Overview
Radioactivity is the process in which unstable atomic nuclei lose energy by emitting particles or electromagnetic radiation. These radiations ionize atoms and molecules along their path and can cause biological harm, including cancer, making them an important health and safety concern in the laboratory.
Understanding atomic structure, the types of radioactive decay, half-life, and the units used to quantify radioactivity is essential background for techniques such as radioimmunoassay (RIA) that historically underpinned many hormone and biomarker assays.
Learning Objectives
After this lesson you will be able to…- Describe the basic structure of the atom and the concept of isotopes.
- Explain radioactive decay and the factors that determine atomic stability.
- Describe the units of radioactivity — Becquerel, Curie, specific activity and counts per minute.
- Explain the characteristics of alpha, beta (negatron/positron) and gamma emissions.
- Calculate half-life and apply the radioactive decay equation.
Clinical Story
Why This MattersA patient is scheduled for a PET (Positron Emission Tomography) brain scan to evaluate seizure activity. The radiology and laboratory teams must understand positron emission, its back-to-back gamma ray production, and radiation safety principles to prepare, administer, and interpret the isotope-based study correctly and safely.
Core Concepts
An atom consists of a positively charged nucleus (protons + neutrons) surrounded by negatively charged electrons. Atomic number (Z) = number of protons = number of electrons; it determines chemical properties. Mass number (A) = protons + neutrons. Isotopes are atoms of the same element (same Z) with different mass numbers (different neutron count), e.g. hydrogen's three isotopes ¹H, ²H, ³H.
Atomic stability depends on the neutron-to-proton ratio (N:P). Lower atomic number elements have N:P ≈ 1; higher atomic number elements need N:P > 1. When this ratio is disturbed, the atom becomes unstable and emits particles or radiation to regain stability — this process is radioactive decay.
Negatron emission: a neutron converts to a proton, ejecting a negatron (β−); N:P decreases, Z increases by 1 (e.g. ¹⁴C). Positron emission: a proton converts to a neutron, ejecting a positron (β+), which annihilates with an electron to produce two back-to-back gamma rays — the basis of PET scanning (e.g. ²²Na). Alpha emission: loss of a helium nucleus (2 protons + 2 neutrons); Z decreases by 2, A decreases by 4 — seen in high atomic number elements, rarely used biologically due to toxicity. Gamma emission: electromagnetic radiation accompanying alpha/beta decay, with no change in Z or A. Electron capture: a proton captures an inner-shell electron, becoming a neutron and emitting X-rays.
Radioactive decay is exponential: dN/dt = −λN, giving Ln(Nt/N0) = −λt, where λ is the decay constant specific to each isotope. Half-life (t½) is the time for the amount of radioactive material to fall to half its original value: t½ = 0.693/λ. Half-lives vary enormously — from 10¹⁹ years for lead-204 to 3×10⁻⁷ seconds for polonium-212. Biologically relevant isotopes: ³H (12.26 years), ¹⁴C (5760 years), ³²P (14.20 days), ³⁵S (87.20 days), ¹²⁵I (60 days).
Becquerel (Bq) is the SI unit, defined as 1 disintegration per second. Curie (Ci) is the traditional unit, defined as the disintegration rate of 1g of radium (3.7×10¹⁰ dps, or 37 GBq). Specific activity is activity per unit weight or volume. Counts per minute (cpm) is the disintegration rate actually detected by a radiation counter, which is always less than true dps due to counter efficiency.
Laboratory Principle
Radioactive isotopes emit alpha particles, negatrons, positrons, gamma rays, or X-rays, all of which cause excitation or ionization of nearby atoms as they pass through matter. Detectors (such as scintillation or Geiger-Müller counters) register these ionization/excitation events as counts, which are then converted to activity units (Bq, Ci, or cpm) to quantify the isotope present — historically the basis for radioimmunoassay (RIA) and continues to underpin nuclear medicine imaging such as PET.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| ³H / ¹⁴C labelled compounds | Radiolabelled tracer, research grade | Labelling organic compounds for tracer studies | Shielded, per radiation safety protocol |
| ³⁵S-methionine | Radiolabelled amino acid | Protein labelling studies | Shielded, -20°C |
| ³²P (nucleic acid label) | Radiolabelled phosphate | Nucleic acid labelling | Shielded, short half-life — use promptly |
| ¹²⁵I | Radiolabelled iodine | Immunoassay tracer (historical RIA use) | Shielded, 2–8°C |
Step-by-Step Procedure
Determine which radioisotope is being used and its predominant mode of decay (negatron, positron, alpha, gamma, or electron capture).
