Development of methods for the preparation of radiopure ${}^{82}$Se sources for the SuperNEMO neutrinoless double-beta decay experiment

A radiochemical method for producing ${}^{82}$Se sources with an ultra-low level of contamination of natural radionuclides (${}^{40}$K, decay products of ${}^{232}$Th and ${}^{238}$U) has been developed based on cation-exchange chromatographic purification with reverse removal of impurities. It includes chromatographic separation (purification), reduction, conditioning (which includes decantation, centrifugation, washing, grinding, and drying), and ${}^{82}$Se foil production. The conditioning stage, during which highly dispersed elemental selenium is obtained by the reduction of purified selenious acid (H${}_2$SeO${}_3$) with sulfur dioxide (SO${}_2$) represents the crucial step in the preparation of radiopure ${}^{82}$Se samples. The natural selenium (600 g) was first produced in this procedure in order to refine the method. The technique developed was then used to produce 2.5 kg of radiopure enriched selenium (${}^{82}$Se). The produced ${}^{82}$Se samples were wrapped in polyethylene (12 μm thick) and radionuclides present in the sample were analyzed with the BiPo-3 detector. The radiopurity of the plastic materials (chromatographic column material and polypropylene chemical vessels), which were used at all stages, was determined by instrumental neutron activation analysis. The radiopurity of the ${}^{82}$Se foils was checked by measurements with the BiPo-3 spectrometer, which confirmed the high purity of the final product. The measured contamination level for ${}^{208}$Tl was 8-54 μBq/kg, and for ${}^{214}$Bi the detection limit of 600 μBq/kg has been reached.