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

The use of nuclear reactions that do not involve a change in element can be advantageous for radionuclide production because enriched targets are rare ly required and the yield of the nuclear reactions is usually adequate. How ever there is a disadvantage to these reactions for medical imaging because the product radioactivity is of low specific activity. In this report we d iscuss the application of radiation chemistry and nucleogenic recoil chemis try to two reactions, O-16(He-3,He-4)O-15 and C-12(He-3,He-4)C-11, to impro ve specific activity of the radioactive oxygen and carbon recovered from th e target. For both reactions, specific activities were improved, with minim al decrease in recovered radioactivity. For the O-16-->O-15 reaction in wat er, results demonstrated that the production of [O-15]-O-2 followed a diffe rent reaction mechanism from that for unlabeled O-2. The unlabeled yield wa s quantitatively predicted from classical radiation chemistry G-values for water. The radiochemical product distribution was a consequence of the comb ined effects of recoil chemistry and radiation chemistry. Studies with thin graphite foils demonstrated that we could irradiate sufficiently thin C fo ils so that a useful fraction of the recoil nucleogenic C-11 atoms escaped the irradiated carbon and reacted with circulating gas to capture an apprec iable fraction of the product C-11 with an improvement in specific activity . Although we have shown the feasibility of producing GBq quantities of rad iopharmaceuticals by recoil techniques, the advantages of even higher speci fic activity using enriched targets outweigh the cost of using enriched tar gets.