Spin assignment and statistical properties of neutron resonances from 161,163Dy(n, gamma) and 167Er(n, gamma) measured at the DANCE facility

Background: Average resonance spacing D0 and spin dependence of nuclear level density (NLD) are essential quantities in nuclear physics, especially important for calculations of nuclear reactions and the normalization ofPurpose: Neutron resonances with different spins in odd-mass targets can form close doublets that are often difficult to resolve. These doublets are not corrected for when accounting for subthreshold resonances during D0 determination. Moreover, different literature sources disagree on the isotopic assignment of some resonances.Methods: The gamma rays following radiative neutron capture on 161,163Dy and 167Er were measured with the highly segmented gamma -ray calorimeter Detector for Advanced Neutron Capture Experiments (DANCE) at the Los Alamos Neutron Science Center. The analysis of spectra using the gamma -multiplicity-based spin assignment method allows checking for the presence of the above-mentioned doublets. The calorimetric sensitivity provides unambiguous isotopic assignment.Results: We were able to assign spin to tens of resonances as well as to identify new ones in all three isotopes. Some isotope assignments from the literature were corrected. Detailed analysis of the number of unobserved resonances, assuming that the resonance positions obey predictions of the Gaussian orthogonal ensemble and reduced neutron widths Porter-Thomas fluctuations, allowed determination of D0 with an uncertainty of a few percent, D0 = 2.15(5), 6.39(24), and 3.86(12) eV for 161,163Dy and 167Er, respectively. Thanks to the spin assignment, the spacings for the resonances with the two spins formed in s-wave neutron capture, D-0 and D+0 , were determined. Their ratio was compared to different NLD models.Conclusions: Our deduced D0 for 163Dy is lower than any 163Dy value found in the literature. Good consistency was found with some literature values for 161Dy and 167Er. The ratios D-0 /D+0 are consistent with several models available in the literature, but in a clear contradiction with a few microscopic NLD models.