Synthetic radioisotope

A synthetic radioisotope is a radionuclide that is not found in nature: no natural process or mechanism exists which produces it, or it is so unstable that it decays away in a very short period of time.^[1] Most known radioisotopes are synthetically made; only 84 out of over 3,000 radioisotopes are found in nature.^[2]

The first synthetic radioisotope was phosphorus-30, which was produced in 1934 by Frédéric Joliot-Curie and Irène Joliot-Curie using aluminum foil and a polonium source .^{[3] [4]} The two won the 1935 Nobel Prize in chemistry for their discovery.^[5] The discovery of artificial radioactivity enabled the development of nuclear weapons based on plutonium-239, including the Fat Man atomic bomb.^[6]

In the modern day, synthetic radioisotopes have many other applications. They are used in medical imaging (such as technetium-99m),^[7] radiotherapy, nuclear energy sources (plutonium-239), and ionization-type smoke detectors (americium-241). These synthetic radioisotopes are manufactured in nuclear reactors using neutron irradiation,^[8] and in particle accelerators using charged particles.^[9][10]

Production

Some synthetic radioisotopes are extracted from spent nuclear reactor fuel rods, which contain various fission products. For example, it is estimated that up to 1994, about 49,000 terabecquerels^{[clarification needed]} (78 metric tons) of technetium were produced in nuclear reactors; as such, anthropogenic technetium is far more abundant than technetium from natural radioactivity.^[11]

Some synthetic isotopes are produced in significant quantities by fission but are not yet being reclaimed. Other isotopes are manufactured by neutron irradiation of parent isotopes in a nuclear reactor (for example, technetium-97 can be made by neutron irradiation of ruthenium-96) or by bombarding parent isotopes with high energy particles from a particle accelerator.^[12][13]

Many isotopes, including radiopharmaceuticals, are produced in cyclotrons. For example, the synthetic fluorine-18 and oxygen-15 are widely used in positron emission tomography.^[14]

Uses

Most synthetic radioisotopes have a short half-life. Though a health hazard, radioactive materials have many medical and industrial uses.

Nuclear medicine

The field of nuclear medicine covers use of radioisotopes for diagnosis or treatment.

Diagnosis

Radioactive tracer compounds, radiopharmaceuticals, are used to observe the function of various organs and body systems. These compounds use a chemical tracer which is attracted to or concentrated by the activity which is being studied. That chemical tracer incorporates a short lived radioactive isotope, usually one which emits a gamma ray which is energetic enough to travel through the body and be captured outside by a gamma camera to map the concentrations. Gamma cameras and other similar detectors are highly efficient, and the tracer compounds are generally very effective at concentrating at the areas of interest, so the total amounts of radioactive material needed are very small.

The metastable nuclear isomer technetium-99m is a gamma-ray emitter widely used for medical diagnostics because it has a short half-life of 6 hours, but can be easily made in the hospital using a technetium-99m generator. Weekly global demand for the parent isotope molybdenum-99 was 440 TBq (12,000 Ci) in 2010, overwhelmingly provided by fission of uranium-235.^[15]

Treatment

Several radioisotopes and compounds are used for medical treatment, usually by bringing the radioactive isotope to a high concentration in the body near a particular organ. For example, iodine-131 is used for treating some disorders and tumors of the thyroid gland.

Industrial radiation sources

Alpha particle, beta particle, and gamma ray radioactive emissions are industrially useful. Most sources of these are synthetic radioisotopes. Areas of use include the petroleum industry, industrial radiography, homeland security, process control, food irradiation and underground detection.^[16][17][18]