Formation of F18-Labeled Fluoro-Organic Compounds by the F19(n, 2n) Reaction

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Abstract
Fluoro-organic compounds were subject to irradiation by fast neutrons produced in a nuclear reactor; fluoroderivatives labeled with F18 were produced and their yield was determined. It was found that labeling yields of fluoro-organic compounds were substantially higher than those of other organic compounds exposed to fluorine 18 produced extramolecularily, either by the O16(H3, n) or by the F19(n, 2n) reactions. The extent of substitution of a hydrogen atom by radiofluorine was found more than 100 times higher than that of a fluorine atom. It was shown that the labeling occurs mainly in the same part of the molecule which carried the original fluorine atom. In some cases it was shown that it is the same atom which is transformed to a radionuclide. When fluoroorganic compounds were irradiated in solution, a constant molecular labeling yield was observed. This yield which was obtained in dilute solutions in water, acetic acid or ethanol, was found independent of solute concentration; moreover, it was not affected by the presence of scavengers. The labeling yield of the aliphatic solvents was lower when exposed to fluorine 18 produced from an organically bound fluorine than from a fluoride ion. It is difficult to interpret the experimental results without assuming that a genuine retention of the transformed fluorine 18 in the parent molecule takes place. It is suggested that the (n, 2n) reaction proceeds, in part at least, as a spallation-type reaction, forming an excited F18 nucleus which does not undergo any recoil. The excited F18 is subsequently converted to the ground state following gamma emission; the recoil energy of many of these gammas is not sufficient to cause an irreversible cleavage of the C–F bond.