The BL3 and BL5 polarimeters provide an essentially non-destructive online measurement of the polarization of the proton beam. Each one is capable of determining the components of the polarization transverse to the beam direction, that is, the normal and sideways components assuming that the analyzing power of the reaction of the proton beam with the polarimeter target is known. At 200 MeV the analyzing power of the polarimeter is [Wel92], but the energy dependence is not necessarily well known. With one polarimeter on each side of the bend in the beam line it is possible to reconstruct the total polarization of the proton beam.
The beamline polarimeters both use coincident p + d scattering as the analyzing reaction. The polarimeters use a target which is thin enough that there is no significant degradation in beam quality. There is one pair of detector arms for each of the four scattering directions: left, right, up, and down. Each pair consists of a proton arm to detect the scattered polarized proton, and a deuteron arm to detect the recoiling deuteron. By gating the signal on the recoiling deuteron the need to subtract the carbon signal is eliminated. The deuteron arm uses a thick scintillator so that in a pulse-height spectrum the recoiling deuteron is cleanly separated from the quasifree-knockout proton continuum. The proton arm uses a thin (E) detector. In addition to the pulse-height information the relative time-of-flight (TOF) between the detected particles can be used as a further constraint to help identify valid p + d elastic events. The polarimeter configuration is shown schematically in figure . A more detailed discussion of the high-energy beamline polarimeters can be found in [Wel92].
Figure: Schematic of the IUCF high-energy beamline polarimeter [Wel92].