Measuring Radiation
Accurate measurement of ionizing radiation is essential for medical applications, nuclear power operations, environmental monitoring, and radiation protection. The field of dosimetry encompasses the instruments, units, and techniques used to quantify radiation exposure and its biological effects.
Types of Ionizing Radiation
Ionizing radiation carries enough energy to remove electrons from atoms, creating ions. The main types relevant to measurement and safety are:
- Alpha particles: Helium-4 nuclei. Highly ionizing (create dense tracks of ion pairs) but very short range (3-7 cm in air, stopped by skin). Dangerous only if inhaled or ingested.
- Beta particles: High-speed electrons or positrons. Moderate ionizing power and range (up to several meters in air, stopped by a few mm of aluminum or plastic).
- Gamma rays and X-rays: High-energy photons. Low ionizing density but high penetrating power (require lead, concrete, or thick water for shielding).
- Neutrons: Uncharged particles that penetrate deeply and cause ionization indirectly by knocking protons from hydrogen-containing materials. Shielded with water, polyethylene, or concrete.
Radiation Units
Three distinct quantities describe radiation, each with its own unit:
Activity measures the rate of radioactive decay: - Becquerel (Bq): 1 decay per second (SI unit) - Curie (Ci): 3.7 x 10^10 decays per second (historical unit, still common in the US)
Absorbed dose measures the energy deposited per unit mass of material: - Gray (Gy): 1 joule per kilogram (SI unit) - Rad: 0.01 Gy (older unit; 1 Gy = 100 rad)
Equivalent dose accounts for the biological effectiveness of different radiation types: - Sievert (Sv): Absorbed dose multiplied by a radiation weighting factor (w_R) - Rem: 0.01 Sv (older unit; 1 Sv = 100 rem)
The radiation weighting factors reflect biological damage potential: gamma rays and beta particles have w_R = 1, protons have w_R = 2, alpha particles have w_R = 20, and neutrons range from 2.5 to 20 depending on energy. A 1 Gy dose of alpha particles thus produces a 20 Sv equivalent dose, reflecting the much greater biological damage from densely ionizing alpha tracks.
Detection Instruments
Geiger-Mueller counters are the most recognizable radiation detectors. A sealed tube filled with inert gas (typically helium, neon, or argon with a halogen quench gas) operates at high voltage. When ionizing radiation enters the tube, it creates ion pairs that trigger an electrical avalanche (discharge), producing a measurable pulse. The characteristic clicking sound comes from the audio amplification of these pulses. Geiger counters are excellent for detecting the presence of radiation and measuring count rates but cannot distinguish between radiation types or measure energies.
Scintillation detectors use materials (sodium iodide crystals, plastic scintillators, or liquid cocktails) that emit visible light when struck by ionizing radiation. A photomultiplier tube converts the light to an electrical signal. The pulse height is proportional to the radiation energy, allowing gamma spectroscopy -- identification of specific isotopes by their characteristic gamma-ray energies.
Semiconductor detectors (high-purity germanium or silicon) provide the best energy resolution for gamma spectroscopy. Radiation creates electron-hole pairs in the semiconductor, producing a current pulse proportional to energy. These detectors can distinguish gamma rays differing by less than 1 keV in energy.
Personal dosimeters track cumulative radiation exposure for workers: - Film badges: Photographic film that darkens proportionally to radiation exposure - Thermoluminescent dosimeters (TLDs): Crystalline materials that store radiation energy and release it as light when heated - Optically stimulated luminescence (OSL) dosimeters: Similar to TLDs but read optically, now the most common type - Electronic personal dosimeters: Provide real-time dose rate and cumulative dose readings with alarm thresholds
Radiation Protection Principles
The three fundamental principles of radiation protection are time, distance, and shielding:
- Time: Minimize exposure duration. Dose is directly proportional to time spent near a source.
- Distance: Maximize distance from the source. Radiation intensity decreases with the square of the distance (inverse square law). Doubling your distance reduces exposure to one-quarter.
- Shielding: Place appropriate material between yourself and the source. Alpha: paper or skin. Beta: plastic or aluminum. Gamma: lead, concrete, or water. Neutrons: water, polyethylene, or boron-containing materials.
Dose Limits and ALARA
Regulatory dose limits for radiation workers and the general public are set by national authorities following recommendations from the International Commission on Radiological Protection (ICRP):
- Occupational: 20 mSv per year averaged over 5 years (50 mSv maximum in any single year)
- General public: 1 mSv per year above natural background
- Emergency workers: Up to 500 mSv for life-saving actions
The guiding philosophy is ALARA -- As Low As Reasonably Achievable. This means that even when exposures are below regulatory limits, every reasonable effort should be made to further reduce them. ALARA considers economic and social factors: radiation doses should be reduced to the point where further reduction would cost more than the benefit gained.
Background Radiation
Humans are constantly exposed to natural background radiation from cosmic rays, terrestrial radioactivity (potassium-40, uranium, thorium in soil and rocks), inhaled radon gas, and internal radioactivity (primarily potassium-40 in our bodies). The worldwide average is about 2.4 mSv per year, though this varies widely by location -- from about 1 mSv in some areas to over 200 mSv in Ramsar, Iran, where natural hot springs concentrate radium.