Characterization of p-wave resonances in $^{139}$La and $^{81}$Br for NOPTREX

Few of the p-wave resonances under consideration for use in NOPTREX have quantified errors for the resonance energy and width. We describe a plan to measure the p-wave resonance energies in $^{139}$La and $^{81}$Br to about 0.1\% accuracy using a $^{10}$B current mode neutron detector array [1] on FP05 at LANSCE. We also present an analysis of preliminary data taken on $^{139}$La. This work is supported by NSF grant PHY-1913789. \newline [1] Y. F. Yen et al., Nucl. Inst. Meth. A 447, 476 (2000).