Dear all, I have a question about the polarization measurement. In the photo production reaction like γp→πp with polarized photon beam of energy GeV. how to measure the polarized components of proton. A paper said that the focal plane polarimeter, consisting of two 55 cm CH2 analyzer blocks, each followed by a pair of drift chambers, measured the asymmetry of the charged particles in p+CH2 -> charged particle + X to extract the proton polarization. I don't understand why CH2 can be used and how the polarization works. Please help me!