Optimised solid-phase extraction of 211At: Activity balance of 211At, 210At and 210Po after wet chemistry target dissolution

Abstract

Several studies have illustrated the significant increase in the production yield of cyclotron produced 211At by irradiating targets at higher beam energies, highlighting that this approach warrants consideration. Recently published data also confirms the importance of the 209Bi(α,x)210Po reaction in addition to the increasing 210At production at higher energies, stressing the importance of determining the total 210Po activity on irradiated targets and the validation of radiochemical separation of 210Po from astatine as an impurity.In this work, two experiments were performed: (i) the dissolution of irradiated Bi-targets to validate the calculated 211At, 210At and 210Po activities on target and (ii) the radiochemical separation of astatine by extraction chromatography to determine the activity balance of 211At, 210At and 210Po after target processing.The calculated 211At, 210At and 210Po activities were validated on targets irradiated at the Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark. 211At and 210At activities were determined by gamma spectrometry, whereas the 210Po activity was measured by liquid scintillation counting (LSC) after target dissolution with concentrated nitric acid. The activity balance during target processing and extraction chromatography was established using targets irradiated at Arronax, Nantes, France. Targets were characterised after the dissolution with an in-house developed dissolution unit. Dissolution yield, extraction yield and separation capability were assessed by determining 211At, 210At and 210Po activities at every step.The results show that more than 88% of the total 210Po activity in targets irradiated at ≈ 28.8 MeV is attributable to the direct production of 210Po. This amounts to about 53% at energies of 29.8 MeV. A maximal discrepancy of 21% was found when comparing the total measured activity to the total calculated 210Po. Activity balance calculations show a mean decay-corrected dissolution yield of 88.9% ± 9.5% and an extraction yield of 87.56% ± 11.4% (n = 5) of 211At/210At resulting in an overall yield of 77.8% ± 10.5% (n = 5). In addition, the radiochemical separability was confirmed by removing >95% of the previously formed 210Po before extraction.This work confirms the important underestimation of the total 210Po component on irradiated targets at 28.8 and 29.8 MeV when calculating solely from 210At measurements. In addition, the validation measurements indicate that calculations can be used to estimate the total 210Po activity.The experimental setup of target dissolution and extraction chromatography presented a very good yield. It allowed the safe isolation and management of 210Po, leaving 210At as the only source for future ingrowth of 210Po. This finding is an important step in moving beyond the conventional approach of irradiating Bi-targets at limited incident energies.