Calibration of the polarization of a frozen spin target

A recent experiment, measuring the spin correlation parameter Ann in neutron-proton elastic scattering to a precision of ±0.03 at several energies in the range 200 to 500 MeV, required the polarization of the proton frozen-spin target used in the experiment to be known to an accuracy of ±2%. The calibration of the polarization of the frozen-spin target was accomplished through an experiment interleaved with the Ann experiment. An unpolarized proton beam, of an intensity comparable to the intensity of the neutron beam in the Ann experiment, was scattered at 240 (lab.) from the frozen-spin target at energies of 468 and 501 MeV. From the measured left-right asymmetries and knowledge of the proton-proton analyzing powers, Ay(240, 468 MeV) = 0.4087 ± 0.0060 and Ay(240, 501 MeV) = 0.4204 ± 0.0059, the target polarizations could be deduced. The weighted average of the ratios of the target polarization deduced in the scattering experiment and obtained from nuclear magnetic resonance measurements was found to be Pt(scattering)/Pt(NMR) = 0.962 ± 0.008(stat) ± 0.021(sys) ± 0.014(sys) at 468 MeV and 0.950 ± 0.005(stat) ± 0.020(sys) ± 0.013(sys) at 501 MeV. By dividing the target cell holding the butanol beads, as illuminated by the incident proton beam, in three parts (top, middle, and bottom), a determination of the average polarization for each of the three parts was made. A clear decrease of the polarization going from the top of the bottom of the target cell was found. (orig.)