Optimized Cyclotron Production of 211at: The Challenge of 210po-Characterization

The yield of ²¹¹At is currently limited by the restriction of the incident He ²⁺-beam energy of the ²⁰⁹Bi(α,2n) ²¹¹At reaction to ≈28 MeV avoiding the co-production of ²¹⁰At with its daughter ²¹⁰Po through ²⁰⁹Bi(α,3n) ²¹⁰At. In parallel, ²¹⁰Po is directly produced as well through the ²⁰⁹Bi(α,x) ²¹⁰Po reaction at a energies as low as 26.7 MeV. Cross-sectional data predict a significant increase of the ²¹¹At-yield at higher energies indicating that this approach warrants optimization. The strategy of using higher energies means that potential solutions to handle ²¹⁰Po during production, target processing and labelling, including the potential ²¹⁰Po-related toxicity in (pre-) clinical studies have to be studied. Here we present the results of the ²¹⁰Po quantification after a non-destructive target characterization. Two 25 μm thick Bi targets were irradiated at the Scanditronix MC 32, Rigshospitalet Copenhagen at 28.8 MeV and 29.8 MeV. For both targets the theoretical yield of ²¹¹At, ²¹⁰At and the directly produced ²¹⁰Po was estimated in a 25 layered 1 μm Bi model using the available cross-sectional data. The ²¹¹At and ²¹⁰At yields were measured with a HPGe-detector 30 min post EOB. After sufficient decay Pb foils and a ²¹⁰Po calibration source were used to calibrate an iQID α-camera and an AMBER α-spectrometer prior to the non-destructive quantification of ²¹⁰Po. Calculations predict a 28% and 1600% increased yield per μA of respectively ²¹⁰At and ²¹¹At for targets irradiated at 28.8 and 29.8 MeV respectively. At the same time the ²¹⁰Po activity increased by 350% up to a total activity of 24.14 kBq from which 60% is attributable to the direct reaction. Measured ²¹¹At and ²¹⁰At activities show an increase of 49% and 1900% per μA for the 29.8 MeV target whereas quantification of ²¹⁰Po with both α-detection systems validates the predicted activities. The results obtained in this study confirm a significantly increased yield of ²¹¹At at higher incident He ²⁺- beam energies but also indicate the importance of directly produced ²¹⁰Po, which impacts the assessment of ²¹⁰Po prior to target processing.