Field of the Invention
The present invention relates to a method of producing radioisotopes used in the field of nuclear medicine, in particular, to a method of removing radioactive europium from solutions of radioactive gadolinium.
Description of Background Information
Radioactive gadolinium (hereinafter abbreviated as .sup.153 Gd) is used as a source of radiation in the field of nuclear medicine for the specific purpose of diagnosing osteoporosis and is commonly produced by irradiating europium with neutrons in nuclear reactors. The produced .sup.153 Gd is chemically separated from other radioactive nuclear species such as .sup.152 Eu, .sup.154 Eu and .sup.156 Eu which occur simultaneously during irradiation with neutrons.
Diagnosis of osteoporosis makes use of the phenomenon that two photons having different energies of 44 keV and 100 keV are liberated from .sup.153 Gd. Since .sup.153 Gd used for this purpose is desirably free of other radioactive nuclear species, it must be purified to a level of at least 99.999%. The method currently practiced at the Oak Ridge National Laboratory to purify gadolinium consists of the following steps: dissolving neutron-irradiated europium in sulfuric acid; reducing the concentration of Eu to about 5.5 mg/ml; reducing Eu.sup.3+ to Eu.sup.2+ by electrolytic reduction; preliminarily separating the radioactive europium by filtration to a decontamination factor of 100; and finely separating the same by means of a cation-exchange resin column. Separation could be accomplished by using a cation-exchange resin alone but when handling a large volume of radioactive europium, the ion-exchange capacity of the resin will decrease by radiation injury. It is therefore necessary to perform preliminary separation of radioactive europium. Thus, in the case of handling radioactive europium in a large volume, the conventional practice has required the adoption of two steps, one being preliminary separation of radioactive europium by electrolytic reduction and the other being purification on a cation-exchange resin column. The decontamination factor of radioactive europium as attained by electrolytic reduction, namely, the ratio of the initial concentration of europium to the europium level after preliminary separation, depends on the solubilities of Eu.sup.3+ and Eu.sup.2+ and would theoretically reach a maximum value at the ratio of the solubility of Eu.sup.3+ to that of Eu.sup.2+, which is estimated to be approximately 200. The Oak Ridge method of electrolytic reduction for preliminary separation employs an apparatus that is chiefly composed of an electrolytic cell with zinc electrodes, a constant current supply unit and a polarity changing unit. The radioactive europium preliminarily separated with this apparatus is subsequently subjected to further purification with a cation-exchange resin. FIG. 1 is a schematic representation of this apparatus of electrolytic reduction.