Радиоактивті ізotоптар және олардың қолданылуы

Chemical compound
A radioactive tracer, radiotracer, or radioactive label is a synthetic derivative of a natural compound in which one or more atoms have been replaced by a radionuclide (a radioactive atom). By virtue of its radioactive decay, it can be used to explore the mechanism of chemical reactions by tracing the path that the radioisotope follows from reactants to products. Radiolabeling or radiotracing is thus the radioactive form of isotopic labeling. In biological contexts, experiments that use radioisotope tracers are sometimes called radioisotope feeding experiments. Radioisotopes of hydrogen, carbon, phosphorus, sulfur, and iodine have been used extensively to trace the path of biochemical reactions. A radioactive tracer can also be used to track the distribution of a substance within a natural system such as a cell or tissue, or as a flow tracer to track fluid flow. Radioactive tracers are also used to determine the location of fractures created by hydraulic fracturing in natural gas production. Radioactive tracers form the basis of a variety of imaging systems, such as, PET scans, SPECT scans and technetium scans. Radiocarbon dating uses the naturally occurring carbon 14 isotope as an isotopic label.

11C decays by positron emission with a half life of ca. 20 min. 11C is one of the isotopes often used in positron emission tomography. 14C decays by beta decay, with a half life of 5730 years. It is continuously produced in the upper atmosphere of the earth, so it occurs at a trace level in the environment. However, it is not practical to use naturally occurring 14C for tracer studies. Instead it is made by neutron irradiation of the isotope 13C which occurs naturally in carbon at about the 1.1% level. 14C has been used extensively to trace the progress of organic molecules through metabolic pathways.

15O decays by positron emission with a half life of 122 sec. It is used in positron emission tomography.

18F decays predominantly by β emission, with a half life of 109.8 min. It is made by proton bombardment of 18O in a cyclotron or linear particle accelerator. It is an important isotope in the radiopharmaceutical industry. For example, it is used to make labeled fluorodeoxyglucose (FDG) for application in PET scans. 123I has also been produced at Oak Ridge National Laboratories by proton bombardment of 123Te. 123I decays by electron capture with a half life of 13.22 hours. The emitted 159 keV gamma ray is used in single photon emission computed tomography (SPECT). A 127 keV gamma ray is also emitted. 125I is frequently used in radioimmunoassays because of its relatively long half life (59 days) and ability to be detected with high sensitivity by gamma counters. 129I is present in the environment as a result of the testing of nuclear weapons in the atmosphere. It was also produced in the Chernobyl and Fukushima disasters. 129I decays with a half life of 15.7 million years, with low energy beta and gamma emissions. It is not used as a tracer, though its presence in living organisms, including human beings, can be characterized by measurement of the gamma rays.

Many other isotopes have been used in specialized radiopharmacological studies. The most widely used is 67Ga for gallium scans. 67Ga is used because, like 99mTc, it is a gamma ray emitter and various ligands can be attached to the Ga3+ ion, forming a coordination complex which may have selective affinity for particular sites in the human body. An extensive list of radioactive tracers used in hydraulic fracturing can be found below.

In metabolism research, tritium and 14C labeled glucose are commonly used in glucose clamps to measure rates of glucose uptake, fatty acid synthesis, and other metabolic processes. While radioactive tracers are sometimes still used in human studies, stable isotope tracers such as 13C are more commonly used in current human clamp studies. Radioactive tracers are also used to study lipoprotein metabolism in humans and experimental animals. In medicine, tracers are applied in a number of tests, such as 99mTc in autoradiography and nuclear medicine, including single photon emission computed tomography (SPECT), positron emission tomography (PET) and scintigraphy. The urea breath test for helicobacter pylori commonly used a dose of 14C labeled urea to detect h. pylori infection. If the labeled urea was metabolized by h. pylori in the stomach, the patient's breath would contain labeled carbon dioxide. In recent years, the use of substances enriched in the non radioactive isotope 13C has become the preferred method, avoiding patient exposure to radioactivity. In hydraulic fracturing, radioactive tracer isotopes are injected with hydraulic fracturing fluid to determine the injection profile and location of created fractures. According to the NRC, some of the most commonly used tracers include antimony 124, bromine 82, iodine 125, iodine 131, iridium 192, and scandium 46.