☢️Radiobiology Unit 13 – Radiation Protection and Dosimetry

Key Concepts and Terminology

• Radiation refers to the emission and propagation of energy through space or a medium in the form of waves or particles
• Ionizing radiation has sufficient energy to ionize atoms or molecules by removing electrons from their orbits
• Non-ionizing radiation lacks the energy to ionize atoms but can still cause biological effects (ultraviolet light, radio waves)
• Radioactivity is the spontaneous emission of radiation from unstable atomic nuclei
• Half-life represents the time required for half of a given quantity of a radioactive substance to decay
• Linear energy transfer (LET) describes the amount of energy deposited per unit length as radiation passes through matter
  • High-LET radiation (alpha particles, neutrons) deposits more energy and causes greater biological damage
  • Low-LET radiation (gamma rays, x-rays) deposits less energy and is less damaging per unit length
• Stochastic effects are probabilistic and have no threshold dose (cancer, genetic mutations)
• Deterministic effects have a threshold dose and severity increases with dose (skin erythema, cataracts)

Radiation Types and Sources

• Alpha radiation consists of heavy, positively charged particles emitted from the nucleus of an atom
  • Short range in air and can be stopped by a sheet of paper or the outer layer of skin
  • Highly ionizing and can cause significant damage if inhaled or ingested (radon gas)
• Beta radiation involves the emission of electrons or positrons from the nucleus during radioactive decay
  • Longer range in air than alpha particles but can be stopped by a few millimeters of aluminum
  • Can penetrate the skin and cause surface burns or damage to shallow tissues
• Gamma radiation is high-energy electromagnetic radiation emitted from the nucleus during radioactive decay
  • Highly penetrating and requires dense materials like lead or concrete for shielding
  • Interacts with matter through photoelectric effect, Compton scattering, and pair production
• X-rays are similar to gamma rays but originate from electron transitions outside the nucleus
  • Produced by x-ray machines and used in medical imaging (radiography, CT scans)
• Neutron radiation occurs when neutrons are ejected from the nucleus during fission or fusion reactions
  • Highly penetrating and can cause activation of stable nuclei, making them radioactive
  • Requires hydrogenous materials (water, concrete) for effective shielding
• Cosmic radiation originates from high-energy particles from space (protons, helium nuclei)
  • Contributes to natural background radiation and increases with altitude
• Terrestrial radiation comes from naturally occurring radioactive materials in the Earth's crust (uranium, thorium, potassium-40)
• Anthropogenic sources include medical procedures, nuclear power plants, and industrial applications

Biological Effects of Radiation

• Radiation interacts with biological molecules through direct and indirect mechanisms
  • Direct effects involve the direct ionization or excitation of critical targets (DNA, proteins)
  • Indirect effects occur when radiation interacts with water to produce free radicals that damage biomolecules
• DNA damage is the primary cause of radiation-induced cellular effects
  • Single-strand breaks are efficiently repaired by base excision repair mechanisms
  • Double-strand breaks are more difficult to repair and can lead to chromosomal aberrations or cell death
• Radiation can induce apoptosis, a programmed cell death mechanism that eliminates damaged cells
• Mitotic catastrophe occurs when cells with unrepaired DNA damage attempt to divide, leading to micronuclei formation and cell death
• Senescence is a state of permanent cell cycle arrest that can be induced by radiation exposure
• Bystander effect refers to the induction of biological effects in unirradiated cells through signaling from nearby irradiated cells
• Acute radiation syndrome (ARS) occurs after whole-body exposure to high doses of radiation
  • Hematopoietic syndrome (>0.7 Gy) affects blood cell production and immune function
  • Gastrointestinal syndrome (>10 Gy) damages intestinal lining and leads to fluid loss and infection
  • Neurovascular syndrome (>50 Gy) causes rapid neurological deterioration and cardiovascular collapse
• Late effects of radiation include increased risk of cancer, cardiovascular disease, and cataracts

Radiation Measurement and Units

• Activity is the rate of radioactive decay and is measured in becquerels (Bq) or curies (Ci)
  • 1 Bq = 1 disintegration per second
  • 1 Ci = 3.7 × 10^10 Bq
• Absorbed dose is the amount of energy deposited per unit mass of material and is measured in grays (Gy) or rads
  • 1 Gy = 1 joule per kilogram
  • 1 rad = 0.01 Gy
• Equivalent dose accounts for the biological effectiveness of different types of radiation and is measured in sieverts (Sv) or rems
  • Calculated by multiplying absorbed dose by a radiation weighting factor (wR)
  • wR = 1 for gamma rays and x-rays, 20 for alpha particles, and varies for neutrons based on energy
  • 1 Sv = 1 Gy × wR
  • 1 rem = 0.01 Sv
• Effective dose is the sum of equivalent doses to individual organs, weighted by tissue weighting factors (wT)
  • Represents the overall risk of stochastic effects from non-uniform exposures
  • Measured in sieverts (Sv) or rems
  • $E = \sum w_T H_T$, where HT is the equivalent dose to tissue T
• Committed dose is the total dose received over a 50-year period following the intake of a radioactive substance
• Collective dose is the sum of individual doses in a population and is measured in person-sieverts (person-Sv) or person-rems

Principles of Radiation Protection

• Justification principle states that any decision that alters radiation exposure should do more good than harm
  • Benefits of a practice should outweigh the associated radiation risks
• Optimization principle (ALARA) requires that radiation exposures be kept as low as reasonably achievable
  • Considers economic and societal factors in addition to radiation protection
• Dose limitation principle sets limits on the maximum permissible dose to individuals from regulated sources
  • Occupational dose limit is 20 mSv per year, averaged over a 5-year period
  • Public dose limit is 1 mSv per year from artificial sources
• