Pure and O2-mixed nitrogen gases were irradiated by 18 MeV protons (14.2 MeV on target) and analyzed directly by a specially designed radio-GC system with a FID coupled to a methanyzer. Non-radioactive carbon, especially CO2 was observed to be generated in the target chamber by irradiation and to decrease gradually with repeated irradiation. The amount of carbon was increased by the use of new materials for the front foil, transport line, and so on. Higher irradiation dose also increased the carrier carbon, where beam current affected carbon generation more strongly than irradiation time. The addition of oxygen gas to nitrogen gas also increased the amount of carrier carbon, nearly all of which was observed to be generated in the chemical form of CO2. The ratio of [11C]CO2 to total C-11 was always low in a pure N2 gas target and increased with higher irradiation dose, addition of oxygen gas and by changing some part of the irradiation system. The ratios of [11C]CO2 were 22.4±1.5%, 77.8±7.1% and 94.0±4.0% in a pure N2 gas, 9.8 ppm O2-mixed N2 gas and 2000 ppm O2-mixed N2 gas, respectively. The specific activity of [11C]CO2 increased by the addition of O2 gas, but the effect of irradiation dose was not clear. Maximal specific activity of 610±150 GBq/μmol was obtained at the irradiation condition of 15 μA and 20 minutes in a 2000 ppm O2-mixed N2 gas. The effect of proton energy on the CO2 generation was also examined by degrading the proton energy with different thick energy absorbers. The amount of CO2 generated in the target depended on the proton energy, i.e., larger amount of CO2 generated with higher proton energy. Surprisingly, some amount of CO2 (1.5±0.3 nmol) was still observed in the target, even in the condition that all the protons stopped in the front foil and did not irradiate the target gas.
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