Medical and Scientific Applications of Radioisotopes

Radioisotopes (radioactive isotopes) are used as tracers to follow processes in the body, plants and industry, as sources of radiation to treat cancer and sterilize equipment, and as clocks for dating archaeological and geological samples; the choice of isotope depends on the type of radiation, the energy and above all the half-life. In brief: medical imaging isotopes (technetium-99m, fluorine-18) have short half-lives and emit gamma rays or positrons that pass out of the body; radiotherapy uses penetrating gamma rays (cobalt-60) or targeted alpha and beta emitters (iodine-131, radium-223); carbon-14 dates organic remains up to about 50,000 years, while uranium-lead and potassium-argon dating measures millions to billions of years.

Isotopes behave chemically like their stable counterparts, but their radiation can be detected, which allows them to be followed. This guide describes the applications of radioisotopes in medicine, industry, agriculture and science, explains how half-life determines the choice of isotope, compares the uses in a table, works through numerical examples and corrects common mistakes.

How are radioisotopes used in medicine?

• Diagnostic imaging: technetium-99m (half-life 6 hours, emits 140 keV gamma rays, no beta particles) is attached to compounds that go to specific organs (bone, heart, kidneys, thyroid, brain) and a gamma camera forms an image (SPECT). PET scans use positron emitters, such as fluorine-18 (half-life 110 minutes) in fluorodeoxyglucose, a glucose analog taken up by active tissues (tumors, brain): each positron annihilates with an electron, producing two 511 keV gamma rays in opposite directions that are detected in coincidence.
• Radiotherapy: external beams of gamma rays from cobalt-60 (half-life 5.27 years, 1.17 and 1.33 MeV), or X-rays/electrons from accelerators, are focused on the tumor from several directions to give a high dose to it and a low dose to the healthy tissue. Internal therapy: iodine-131 (half-life 8 days; beta emitter) accumulates in the thyroid and destroys overactive or cancerous cells; radioactive seeds (iodine-125, palladium-103) are implanted in the prostate; radium-223 (alpha emitter) is used for bone metastases.
• Sterilization: gamma rays from cobalt-60 sterilize syringes, dressings and surgical equipment in sealed packaging.
• Tracers to study blood flow, kidney function, or the absorption of drugs and iron (iron-59).

Key requirements for medical isotopes: a half-life long enough to prepare and use it but short enough to limit the dose (hours to days), a suitable radiation (gamma or positron for imaging; beta/alpha for therapy), and a biologically useful chemical form. Short-lived isotopes are produced close to the hospital in a cyclotron or a generator (a technetium generator contains molybdenum-99, half-life 66 hours, which decays to technetium-99m).

How are they used in industry and agriculture?

• Thickness and level gauges: a beta source on one side of a moving sheet of paper, plastic or metal, and a detector on the other. The count rate is related to the thickness, and the system adjusts the rollers. Gamma gauges measure the level of liquid in sealed containers.
• Smoke detectors: a small americium-241 source (alpha emitter, half-life 432 years) ionizes the air between two electrodes, giving a small current. Smoke particles reduce the current and trigger the alarm.
• Leak detection and flow tracing: a short-lived gamma emitter is added to a fluid in a pipeline, and detectors outside the pipe locate the leaks.
• Industrial radiography: gamma rays (iridium-192, cobalt-60) detect cracks in welds and castings.
• Agriculture and food: tracers (phosphorus-32) show how plants take up fertilizer; irradiation of food with gamma rays kills bacteria and delays spoilage; sterile insect techniques use irradiated male insects to control pests.
• Power sources: plutonium-238 (half-life 87.7 years) provides heat and electricity for spacecraft such as Voyager and the Mars rovers.

How are they used for dating?

Radiocarbon dating: cosmic rays produce carbon-14 in the upper atmosphere, which becomes part of carbon dioxide and enters living things; the ratio of carbon-14 to carbon-12 in living organisms is constant (about 1 in 10¹²). After death, no more carbon-14 is taken in, and it decays with a half-life of 5,730 years. Measuring the remaining fraction gives the age, up to about 50,000 years. Calibration against tree rings corrects for variations in atmospheric carbon-14. Radiometric dating of rocks: uranium-238 (half-life 4.47 billion years) to lead-206, potassium-40 (1.25 billion years) to argon-40, and rubidium-87 to strontium-87 determine the ages of rocks, fossils (by dating volcanic layers) and meteorites; the age of the Earth is about 4.54 billion years.

Applications compared

Worked example: dating, doses and gauges

1. Radiocarbon age. A wooden artifact has 60 percent of the carbon-14 activity of living wood. t = T_½ × log₂(1 ÷ 0.60) = 5,730 × 0.737 = 4,220 years. A bone with 12.5 percent remaining is 3 half-lives, or 17,190 years, old.
2. Medical dose decay. A hospital receives 600 MBq of technetium-99m at 8:00. By 14:00 (6 hours, one half-life), the activity is 300 MBq; by 20:00, 150 MBq. The patient receives a dose scheduled by the activity at the time of the injection.

3. Waste decay. Fluorine-18 (110 minutes) from a PET scan: after 11 hours (6 half-lives), the activity is down to (1/2)⁶ = 1/64, about 1.6 percent, so samples are stored for a day before disposal.
4. Thickness gauge. Radiation passes through a sheet with an intensity I = I₀e^−μx. For μ = 0.5 mm⁻¹ and a thickness of 2.0 mm: I ÷ I₀ = e^−1.0 = 0.37. If the thickness increases by 0.1 mm to 2.1 mm: e^−1.05 = 0.35, a change of 5 percent in the count rate, which a detector can sense.
5. Rock dating. A rock contains uranium-238 and lead-206 (from decay) in a ratio of 1 : 1 by number of atoms; therefore, the original number of uranium atoms is twice the number now remaining, i.e. exactly one half-life has passed: the age is 4.47 billion years (assuming no initial lead).
6. Smoke detector. An americium-241 source has an activity of 37 kBq. Its half-life of 432 years means the activity falls only by 1.6 percent in 10 years (e^{−0.693 × 10/432} = 0.984), so the detector works for decades.

How did these applications start?

George de Hevesy used radioactive lead as a tracer in the 1910s and received the Nobel Prize in Chemistry in 1943. Ernest Lawrence’s cyclotron made artificial radioisotopes available in the 1930s, and the reactors of the 1940s and 1950s produced them in quantity. Cobalt-60 therapy began in the 1950s. Willard Libby published radiocarbon dating in 1949. Technetium-99m was introduced in nuclear medicine in the 1960s by Powell Richards and Walter Tucker. PET scanners were developed in the 1970s by Michael Phelps and Edward Hoffman.

Which mistakes are common?

Mistake 1: Choosing a long half-life for medical imaging

A long half-life would deliver an unnecessarily high dose over a long time. Short half-lives are preferred.

Mistake 2: Using carbon-14 to date very old rocks

Its half-life is too short. After about 10 half-lives (57,000 years), too little remains. Use isotopes with half-lives of millions to billions of years.

Mistake 3: Thinking that irradiated food becomes radioactive

Gamma rays and electron beams used for food irradiation do not make the food radioactive.

Mistake 4: Assuming that all radioisotopes in medicine are dangerous to the patient’s surroundings

The isotopes and doses used in diagnostics are chosen to minimize exposure. Some therapies need precautions for a few days.