Effect of dissolved gas on the specific activity of N-13 labeled ions generated in water by the 16O(p,α)13N reaction

A flow target system was constructed to study the chemical forms and specific activities of ions containing nitrogen-13 and non-radioactive nitrogen generated in water by the O-16(p,alpha)N-13 nuclear reaction. Ultra-pure water saturated with air, hydrogen, nitrogen, oxygen, and helium gases was irradiated with 18 MeV protons (17.4 MeV on target) accelerated with a cyclotron under a fixed flow of water. The irradiated water was collected in polypropylene sampling vessels at fixed time intervals, and ions containing radioactive and non-radioactive nitrogen were analyzed by radio-ion chromatography. While the concentrations of ions containing non-radioactive nitrogen (NRNI) was below the detection limit (similar to 100 nM) in unirradiated samples regardless of the dissolved gas, they were markedly higher in all irradiated samples. The total concentrations of NRNI in samples saturated with various dissolved gases were 1700nM (air), 900nM (hydrogen, oxygen, helium), and 3000 nM (nitrogen) at the beam current of 5 mu A. These results suggest that dissolved nitrogen gas is most closely related to the increase of NRNI. However, clear increases were observed also in the samples saturated with hydrogen, oxygen, and helium gases not containing nitrogen gas. From these observations, the increase of NRNI due to irradiation is considered to be primarily due to dissolved nitrogen gas but also partly due to other factors.