Isotopic Enrichment and Thermal Neutron Cross Section Measurement of Erbium-167

The thermal neutron cross section of Er167 has been obtained by measuring the relative number of Erl68 atoms formed by radiative neutron capture, when isotopically pure Er167 samples were exposed to a highly-thermalized neutron flux. A four-stage mass spectrometer was used to provide a 106/1 isotopic enrichment of Er167. It was also employed it measure the postirradiation Er168/Er167 isotopic ratios of the samples, which were placed in the D2O moderated Argonne CP-5 reactor. The integrated neutron flux, and effective neutron temperature in this reactor were also monitored mass spectrometrically, by observing isotopic ratio changes in nuclides whose cross section as a function of temperature are known. Two rare earth isotopes Sm149 and Gd157, were selected as the "non-l/v" absorbers that yielded this data. The effective neutron cross section of Er167 was found to be 740 ± 21 barns, at an effective neutron temperature of 42.8 + 2.5°C, with an integrated neutron flux of 3.37 ± 0.08 × 1019 neutron/cm2. The thermal (2200 m/sec) neutron cross section of Er167 was then calculated to be 699 ± 20 barns.