What Is Radioactive Iodine?

Radioactive iodine refers to the radioactive isotope of iodine, which is other known isotopes than iodine-127, mainly including iodine-129, iodine-131, iodine-123, iodine-124, and iodine-125 ^[1] . Radioactive iodine is widely used in medical diagnosis and treatment, detection of nuclear accidents, finding groundwater, measuring the flow rate and direction of groundwater, and finding underground pipeline leaks. At the same time, its radioactivity will cause harm to human beings, so protective measures need to be taken.

Iodine (I, atomic weight: 126.90447 (3)) has 37 known isotopes, of which only iodine-127 is a stable isotope and the others are radioactive, so iodine is a single isotope element. Naturally occurring iodine contains two isotopes, mainly iodine-127, and trace amounts of iodine-129.

All but iodine-127 are radioactive isotopes of iodine. Among the radioisotopes of iodine, the longest-lived is iodine-129, with a half-life of 15.7 million years, but it is still much lower than the native radioisotope. There are trace amounts of iodine-129 in nature, but iodine-129 generated based on cosmic rays is also very small. Most of the iodine-129 present on the earth is almost anthropogenic, mainly because iodine-129 is an unnecessary long-lived product of early nuclear tests and nuclear accidents. Except for iodine-129, the half-lives of the other radioactive isotopes of iodine are less than 60 days, of which four isotopes are present in

Iodine-129 is one of the radioactive isotopes of iodine, with a half-life of 15.7 million years. It will decay to xenon-129 through beta decay, which is an extinct natural radioisotope ^[3]

Iodine-131 ^[5]

Iodine-123 is a radioactive isotope of iodine. There are 70 neutrons in the nucleus, which is 4 fewer than the number of neutrons in the stable isotope of iodine. Gamma rays are emitted at the same time, which is a source of gamma radiation.

application

Iodine-123 is most suitable for the preparation of radiopharmaceuticals for nuclear medicine diagnosis. Iodine-123 radiopharmaceuticals can be used in thyroid function diagnosis, cerebral blood flow imaging, myocardial imaging, renal function diagnosis and adrenal imaging and various brain receptor imaging ^[8]

Iodine-124 is a radioactive isotope of iodine with 71 neutrons in the nucleus. The half-life is 4.17 days, and beta rays are emitted when iodine-124 decays into tellurium-124.

use

Iodine-124 is used in CT scans of iodine-containing contrast, which has less damage to patients; applied medical molecular imaging ^[9]

Iodine-125 is a radioactive isotope of iodine ^[5] . There are 72 neutrons in the nucleus, and the half-life is 60.14 days. Orbiting electrons capture decay nuclides and emit -ray energy of 0.03548Mev. 3.7 × 10 Beco iodine 125 weighs 5.76 × 10 grams. The exposure rate of a 3.7 × 10 Beco iodine-125 point source at a distance of 1 cm was 0.66 roentgen / hour. Iodine-125 belongs to the toxic nuclides. The most important organ is the thyroid gland. The effective half-life for humans is 41.7 days. Iodine-125 has the same chemical properties as elemental iodine.

Radioactive iodine production

The nuclear reactions that generate iodine 125 include Xe (n, ) XeI, Sb (, 2n) I, Te (p, n) I, Te (d, 2n) I, I (p, 3n) XeI, and the like.

Xe (n, ) XeI reaction has the most practical value. The preparation process is ^[1] :

Using natural xenon (Xe's abundance is 0.10%) gas as the target material, under the conditions of liquid nitrogen freezing, fill the xenon gas into a thick-walled zirconium-2 alloy or aluminum target tube, weld and seal it, and then send it to the reactor for irradiation. This produces iodine-125. Because the cross section of iodine-125 absorbs thermal neutrons is quite large, iodine-125 products always contain a certain amount of iodine-126. The decay type of iodine 126 is more complex and more energy-intensive than iodine-125, which affects both labeling and application. Therefore, the iodine 125 product is often left for a period of time (cooling) to decay the iodine 126, which is much shorter than the half-life of iodine 125, and the content of iodine 126 is generally limited to less than 2%. Iodine-125 with low iodine-126 content and high specific activity can be directly prepared by the reactor loop method. The method is to let xenon circulate inside and outside the reactor in a closed loop. When the gas flows through the active area inside the reactor and is irradiated by neutrons, iodine-125 is produced; when the iodine-125 follows the gas flow to the outside of the reactor, it is collected, and this part of the iodine-125 will no longer return to the internal activity The zone is irradiated with neutrons, so there is little chance of producing iodine 126, so the product is not cooled. Its iodine-126 content is also below 1%. After transferring iodine-125 from the target tube or closed circuit, it is usually made into NaI solution. NaI solution is the primary product of iodine-125.

Radioiodine application

Iodine-125 has a wide range of applications:

(1) Using its low-energy conversion electrons, autoradiography can be performed, such as thyroid tumor biopsy;

(2) Iodine 125 can emit single-energy photons (ie, low-energy gamma rays) with appropriate energy, which can be used to make a simple, high-accuracy, low-dose-rate accurate bone density measurement device;

(3) The low-energy photon source made of iodine 125 can also be used for X-ray fluorescence analysis to determine the content of many elements on the periodic table from arsenic to cadmium.

(4) However, the most widely used iodine 125 is as a labeling reagent to label a variety of compounds, especially preparations for in vitro radioimmunoassay.

(5) Used as a radioactive tracer.