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

摘要

The parity nonconserving spin rotation of neutrons in the 0.734-eV p-waveresonance of $^{139}La$ was measured with the neutron transmission method. Twooptically polarized $^3He$ cells were used before and behind a a 5-cm long$^{139}La$ target as a polarizer and an analyzer of neutron spin. The rotationangle was carefully measured by flipping the direction of $^3He$ polarizationin the polarizer in sequence. The peak-to-peak value of the spin rotation wasfound to be $ (7.4 \pm 1.1) \times 10^{-3} $ rad/cm which was consistent withthe previous experiments. But the result was statisticallly improved. The s-pmixing model gives the weak matrix element as $xW = (1.71 \pm 0.25)$ meV. Thevalue agrees well with the one deduced from the parity-nonconservinglongitudinal asymmetry in the same resonance.