Physicochemical and radiochemical aspects of separation of radioiodine from TeO2-targets

The radio-isotopes of iodine with mass-numbers 120, 123, and 124 are available via proton or deuteron induced reaction on enriched TeO2 targets with medical PET cyclotrons. The radioiodine can easily be separated from irradiated TeO2 targets using a dry thermo-chromatographic process. The half-time for the release of radioiodine from molten TeO2 samples as a function of the sample geometry and temperature has been studied systematically. Only 2 minutes are needed to release the radioiodine with >95% yield. Another 10 minutes are needed to trap the released iodine from the gas phase. An Al2O3 filter can be used to trap the released iodine intermediately and to clean the iodine from traces of evaporated TeO2. The activation energy for the release process has been determined to be Q(A) = (170 +/- 30) kJ/mol which indicates, that convection contributes significantly to the release. For the adsorption-desorption process of iodine from Al2O3 the activation energy was measured to be Q(ad) = (90 +/- 14) kJ/mol. The desorption half-time was <5 sec for T = 800 degrees C. Dry Al2O3 columns can be used as a thermochromatographic Te-132-I-132 generator. The released iodine exhibits electrophilic properties, suitable for labelling procedures directly from the gas phase. Most likely the iodine is transported in the gas phase in the form of free radicals I*.