerator beam energy measurement is proposed. Coordinate of an electron (or a positron) in a focusing magnetic spectrometer built in a circular accelerator depends on its energy, two spectrometer parameters, and the circulating beam energy. The spectrometer parameters can be defined using minimum electron energies from Compton backscattering with two laser wavelengths, and an additional photon coordinate detector. The photon detector is calibrated at once at well-known beam energy. The VEPP-4M collider has appropriate equipment for the method implementation: built-in focusing magnetic spectrometer, Compton calibration system with two lasers and a photon coordinate detector. Thus, the proposed technique could be implemented with minimum efforts; tests and further upgrade are planned. The beam energy can be defined with expected uncertainty better than $10^{-4}$.