In-target chemistry during the production of ¹⁵O and ¹¹C using ³He reactions

The use of nuclear reactions that do not involve a change in element can be advantageous for radionuclide production because enriched targets are rarely required and the yield of the nuclear reactions is usually adequate. However there is a disadvantage to these reactions for medical imaging because the product radioactivity is of low specific activity. In this report we discuss the application of radiation chemistry and nucleogenic recoil chemistry to two reactions, ¹⁶O(³He,⁴He)¹⁵O and ¹²C(³He,⁴He)¹¹C, to improve specific activity of the radioactive oxygen and carbon recovered from the target. For both reactions, specific activities were improved, with minimal decrease in recovered radioactivity. For the ¹⁶O → ¹⁵O reaction in water, results demonstrated that the production of [¹⁵O]-O₂ followed a different reaction mechanism from that for unlabeled O₂. The unlabeled yield was quantitatively predicted from classical radiation chemistry G-values for water. The radiochemical product distribution was a consequence of the combined effects of recoil chemistry and radiation chemistry. Studies with thin graphite foils demonstrated that we could irradiate sufficiently thin C foils so that a useful fraction of the recoil nucleogenic ¹¹C atoms escaped the irradiated carbon and reacted with circulating gas to capture an appreciable fraction of the product ¹¹C with an improvement in specific activity. Although we have shown the feasibility of producing GBq quantities of radiopharmaceuticals by recoil techniques, the advantages of even higher specific activity using enriched targets outweigh the cost of using enriched targets.