Performance of a neutron polarimeter to measure the electric form factor of the neutron

We measured the analyzing power and the efficiency of a neutron polarimeter that was designed and constructed to measure the neutron electric form factorGEn. The measurement was carried out at the Indiana University Cyclotron Facility (IUCF) with the 14C(p,n)14N reaction at a proton beamenergy of 138.4 MeV. The analyzing power, 〈Ay〉, averaged over the acceptance of the polarimeter is typically 0.44±0.02−0.03+0.04 at a neutron energy of 135 MeV. With a lead-steel wall, consisting of 4 in. of lead sandwiched between two 138-in. steel plates, ahead of the polarimeter, the average analyzing power is the same within statistics, namely, 〈Ay〉 = 0.43 ± 0.02−0.03+0.04. The large systematic uncertainty quoted here is dominated by an uncertainty from the inability at the time of the experiment of measuring the longitudinal polarization component of the sideways polarized protons; this systematic uncertainty can be reduced significantly to 0.02 in any future calibration at the IUCF. In each case, the efficiency of the polarimeter is about 0.26% for 135 MeV neutrons at a pulse-height threshold of 4 MeV equivalent electron energy (MeVee) on the front detectors and 10 MeVee on the rear detectors. This analysis reveals a small analyzing power from the carbon in the front-analyzer detectors.