Measurement of 100 GeV Proton Beam Polarization By Elastic Carbon Scattering
in CNI-Region at RHIC

 
Itaru Nakagawa
RIKEN/RBRC
 
 The polarization of the proton beams at RHIC are measured using both a
hydrogen jet and carbon polarimeter.  The carbon polarimeter measures
relative polarization to a few percent statistical accuracy within 20 to 30
seconds using an ultra-thin (~3.5 mg/cm2) carbon ribbon target, providing
fast feedback to beam operations and experiments.  The absolute
normalization is provided by the hydrogen polarimeter which has an even
thinner gas target  takes a longer time to accumulate sufficient statistics.
Taking advantage of the maximized analyzing power in the Coulomb-Nuclear
Interference (CNI) region at very small momentum transfer (-t ~ 0.003
(GeV/c)2), both polarimeters detect the recoil proton/carbon through the
elastic scattering process at 90 degrees along the beam direction.  In this
talk, I will focus on the RUN05 analysis of the carbon target pC
polarimeter.
  RUN05 is the first extended operation of polarized proton-proton beams at
RHIC.  We accumulated nearly 1200 polarization measurements in physics
stores using the pC polarimeters during the three months running period.
Continuous efforts were made in the accelerator tuning in order to improve
the quality of the beam, for example polarization, intensity, emittance and
backgrounds. Thus these measurements were not necessarily performed always
under the same accelerator/detector conditions and therefore careful
analysis was required in order to study possible effects on the measured
polarizations. Slowly evolving changes, for example, an accumulated
radiation damage to the detectors is the one of the major concerns because
of the longer exposure to the relatively higher beam intensity compared to
previous runs. I will introduce how the polarizations are evaluated through
elastic pC scattering, as well as addressing the source of present dominant
systematic errors which limits the precision of the polarization
measurements at RHIC.