Measurement of parity-nonconserving rotation of neutron spin in the 0.734-eV p-wave resonance of 139La

The parity nonconserving spin rotation of neutrons in the 0.734-eV p-wave resonance of 139La was measured with the neutron transmission method. Two optically polarized 3He cells were used before and behind a 5-cm long 139La target, each as a polarizer and an analyzer for neutron spin. The rotation angle was carefully measured by flipping the direction of 3He polarization in the polarizer in sequence. The peak-to-peak value of the spin rotation was determined to be (7.4±1.1)×10−3 rad/cm. The result is found to be consistent with the previous experiments, but the result was statistically improved. The s–p mixing model gives the weak matrix element as xW=(1.71±0.25) meV. The value agrees well with the one deduced from the parity-nonconserving longitudinal asymmetry in the same resonance.