Neutron Capture and Transmission Measurements and Resonance Parameter Analysis of Samarium

The purpose of the present work is to measure the neutron cross sections of samarium accurately. The most significant isotope is ¹⁴⁹Sm, which has a large neutron absorption cross section at thermal energies and is a ²³⁵U fission product with a 1% yield. Its cross sections are thus of concern to reactor neutronics.

Neutron capture and transmission measurements were performed by the time-of-flight technique at the Rensselaer Polytechnic Institute (RPI) LINAC facility using metallic and liquid Sm samples. The capture measurements were made at the 25-m flight station with a multiplicity-type capture detector, and the transmission total cross-section measurements were performed at 15- and 25-m flight stations with ⁶Li glass scintillation detectors. Resonance parameters were determined by a combined analysis of six experiments (three capture and three transmission) using the multilevel R-matrix Bayesian code SAMMY version M2.

The significant features of this work are as follows. Dilute samples of samarium nitrate in deuterated water (D₂O) were prepared to measure the strong resonances at 0.1 and 8 eV without saturation. Disk-shaped spectroscopic quartz cells were obtained with parallel inner surfaces to provide a uniform thickness of solution. The diluent feature of the SAMMY program was used to analyze these data. The SAMMY program also includes multiple-scattering corrections to capture yield data and resolution functions specific to the RPI facility.

Resonance parameters for all stable isotopes of samarium were deduced for all resonances up to 30 eV. Thermal capture cross-section and capture resonance integral (RI) calculations were made using the resultant resonance parameters and were compared to results obtained using resonance parameters from ENDF/B-VI updated through release 3. Extending the definition of the capture RI to include the strong 0.1-eV resonance in ¹⁴⁹Sm, present measurements agree within estimated uncertainties with ENDF/B-VI release 3. The thermal capture cross section was calculated from the present measurements of the resonance parameters and also agrees with ENDF within estimated uncertainties. The present measurements reduce the statistical uncertainties in resonance parameters compared to prior measurements.