Separation and recovery of exotic radiolanthanides from irradiated tantalum targets for half-life measurements

The current knowledge of the half-lives (T1/2) of several radiolanthanides is either affected by a high uncertainty or is still awaiting confirmation. The scientific information deriving from this imprecise T1/2 data has a significant impact on a variety of research fields, e.g., astrophysics, fundamental nuclear sciences, and nuclear energy and safety. The main reason for these shortcomings in the nuclear databases is the limited availability of suitable sample material together with the difficulties in performing accurate activity measurements with low uncertainties. In reaction to the urgent need to improve the current nuclear databases, the long-term project "ERAWAST"(Exotic Radionuclides from Accelerator Waste for Science and Technology) was launched at Paul Scherrer Institute (PSI). In this context, we present a wet radiochemical separation procedure for the extraction and purification of dysprosium (Dy), terbium (Tb), gadolinium (Gd), and samarium (Sm) fractions from highly radioactive tantalum specimens, in order to obtain 154Dy, 157-158Tb, 148,150Gd, and 146Sm samples, needed for T1/2 determination studies. Ion-exchange chromatography was successfully applied for the separation of individual lanthanides. All separations were conducted in aqueous phase. The separation process was monitored via γ-spectrometry using suitable radioactive tracers. Both the purity and the quantification of the desired radiolanthanides were assessed by inductively coupled plasma mass spectrometry. Test experiments revealed that, prior to the Dy, Tb, Gd, and Sm separation, the removal of hafnium, lutetium, and barium from the irradiated tantalum material was necessary to minimize the overall dose rate exposure (in the mSv/h range), as well to obtain pure lanthanide fractions. With the herein proposed separation method, exotic 154Dy, 157-158Tb, 148,150Gd, and 146Sm radionuclides were obtained in sufficient amounts and purity for the preparation of samples for envisaged half-life measurements. During the separation process, fractions containing holmium, europium, and promethium radionuclides were collected and stored for further use.