Patent Number: 056339003
Section: summary

FIELD OF INVENTION The present invention relates to the production of radioactive iodine and, in particular, to a novel procedure and apparatus for effecting the same on a large scale in safety. BACKGROUND TO THE INVENTION Iodine-125 (.sup.125 I) is a radioactive isotope of iodine with a relatively long half-life of 60 days. This material is used for medical diagnostic studies and for medical and biological research. This iodine isotope is valuable because the radiation it emits is less damaging than that from other isotopes of iodine. It is known to produce such material by irradiating .sup.124 Xe with thermal neutrons, according to the following scheme: ##STR1## .sup.125 I decays to form .sup.125 Te or may be converted to .sup.126 I which decays to .sup.126 Te, as follows: ##STR2## Supplies of .sup.125 I isotope are limited and there is an increasing demand for this material. Iodine-126 that is present with .sup.125 I is a contaminant. Because of the emission of more damaging radiation by .sup.126 I, the Food and Drug Administration, U.S.A., requires that .sup.125 I for use in the human body contains less than 5 parts per million of .sup.126 I. SUMMARY OF INVENTION The present invention provides a novel method and apparatus for the production of .sup.125 I, which is amenable to large-scale production. The procedure is effected on a batch basis with .sup.124 Xe gas being irradiated periodically with a neutron flux over a period of time and permitting .sup.125 Xe so provided to be transferred remotely and in safety to a different portion of the apparatus, where the .sup.125 Xe decays to form .sup.125 I. For example, for a one-week cycle, approximately 5 g of .sup.124 Xe gas is irradiated for up to about 15 hours a day for three to five days in a flux of approximately 5.times.10.sup.12 neutrons cm.sup.-2 s.sup.-1, to produce about 0.3 TBq (8 Ci) of .sup.125 I which is free from .sup.126 I. The quantity of .sup.125 I can be increased by irradiating larger amounts of .sup.124 Xe or by locating the apparatus in a higher flux. The upper limit of production of .sup.125 I using the batch procedure of the present invention is about 0.74 TBq (20 Ci) of .sup.125 I per batch, by employing a suitable combination of target amount, neutron flux and irradiation time. Limits of the individual parameters of the process are irradiating up to 6 g of .sup.125 Xe, using fluxes of up to 2.times.10.sup.13 neutrons cm.sup.-2 s.sup.-1 and irradiating for up to five 15-hour days. In one aspect, the present invention provides a method of producing radioactive .sup.125 I, which comprises feeding .sup.124 Xe from a source thereof to an irradiation zone located within an enclosure, irradiating the .sup.124 Xe in the enclosure with neutrons to cause the formation of .sup.125 Xe therefrom, transferring irradiated gas from the irradiation zone to a decay zone within the enclosure and free from neutron flux, and permitting .sup.125 Xe to decay to form .sup.125 I in the decay zone. The location of the decay zone free from neutron flux ensures that the .sup.125 I is produced free from .sup.126 I. The invention also includes an apparatus for producing radioactive .sup.125 I comprising a housing which is gas-tight and submersible in a nuclear reactor water pool and defining an interior chamber, the housing having upper and lower separable portions to permit access to the interior chamber. A first enclosure is provided within the chamber and is arranged to permit neutron irradiation of .sup.124 Xe contained therein by the nuclear reactor. A second removable enclosure is provided within the chamber and is connected in interruptible fluid flow relationship with the first enclosure for transfer of irradiated xenon gas from the first enclosure to the second enclosure to permit decay of .sup.125 Xe to .sup.125 I in the second enclosure free from neutron flux. The second enclosure has valved inlet/outlet port means to permit .sup.124 Xe to be received into the apparatus, to permit .sup.125 I solution to be discharged from the second enclosure, and to permit the passage of xenon gas between the first and second chambers. First pump means is operably connected to the first enclosure for precipitating .sup.124 Xe received into apparatus through the valved port means when the first and second enclosures are in fluid flow relationship and for providing gaseous xenon in the first enclosure when the first and second enclosures are out of fluid flow relationship. Second pump means is operably connected to the second enclosure for precipitating irradiated xenon received from the first enclosure when the first and second enclosures are in fluid flow relationship and for providing gaseous irradiated xenon in the second enclosure when the first and second enclosures are out of fluid flow relationship.