Analysis of neutron resonance parameters for dysprosium isotopes

Abstract

Neutron capture yields of dysprosium isotopes were measured using the time-of-flight method with a 16-segment sodium iodide multiplicity detector. The measurements were made at the 25-m flight station at the Gaerttner LINAC Center at Rensselaer Polytechnic Institute. We used four different kind of enriched dysprosium isotopes and one natural dysprosium sample. Neutron transmission data from the natural dysprosium sample were used to normalize the neutron capture data. Resonance parameters were obtained using the multilevel R-matrix Bayesian code SAMMY. The neutron capture data for four enriched dysprosium isotopes and one natural dysprosium were sequentially fitted. There were 29 and 17 new resonances not listed in ENDF/B-VII.1 from 161-Dy and 163-Dy isotopes, respectively. However, 6 resonances from 161-Dy isotope, 2 resonances from 163-Dy, and 4 resonances from 164-Dy were not observed because present measurements did not support the existence of the resonances which are too small. The capture resonance integrals of the each isotope were calculated with the resulting resonance parameters in the energy region from 0.5 eV to 20 MeV, and the results were compared to the capture resonance integrals with the resonance parameters from ENDF/B-VII.1. A resonance integral value of the natural dysprosium calculated with present resonance parameters was 1,405 ± 3.5 barn. This value is approximately 0.3% higher than that obtained with the ENDF/B-VII.1 parameters. The distributions of the present and ENDF/B-VII.1 neutron widths were compared to a Porter-Thomas distribution. Neutron strength functions for 161-Dy and 163-Dy were calculated with the present resonance parameters and both values were in between the values of “Atlas of Neutron Resonances” and ENDF/B-VII.1. The present radiation width distributions of 161-Dy and 163-Dy were fitted with χ2 distribution by varying the degrees of freedom.