Improved target system for production of high purity [18F]fluorine via the 18O(p,n)18F reaction

An improved aluminium target system for production of elemental fluorine via the O-18(p,n)F-18 reaction using a two-step irradiation protocol is described. In the first step highly enriched gaseous oxygen-18 is irradiated with protons to form fluorine-18 which gets deposited on the inner target surface. In the second step, after cryogenic recovery of oxygen-18 target gas, a mixture of elemental 'cold' fluorine and krypton is introduced and a short proton irradiation is done, whereby an isotopic exchange between the gaseous fluorine and the deposited radiofluorine occurs. The second step leads to the recovery of the radiofluorine as [F-18]fluorine. Optimisation studies were performed regarding the yield and specific radioactivity of [F-18]fluorine. Furthermore, some irradiation parameters relevant to the recovery step were investigated. It was found that a 15 to 20 min irradiation with a beam current of 20 CIA is sufficient for the isotopic exchange between the fluorine-carrier and the F-18-radioactivity deposited on the inner wall of the target. The distribution of the F-18-radioactivity deposited on the inner target surface is inhomogeneous, probably due to convection effects. Extensive radioanalytical techniques were applied to characterise the reactivity of [F-18]fluorine and to identify undesired nonreactive F-18-compounds, mainly [F-18]tetrafluoromethane and [F-18]nitrogentrifluoride. The [F-18]fluorine produced in the system used has the distinction of having a negligible contamination from those inert F-18-compounds. This is a combined effect of the use of highest purity gases and a welded target construction, which avoids any contact of the gases with organic material during irradiations. The target has proved to be very reliable for production of [F-18]fluorine in high yields of up to 34 GBq and specific activities of 350-600 GBq/mmol, both at 30 min after end of activation bombardment. (C) 2000 Elsevier Science Ltd. All rights reserved.