Use appropriate shielding, personal dosimeters, and time/distance precautions based on the type and energy of the emission.
Use a scintillation or Geiger-Müller counter to detect disintegrations and record counts per minute (cpm).
Convert measured cpm to activity units (Bq or Ci), correcting for counter efficiency and background radiation.
Apply the decay equation Ln(Nt/N0) = −λt or t½ = 0.693/λ to predict remaining activity at a future time point, important for dosing and disposal planning.
Flow Diagram
Quality Control
Calibrate counters regularly using certified reference sources of known activity. Monitor and subtract background radiation counts from every reading. Maintain a radiation exposure log and dosimeter checks for all personnel handling isotopes.
Comply with national radiation safety authority inspections and proficiency testing for radioisotope handling, storage, and waste disposal to ensure regulatory compliance.
Reference Values
Key Constants & Half-Lives⚠️ These are physical/technical constants, not patient reference ranges.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| High background count rate | Contamination of work area or detector malfunction | Decontaminate area; recalibrate detector |
| Rapid decline in counts over short period | Isotope with very short half-life (e.g. ³²P) approaching depletion | Plan experiments/administration around remaining half-life |
| Personnel dosimeter exceeding threshold | Excess radiation exposure risk | Investigate exposure source; review shielding and handling protocols |
Common Errors & How to Avoid Them
Cause: Failing to subtract ambient background counts leads to falsely elevated activity readings.
Prevention: Always measure and subtract background counts before reporting sample activity.
Cause: Isotopes like ³²P decay rapidly (14.2-day half-life), so old stock gives unreliable low-activity results.
Prevention: Track receipt dates and calculate remaining activity before use; discard per half-life schedule.
Cause: Using inappropriate shielding material or thickness for gamma versus beta emitters.
Prevention: Match shielding material (e.g. lead for gamma, acrylic for beta) to the specific radiation type and energy.
Laboratory Tips from the Bench
Always calculate the current activity of a stock isotope using its half-life before an experiment — old ³²P or ¹²⁵I stock may be significantly weaker than the label suggests.
Positron emitters are always followed by gamma ray production (from annihilation), so positron-emitting isotopes require gamma-level shielding even though the positron itself has limited range.
Remember "t½ = 0.693/λ" — 0.693 is simply the natural log of 2 (ln 2), since half-life is the time for the amount to fall to exactly half.
Important Notes
All radioactive emissions can ionize tissue and cause cellular/DNA damage. Time, distance, and shielding are the three fundamental principles of radiation protection for anyone handling isotopes.
PET scanning detects the two back-to-back 511 keV gamma rays produced when an emitted positron annihilates with an electron, allowing precise localization of metabolically active tissue such as brain regions or tumours.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgePatient with recurrent focal seizures is referred for a PET scan using a positron-emitting radiotracer to localize the epileptogenic focus.
The positron-emitting tracer accumulates in metabolically active tissue; annihilation with electrons produces detectable back-to-back gamma rays, and reduced uptake (hypometabolism) in the temporal lobe correlates with the seizure focus.
- →PET imaging relies on detecting the gamma rays from positron-electron annihilation, not the positron itself.
- →Radiotracer dosing must account for isotope half-life to ensure adequate activity at the time of imaging.
- →Radiation safety monitoring protects both patients and staff during nuclear medicine procedures.
Frequently Asked Questions
Alpha particles have very high mass and ionizing power, making them highly toxic if internalized, even though their external penetrating power is low. This makes them unsuitable for routine biological tracer work.
³²P has a short half-life of only 14.2 days, so its activity decays significantly within days to weeks, requiring prompt use for reliable experimental results.
The Becquerel is the modern SI unit (1 disintegration per second), while the Curie is the older, larger traditional unit (3.7×10¹⁰ disintegrations per second, equivalent to 37 GBq) still used in some clinical contexts.
Quick Revision
10-Minute ReviewKey Takeaways
- Radioactivity results from unstable atomic nuclei emitting particles or electromagnetic radiation to regain stability.
- The five major decay types relevant to biochemistry are negatron, positron, alpha, gamma emission, and electron capture.
- Half-life determines how quickly an isotope's activity declines, calculated as t½ = 0.693/λ.
- Becquerel and Curie are the SI and traditional units of radioactivity, respectively.
- Positron emission underlies PET imaging via detection of annihilation gamma rays.
- Radiation safety principles (time, distance, shielding) are essential whenever handling radioisotopes.
Competency Checklist
Track Your MasteryReferences
- NIOS Biochemistry Course Material, Lesson 19: Basic Principles of Radioactive Measurements.
- Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, latest edition.
- Harper's Illustrated Biochemistry, latest edition.