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

An improved aluminium target system for production of elemental fluorine vi a the O-18(p,n)F-18 reaction using a two-step irradiation protocol is descr ibed. In the first step highly enriched gaseous oxygen-18 is irradiated wit h protons to form fluorine-18 which gets deposited on the inner target surf ace. In the second step, after cryogenic recovery of oxygen-18 target gas, a mixture of elemental 'cold' fluorine and krypton is introduced and a shor t proton irradiation is done, whereby an isotopic exchange between the gase ous fluorine and the deposited radiofluorine occurs. The second step leads to the recovery of the radiofluorine as [F-18]fluorine. Optimisation studie s were performed regarding the yield and specific radioactivity of [F-18]fl uorine. Furthermore, some irradiation parameters relevant to the recovery s tep 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 inn er target surface is inhomogeneous, probably due to convection effects. Ext ensive radioanalytical techniques were applied to characterise the reactivi ty of [F-18]fluorine and to identify undesired nonreactive F-18-compounds, mainly [F-18]tetrafluoromethane and [F-18]nitrogentrifluoride. The [F-18]fl uorine produced in the system used has the distinction of having a negligib le 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 Sci ence Ltd. All rights reserved.