Cyclotron production of 225Ac from an electroplated 226Ra target

Abstract

Purpose: We demonstrate cyclotron production of high-quality ²²⁵Ac using an electroplated ²²⁶Ra target.

Methods: ²²⁶Ra was extracted from legacy Ra sources using a chelating resin. Subsequent ion-exchange purification gave pure ²²⁶Ra with a certain amount of carrier Ba. The radium target was prepared by electroplating. We successfully deposited about 37 MBq of ²²⁶Ra on a target box. Maximum activation was achieved using 15.6 MeV protons on the target at 20 µA for 5 h. Two functional resins with various concentrations of nitric acid purified ²²⁵Ac and recovered ²²⁶Ra. Cooling the intermediate ²²⁵Ac for 2-3 weeks decayed the major byproduct of ²²⁶Ac and increased the radionuclidic purity of ²²⁵Ac. Repeating the same separation protocol provided high-quality ²²⁵Ac.

Results: We obtained ²²⁵Ac at a yield of about 2.4 MBq at the end of bombardment (EOB), and the subsequent initial purification gave 1.7 MBq of ²²⁵Ac with ²²⁶Ac/²²⁵Ac ratio of < 3% at 4 days from EOB. Additional cooling time coupled with the separation procedure (secondary purification) effectively increased the ²²⁵Ac (4n + 1 series) radionuclidic purity up to 99 + %. The recovered ²²⁵Ac had a similar identification to commercially available ²²⁵Ac originating from a ²²⁹Th/²²⁵Ac generator.

Conclusion: This procedure, which involves the ²²⁶Ra(p,2n)²²⁵Ac reaction and the appropriate purification, has the potential to be a major alternative pathway for ²²⁵Ac production because it can be performed in any facility with a compact cyclotron to address the increasing demand for ²²⁵Ac.

Keywords: Actinium-225; Alpha emitter; Radium-226; Targeted alpha therapy.

Fig. 1

Decay chains for the 4n, 4n + 1, and 4n + 2 series

Fig. 2

²²⁶Ra target assembly

Fig. 3

Separation diagram for ²²⁵Ac from the ²²⁶Ra matrix

Fig. 4

Deposition profile of (a) approximately 37 MBq (1 mCi) of ²²⁶Ra on the target surface and (b) 5 mg of Ba. Both electrodepositions are performed under the same conditions (100 mA constant DC for 3 h)

Fig. 5

Gamma spectrum for various ²²⁵Ac samples: (a) ²²⁵Ac product (initial separation) after 4 days from the end of bombardment (EOB), (b) ²²⁵Ac product (secondary separation) after 20 days from EOB, and (c) commercially available authentic ²²⁵Ac (generator-made)

Fig. 6

Alpha spectrum for the ²²⁵Ac product. Aliquot of 0.37–1.85-kBq ²²⁵Ac dried on an Al disk measured without a covering for (a) purified ²²⁵Ac product after 19 days from the end of bombardment (EOB) and (b) commercially available authentic ²²⁵Ac

Fig. 7

Diagram of ²²⁶Ra recycling