Neutron-proton analyzing power measurements from 375 to 775 MeV.

Abstract

As part of an experimental study of the nucleon-nucleon interaction at medium energy, the free neutron-proton analyzing power A„{0„, T„) has been measured at nine incident neutron energies in the range 375 ~ T„»775 MeV and for neutron c.m. angles in the range 57' ~ 0„*~ 159'. Unpolarized neutrons with a broad continuum of energies, produced by interaction of an 800 MeV proton beam with a beryllium target, were scattered from a polarized proton target. At each angle, for the whole energy region, the scattered neutron and conjugate recoil proton were detected in coincidence. A previously unseen minimum is observed in the energy dependence of A„(0*— 100') near 625 MeV. I. INTRODUCTION Understanding of the isoscalar (I=O) part of the nuclear force at medium energies requires comprehensive measurements of neutron-proton (n-p ) scattering observables, including those involving the spins and relative orbital angular momenta of the nucleons. Experimental activity in this field began more than 20 years ago, ' although it was recognized-much earlier that information on the- noncentral nature of the nuclear force must be obtained through measurements of spin dependence. Nucleon-nucleon (N S) phase--shift analyses (PSA's) combine the proton-proton (p-p ) and n-p data in such a way that the I = 1 phases are determined by the generally more accurate p-p data; the I=0 phases can only be determined after n -p experiments have been done. The analyzing power is directly sensitive to those phase shifts which depend on spin-orbit terms. Analyzing power measurements in the medium-energy region will contribute to an understanding of the energy dependence of spin-orbit forces in the I =0 D and 6 waves. Measurements of the analyzing power are also of particular importance because the measurements of other spin observables almost always hinge upon knowledge of the analyzing power. Most of the previous n-p*' data in the 500— 800 MeV region were actually p-n data obtained by quasi-free proton-deuteron scattering. The experiment reported here is a measurement of the free n panalyzing -power A„(0„*,T„) for neutron c.m. angles 57'~ 8„*~ 159 and laboratory energies 375 ~ T„~775 MeV. In this experiment a neutron beam of large energy spread was collimated onto a polarized proton target, so that free n -p analyzing power data for a given angular range of about 16 deg in the center of mass were accumulated simultaneously for the wide region of neutron energies between 375 and 775 MeV. Since normalization errors caused by uncertainties in the target polarization were the same for all energies at each angle setting, the shape of the energy dependence of A„(O„*,T„) at each angle was reliably determined in this experiment, since energy-dependent false asymmetries were considered to be negligible.