Mechanism of nitrogen-13-labeled ammonia formation in a cryogenic water target

Methods for producing N-13 ammonia via the ¹⁶O(p,α)¹³N nuclear reaction utilizing a cryogenic target have been investigated. These targets included frozen carbon dioxide and pure frozen water. Results from these targets were compared with the more traditional liquid water target with and without additives. A very dramatic difference was found between the pure water target in the frozen state when compared with the liquid state. When frozen, more than 95% of the nitrogen-13 activity is in the chemical form of ammonia at all radiation doses. In contrast, the liquid water target yielded predominately nitrates and nitrites at high radiation doses. When frozen carbon dioxide was irradiated under these conditions, more than 95% of the nitrogen-13 activity was in the form of nitrate and nitrite. The nitrogen oxides remained on the surface of the target and could be easily removed from the surface with pure water. The wash solution was converted to [¹³N]ammonia using the DeVarda's alloy method for reduction. It was determined that levels of [¹³N]ammonia sufficient for diagnostic medical procedures could be produced directly using the frozen water targets or from frozen carbon dioxide with a wet chemical reduction. These results have significance particularly in the design of targetry for low-energy, high- beam current accelerators, because targets of this design can be used with either no vacuum isolation window or a very thin window. The substitution of carbon-13-enriched carbon dioxide for natural carbon dioxide gives access to the ¹³C(p,n)¹³N nuclear reaction, which allows protons energies as low as 6 MeV to be used to produce useable quantities of N-13 ammonia. The mechanism of these reactions has been explored to determine why there are such dramatic differences in the product distribution between the frozen state and the liquid or gaseous state.