Calibration of the 2nd focal-plane polarimeter for low-energy protons using polarized <i>p</i> –C scattering and GEANT4 simulation

Abstract The 2nd-FPP, a focal-plane polarimeter optimized for proton energies below 100 MeV, has been developed for polarization measurements of backward-scattered protons in $(\vec{p},2p)$ experiments. A calibration experiment was performed at the Research Center for Nuclear Physics (RCNP), Osaka University, using a 64.2-MeV proton beam. Highly polarized protons produced by p–C elastic scattering were transported to the polarimeter by the Large Acceptance Spectrometer. To interpret the detector response quantitatively, we developed a dedicated GEANT4 simulation. Because the standard GEANT4 processes do not provide the azimuthal asymmetry associated with polarized proton scattering, the hadronic elastic process based on G4HadronElastic() was modified in GEANT4 to incorporate polarization-dependent scattering probabilities using known analyzing-power data for p–C and p–p scattering. An improved track-reconstruction procedure for multi-wire drift chambers was applied to the data, and elastic p–C events were selected using the correlation between the reconstructed horizontal scattering angle and the scintillator energy-loss information. The measured asymmetry and the effective analyzing power, evaluated as functions of the proton incident angle, are consistent with the GEANT4 simulation. These results demonstrate that the 2nd-FPP provides reliable low-energy proton polarimetry and that the polarization-aware GEANT4 simulation gives a consistent description of the detector response and calibration.