Measurement of the Free Neutron-Proton Elastic Differential Cross Section at 212 and 418 Mev Over the Full Angular Range.

The free neutron-proton elastic differential cross section has been measured at neutron beam energies of 21 and 418 MeV. The experiment determined the angular distribution over the range 15(DEGREES)-180(DEGREES) (CM) in two segments, having the same overall normalization. The 15(DEGREES) -90(DEGREES) range was measured by detecting scattered neutrons in a scintillator and MWPC array. Normalization was obtained by moving the array into the direct neutron beam. Energy selection was made via a time of Flight (TOF) measurement relative to the TRIUMF cyclotron RF. The neutron detector was calibrated absolutely using an associated particle technique, in which the recoil proton was detected. The ratio of coincidentally observed neutron-proton pairs to protons alone yielded the efficiency, which was used to calibrate neutron beam monitors. The remainder of the angular distribution was obtained by detecting the scattered protons in a magnetic spectrometer. Proton selection was made using the TOF through the spectrometer and the momentum. Energy selection was made via the RF TOF. Normalization was provided by the same monitors as used in the measurement of the forward half of the angular distribution. The normalization of d(sigma)/d(OMEGA) has been determined to about 3 , with statistical accuracies of about 1-3 on the individual data points. These data have been included in a phase shift analysis together with all world data, showing an improvement in the energy dependence of the differential cross section near 0(DEGREES) CM and in the phase shifts, notably(' )(zeta)(,3). Together with the previously measured Wolfenstein parameters, unambiguous phase shifts in the I = 0 system are obtained for the first time in the TRIUMF energy range.