Dry distillation of astatine-211 by electromagnetic induction

Introduction: Astatine-211 (211At) is a promising alpha-emitter with properties that are well suited for targeted alpha-particle therapy; due to its short half-life of 7.2h it is typically produced on-site using bismuth-209 (209Bi) targets. Isolation of 211At from the 209Bi target is generally done in one of two ways, either by dry distillation or wet distillation. Dry distillation involves heating the target to 650°C and trapping the volatilized 211At, whereas, wet distillation is completed by dissolving the target in concentrated acid and then isolating the 211At via liquid-liquid extraction. Dry distillations are less operator intensive and take less time to perform compared to wet distillation (10-20min vs. 2h) with the drawbacks of lower recovery (50-75% vs 80-100%) and inconsistency. To improve the dry distillation process, we utilized electromagnetic induction in place of a conduction heater to rapidly heat the target. Induction operates using an alternating magnetic field to generate electric currents within the material (eddy currents). The resistance of the material generates heat from the currents - heating the target directly, rapidly, and without the need for external contact. In this work, we demonstrate the use of induction heating in place of traditional tube furnaces for the dry distillation of 211At.

Methods: Astatine-211 was produced on a JSW BC3015 cyclotron - via the nuclear reaction 209Bi(α,2n)211At - by irradiating elemental bismuth targets with aluminum backings with a 7µA beam of alpha-particles at 29MeV. Coin targets are composed of a thin layer of 209Bi (approximately 150mg) located in a shallow cavity milled into a larger aluminum body which makes up the majority of the target. The distillation setup consists of a quartz glass tube placed within the coils of an induction heater and connected to a solvent trap. The induction heater utilizes 48 volts and 18A of current passed through a 9 AWG copper solenoid coil (100mm in length, 8 turns) to generate the alternating magnetic field. During distillation, the target is placed in the quartz tube and within the coil. A vacuum pump pulls ambient air through the tube and the target is heated. Current is induced within the aluminum backing of the target due to its paramagnetic properties - bismuth metal is diamagnetic, making it a poor inductor - generating heat that transfers to the bismuth via conduction.

Results: Accurate temperature measurements are difficult to acquire as thermocouples induce heat and infrared measurements are distorted by the quartz tube. Astatine-211 begins to accumulate in the solvent trap after 30 seconds (boiling point of 337°C) and the aluminum can be observed melting after 90 seconds (melting point of 660°C). Distillations using the induction heater achieve an average recovery of 51% in 10 minutes. The coils of the induction heater reach a maximum temperature of 250°C during distillation and once completed, the entire system, including the target, cool to room temperature in 5 minutes. In comparison, the tube furnace reaches its target temperature of 750°C after 10-15 minutes. The tube furnace also yields a 51% recovery. However, the distillation requires 20 minutes and takes over 30 minutes to return to safe temperatures. Overall, the induction heater reduces the time required for 211At distillations while being less hazardous and maintaining similar recovery rates.

Conclusions: This study provides proof of concept for induction heating as a useful technique for the isolation of 211At. Induction heating is a viable replacement for standard conduction heating methods used in dry distillations of 211At. Furthermore, the technology has the potential for better dry distillation setups that would not be possible using standard tube furnaces allowing for better recoveries of 211At. Additional Data:

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