Europium resonance parameters from neutron capture and transmission measurements in the energy range 0.01-200 eV

摘要

Europium is a good absorber of neutrons suitable for use as a nuclear reactor control material. It is also a fission product in the low-yield tail at the high end of the fission fragment mass distribution. Measurements have been made of the stable isotopes with natural and enriched samples. The linear electron accelerator center (LINAC) at the Rensselaer Polytechnic Institute (RPI) was used to explore neutron interactions with europium in the energy region from 0.01 to 200 eV. Neutron capture and transmission measurements were performed by the time-of-flight technique. Two transmission measurements were performed at flight paths of 15 and 25 m with ~6Li glass scintillation detectors. The neutron capture measurements were performed at a flight path of 25 m with a 16-segment sodium iodide multiplicity detector. Resonance parameters were extracted from the data using the multilevel R-matrix Bayesian code SAMMY. A table of resonance parameters and their uncertainties is presented. To prevent air oxidation metal samples were sealed in airtight aluminum cans in an inert environment. Metal samples of natural europium, 47.8 atom% ~(151)Eu, 52.2 atom% ~(153)Eu, as well as metal samples enriched to 98.77 atom% ~(153)Eu were measured. The measured neutron capture resonance integral for ~(153)Eu is (9.9 ± 0.4)% larger than ENDF/B-Ⅶ.1. The capture resonance integral for ~(151)Eu is (7 ± 1)% larger than ENDF/B-Ⅶ.1. Another significant finding from these measurements was a significant increase in thermal total cross section for ~(151)Eu, up (9 ± 3)% from ENDF/B-Ⅶ.1. The thermal total cross section for ~(153)Eu is down (8 ± 3)% from ENDF/B-VII.l, but it is larger than that of ENDF/B-Ⅶ.0. The resolved resonance region has been extended from 100 eV to 200 eV for both naturally-occurring isotopes. Uncertainties in resonance parameters have been propagated from a number of experimental quantities using a Bayesian analysis. Uncertainties have also been estimated from fitting each Eu sample measurement individually.