Low-Energy Neutron Resonances in Erbium and Gadolinium

The two neutron resonances in Er at 0.46 and 0.58 ev have been measured using samples enriched in Er¹⁶⁶, Er¹⁶⁷, and Er¹⁶⁸. It was found that both resonances occur in the target nucleus Er¹⁶⁷. The isotopic cross sections of Gd¹⁵⁵ and Gd¹⁵⁷ were measured from 0.02 to 0.30 ev and resonances were found in Gd¹⁵⁵ at 0.0268 and in Gd¹⁴⁷ at 0.0314 ev. Parameters for these resonances were derived by fitting the data to single-level Breit-Wigner formulae. Isotopic assignments were made for fifteen Gd resonances between 1 and 25 ev from previously unpublished data using samples enriched in 155, 156, 157, and 158. The Breit-Wigner parameters for three of these were obtained.