Determining ¹⁷⁶Yb Enrichment in Materials Used for ¹⁷⁷Lu Production

¹⁷⁷Lu is now being used to treat metastatic prostate cancer. For this medical application, high specific activity and radiopurity is required. One method to make such material is to neutron activate highly enriched ¹⁷⁶Yb to produce ¹⁷⁷Yb which beta decays to ¹⁷⁷Lu. Chemical separation then allows the purified ¹⁷⁷Lu to be delivered. Mass spectrometry can be used to measure the ¹⁷⁶Yb abundance in the starting material, but may not have the accuracy required for this application. As a result, two new methods have been developed to determine the isotopic enrichment of ¹⁷⁶Yb. Small samples of natural isotopic composition and ¹⁷⁶Yb-enriched Yb₂O₃ are irradiated with thermal neutrons. Following suitable cooling periods, gamma-ray spectra from the samples are collected using large germanium detectors. In the first method, by measuring the ratios of ¹⁷⁷Yb/¹⁷⁵Yb in both the natural and enriched samples the isotopic abundance of ¹⁷⁶Yb in the enriched material is determined. In the second method, after allowing all of the ¹⁷⁷Yb to decay, the ratios of ¹⁷⁷Lu/¹⁷⁵Yb are used. By utilizing these ratios of ratios, one does not need to know the neutron capture cross sections or fluences, sample masses, or detector efficiencies. Results from both of these techniques will be presented.