The n + ¹⁴⁹Sm study with DANCE and DICER at LANSCE and abrupt changes in width distributions

¹⁴⁹Sm has a thermal capture cross section of 40 kb and is an important stable fission product (FP) for burnup credit. ¹⁴⁹Sm builds up in power reactor fuel, similar to ¹³⁵Xe but does not decay out of spent nuclear fuel. Total and capture cross sections, have been identified as inadequate for burnup credit applications. With regard to benchmark performance, critical experiment model calculations indicate an under-prediction of the ¹⁴⁹Sm reactivity worth measurements. In addition, the calculated ¹⁴⁹Sm capture rate underestimates measured capture rates for Pressurized Water Reactor (PWR) experiments by a few percent. Based on the assessment, total and capture ¹⁴⁹Sm cross-section measurements are needed from meV and extending through the resonance region. The DANCE (Detector for Advances Neutron Capture Experiments) and DICER (Device for Indirect Capture Experiments on Radionuclides) instruments have been used at the Los Alamos Neutron Science CEnter (LANSCE) to measure spin separated neutron-capture capture cross section and neutron transmission, respectively using the time-of-flight technique. A combined R-Matrix analysis has been performed to extract resonance parameters. Data have been collected and analyzed from 8 − 520 eV for capture and 0.8 meV - 1.5 keV for transmission. Resonance parameters have been extracted up to 520 eV. Significant discrepancies have been observed with evaluation libraries. Average neutron capture has been extracted up to 200 keV.