Uses of radioactivity

Key ideas

Radioactive isotopes have useful applications because of the specific properties of the radiation they emit — its penetrating power, ionising ability, and predictable half-life.

Medical uses

• Medical tracers — a short-half-life, gamma-emitting isotope is attached to a chemical the body absorbs (e.g. taken up by a particular organ). A gamma camera detects where it collects, showing how an organ is functioning.
  • Gamma is used because it penetrates the body to be detected externally, without needing to be surgically removed, and does not ionise tissue as heavily as alpha or beta on its way out.
  • A short half-life is chosen so the isotope decays away quickly, minimising the radiation dose the patient receives.
• Radiotherapy (cancer treatment) — a source is used to direct radiation at a tumour to kill cancerous cells. The ionising property of the radiation, which is harmful in general, is used deliberately here to damage the DNA of the target cells.

Industrial and household uses

• Smoke detectors — a weak alpha source ionises the air inside a small chamber, allowing a steady current to flow. Smoke particles entering the chamber absorb the alpha radiation, reducing the ionisation and the current, which triggers the alarm.
  • Alpha is used because it is easily absorbed by smoke particles (and is not dangerous outside the sealed unit, since its short range means it cannot escape the detector casing).
• Thickness gauging — a beta source is placed on one side of a manufactured sheet (e.g. paper or metal) with a detector on the other side. The amount of radiation getting through depends on the sheet's thickness, so the reading can control rollers to keep the thickness constant.
  • Beta is chosen because its penetrating power is sensitive to small changes in thickness over the typical range of the material — alpha would be blocked completely and gamma would barely be affected.

Dating and tracing

• Carbon dating — living organisms constantly exchange carbon with the environment, keeping a steady proportion of radioactive carbon-14 to stable carbon-12. Once an organism dies, carbon-14 decays with its known half-life (about $5{,}730$ years) and is not replaced.
  • By measuring the remaining proportion of carbon-14 in a sample (e.g. from an archaeological find), scientists calculate how many half-lives have passed and estimate its age.
• Radioactive tracers are also used in industry and agriculture — for example, tracking the movement of fertiliser uptake in plants, or detecting leaks in underground pipelines, by following the path of a radioactive isotope added to the system.

Practice question

Explain why an alpha-emitting source, rather than a beta or gamma source, is used inside a household smoke detector.

Worked solution: alpha radiation is easily absorbed — it ionises the air inside the chamber to create a small current, and this ionisation is easily blocked by smoke particles when they enter, changing the current and triggering the alarm. Its very short range also means it poses little risk once sealed inside the unit.