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Production of Carrier-Free ¹²³I Using the ¹²⁷I(p,5n)¹²³Xe Reaction

University of California at Davis, Davis, California

Correspondence: For reprints contact: Neal Peek, Crocker Nuclear Laboratory, University of California, Davis, Calif. 95616.

ABSTRACT

A cyclotron procedure for the production of high-purity, carrier-free ¹²³I suitable for immediate radio-pharmaceutical use has been developed. Proton irradiation of natural elemental iodine results in the production of ¹²³Xe which subsequently decays to ¹²³I. The primary reaction for 57-MeV protons is ¹²⁷I(p,5n)¹²³Xe; other reactions yield ¹²⁵Xe and the neutron-deficient radioisotopes of iodine. At the end of bombardment, the target iodine is dissolved in aqueous KI, and helium gas is bubbled through the solution, sweeping the xenon through a recirculating system. The xenon gas is collected in a liquid nitrogen-cooled trap. The isolated xenon is contained for 6 hr during which time the ¹²³Xe (T_1/2, 2.1 hr) decays to ¹²³I. The ¹²³I may be recovered by adding a small volume of physiological saline.

An irradiation on the Crocker Nuclear Laboratory isochronous cyclotron of 1 hr, using a 1 gm/cm² thickness of iodine (7-MeV proton energy loss in the target) and 57.5-MeV protons at 6 µA, resulted in 24 isolable mCi of ¹²³I. The only detectable contaminant in the final product was ¹²⁵I which was about 0.1% of the ¹²³I activity.

A reasonable extrapolation of our irradiation experience indicates that a yield of 89 mCi can be obtained in practice. This production figure is based on a target thickness of 1 gm/cm² and an irradiation time of 3 hr at 10 µA.

The advantages of this method over existing methods are (A) readily available, inexpensive, isotopically pure targets; (B) multimillicurie yields of carrier-free ¹²³I, and (C) significantly reduced radio-nuclidic contamination. When used in vivo the radiation dose to the patient is reduced and optimal spatial resolution is possible for imaging procedures. This method, however, requires 50–60-MeV protons which are not universally available.