Absolute polarization measurement of the 200 MeV proton beam at BNL Linac.

For absolute calibration of the vertically polarized \unit{200}{\MeV} proton beam at BNL Linac, we detect in left/right symmetric scintillator detectors the elastic protons from the beam scattering on the Carbon target. The detectors are located at angles $\theta_\text{Lab}=\pm16.2^\circ$, at which analyzing power of the elastic scattering has local maximum of 99.35\%. Variable length copper absorbers of $\sim$\unit{40}{\mm} thickness are installed in both arms of the polarimeter to suppress the inelastic protons. During RHIC Run 2017 we upgraded the polarimeter DAQ with a \unit{250}{\MHz}12 bit WFD. The detailed analysis of all detected events allowed us to properly isolate scattered proton signals and to control in-situ the efficiency of inelastic proton suppression. The systematic error in absolute polarization measurement was confirmed to be $\sigma_P^\text{syst}/P\sim0.5\%$.