The beam energy measurement system for the Beijing electron-positron   collider
  The beam energy measurement system (BEMS) for the upgraded Beijing electron-positron collider BEPC-II is described. The system is based on measuring the energies of Compton back-scattered photons. The relative systematic uncertainty of the electron and positron beam energy determination is estimated as 2 \cdot 10^{-5}. The relative uncertainty of the beam's energy spread is about 6 %. 
E.V.Abakumova, M.N.Achasov, V.E.Blinov, X.Cai, H.Y.Dong, C.D.Fu,   F.A.Harris, V.V.Kaminsky, A.A.Krasnov, Q.Liu, X.H.Mo, N.Y...
physics.acc-ph hep-ex