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].