## 2 Experimental Section
### Procedure for Wet Chemistry Isolation of NaI\({}^{211}\)At]At  
The wet chemistry isolation process is conducted in a charcoal-filtered radiochemical glovebox designed to prevent any release of \({}^{211}\)At into the atmosphere. A picture of the glovebox is shown in the Supplemental Material (Figure S1). Most reagents, glassware and equipment (e.g., heater-stirrs, lab jacks, _etc._) required for the isolation are placed in the glovebox prior to introduction of the irradiated target. A list of those items is provided in the Supplemental Material (List S2). Prior to the introduction of the plastic bag containing the irradiated target into the glovebox, the fan removing air through the charcoal filter is turned on to provide negative pressure in the glovebox. The exhaust from the fan passes through a charcoal filter within the glovebox and then through tubing, which is vented into a Plexiglas box that has two stacked charcoal filters. The exhaust-facing surfaces of both charcoal filters are examined to determine if \({}^{211}\)At has been released from the glovebox. A picture of the Plexiglas enclosure with the vent tubing is shown in the Supplemental Material (Figure S3). Just prior to beginning the procedure, the irradiated bismuth target, inside of a plastic zipper bag used while transporting from the cyclotron vault to the radiochemistry laboratory, was placed in a dose calibrator to estimate the amount of \({}^{211}\)At produced. It was then passed through the antechamber into the glovebox. There is a second dose calibrator in the glovebox, so the target is read in that dose calibrator as well. Any \({}^{211}\)At measurements performed on material within the glovebox are compared with the target reading obtained from the dose calibrator within the glovebox. Step 1: Dissolution of Bi target in HNO\({}_{3}\). The irradiated bismuth target was removed from the plastic zipper bag and placed bismuth face down in a 1L modified polypropylene bottle. A 10 mL aliquot of conc. HNO\({}_{3}\) (ThermoFisher, Waltham, MA, USA) was then added by pipet to the bottle. The dissolution of the bismuth target was allowed to proceed for 10 min. The HNO\({}_{3}\) solution was transferred by a pipet to a 50 mL round bottom flask. An additional 5 mL of HNO\({}_{3}\) was added to the dissolution bottle to rinse the target and container. After gently shaking the flask, the wash HNO\({}_{3}\) was also transferred to the round bottom flask. Step 2: Removal of HNO\({}_{3}\). The 50 mL flask containing Bi and \({}^{211}\)At dissolved in conc. HNO\({}_{3}\) was connected to a short-path distillation head. That glassware was lowered into a 50 mL round bottom aluminum heating block on a hot plate at \(\sim\)300 \({}^{\circ}\)C. The distillation flask and neck of distillation head were covered with aluminum foil. After rapidly heating the HNO\({}_{3}\), it was distilled with gentle stirring over a 20-30 min period. The distillation was stopped when no further HNO\({}_{3}\) distillate drops were visible. At that time the distilling glassware was raised from the heating block, and the white residue in the distillation flask was allowed to cool for 10 min. Step 3: Dissolution in HCl. The 50 mL distillation flask containing the white residue was removed from distillation head. To that flask was added 8 mL of 8 M HCl (ThermoFisher, Waltham, MA, USA), and the mixture was agitated until all solid was dissolved (5-10 min). The solution was transferred via pipet to a 20 mL glass scintillation vial. The distillation flask was rinsed with an additional 2 mL of 8 M HCl as a wash, and that solution was also transferred to the scintillation vial. Step 4: DIPE Extraction and HCl Washes. A 4 mL aliquot of freshly distilled diisopropyl ether (DIPE) (Sigma-Aldrich, St. Louis, MO, USA) that had been saturated with 8 M HCl was added to the scintillation vial and the resulting biphasic mixture was stirred for 10 min. Following that, the aqueous layer was removed and counted in a dose calibrator.