Resonance Parameters and Uncertainties Derived from Epithermal Neutron Capture and Transmission Measurements of Natural Molybdenum
Abstract The electron linear accelerator facility at the Rensselaer Polytechnic Institute was used to explore neutron interactions with molybdenum in the energy region from 10 eV to 2 keV. Neutron capture and transmission measurements were performed by the time-of-flight technique. 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. Two transmission measurements were performed at a flight path of 25 m with a 6Li glass scintillation detector. The neutron capture measurements were performed at a flight path of 25 m with a 16-segment sodium iodide multiplicity detector. Nine different thicknesses of elemental molybdenum metal samples ranging from 0.051 mm (0.002 in.) to 6.35 mm (0.250 in.) were measured in either capture or transmission. Reductions in resonance integrals were observed when compared to ENDF/B-VII.0 for six of the seven stable isotopes. The largest reductions were 9% in 97Mo and 11% in 100Mo. The one measured increase in resonance integral relative to ENDF/B-VII.0 occurred in 95Mo, and it was significant (10%). The measured distribution of neutron widths for 95Mo and 97Mo are a better match to a Porter-Thomas distribution than those of ENDF/B-VII.0. Neutron strength functions for 95Mo and 97Mo were measured and compared to ENDF/B-VII.0. The strength of 95Mo and 97Mo are within uncertainties of each other. The measured radiation width distribution for 95Mo and 97Mo are compared to those of ENDF/B-VII.0 and to χ2 distributions. Significant aspects of this analysis are the assignment of radiation widths, the determination of the transmission resolution function, and the propagation of experimental uncertainties into resonance parameter uncertainties.
Minimizing the statistical error of resonance parameters and cross-sections derived from transmission measurements
Total neutron cross-sections are usually measured by a transmission experiment. In this experiment the transmission through a sample is measured by taking the ratio of the background corrected counts measured with and without the sample in the beam. This procedure can be optimized to reduce the statistical error in the measured cross-section. The objective is to find the optimal sample thickness and time split between the open beam, sample and background measurements. An optimization procedure for constant cross-section measurement is derived and extended to the area under the total cross-section curve of an isolated resonance. The minimization of the statistical error in the measured area also minimizes the statistical error in the inferred neutron width. Comparison of the analytical expression developed in this paper and resonance parameters obtained from the SAMMY (Updated users’s guide for SAMMY: Multilevel R-Matrix fits to neutron data usingBays’ equation, version m2, ORNTL/TM/-9179/R4) code is shown. The comparison was done with both simulated data and data from transmission experiments that were previously done at RPI. It is shown that the analytical expression can be used as a design tool for optimizing transmission experiments. This will consequently result in more accurate measurements of resonance parameters and can shorten the time required to reach a given accuracy. r 2001 Elsevier Science B.V. All rights reserved.
Calculation of the ORELA neutron moderator spectrum and resolution function
Abstract The yield, spectrum, and delay-time distributions (resolution function) of escape neutrons from the ORELA water-moderated and water-cooled tantalum target assembly are calculated using-dependent, three-dimensional Monte Carlo techniques. The resulting spectrum and delay-time distributions of the moderated neutrons are compared with measured values.