- Explain radioactive tracers and common diagnostic, treatment, and industrial applications
- Choose suitable isotope chemistry, emission type, and half-life
Core Concepts & Principles
A radioactive tracer (radioactive label) replaces a stable atom with a detectable radioisotope. Isotopes of the same element have nearly the same chemical behavior, so the label follows the substance through a reaction, body, or machine. Choose a suitable chemical form, emission type, and half-life.
Diagnosis and Biological Research
- Radioimmunoassay (RIA): Radioactive labels detect tiny amounts of antigens or hormones; applications include hormone measurements, drug testing, and blood screening.
- Tc-99m: Gamma-emitting compounds image heart, liver, and lung tissue. Its 6.01-hour half-life favors local generation from Mo-99 (66-hour half-life). Column chromatography retains molybdate, MoO₄²⁻, on alumina while pertechnetate, TcO₄⁻, is collected. The source sometimes says Tc-99; the short-lived imaging form is Tc-99m.
- Tl-201: Concentrates in healthy heart tissue and is used in cardiac imaging. I-131: Concentrates in the thyroid and can trace or treat thyroid conditions. The source also describes iodine compounds for liver and brain tumors; uptake depends on the compound. Na-24: Traces blood circulation and flow obstructions.
- C-14: Labeling CO₂ and testing plant products at timed intervals revealed the photosynthesis pathway. Tracers also follow fertilizer uptake, digestion, and metabolism.
- PET: Positrons annihilate with electrons, producing two gamma photons traveling in opposite directions; detecting them locates the tracer.
Treatment and Commercial Uses
Radiation therapy damages cancer-cell DNA. External beam radiation therapy uses a source outside the body (for example, Co-60 gamma rays); internal radiation therapy, or brachytherapy, places a radioactive source inside. Chemotherapy uses drugs instead of radioactive emissions.
Industrial uses include tracing leaks and engine wear, studying oil-well formations, measuring sheet thickness by transmission, inspecting metal flaws with gamma rays, sterilizing male insect pests, and killing food-spoilage organisms.
Am-241 smoke detectors: Alpha particles ionize air between electrodes, allowing a small current. Smoke impedes ion movement, so the current drops and triggers the alarm. Am-241's half-life is about 432 years; the source's 458-year value is outdated.
The isotope's chemistry determines where it goes; its emissions make it detectable. The half-life must allow the procedure while limiting unwanted remaining activity.
Problem-Solving Routines & Methods
Neutron activation produces Co-60; its beta decay and subsequent gamma emissions provide a treatment source.
- 1Choose a chemical form that follows the desired substance or targets the intended tissue.
- 2Choose detectable emissions for imaging, suitable radiation for treatment, or an ionizing source for a detector.
- 3Match half-life to the task: hours for many diagnostic tracers, much longer for a smoke-detector source.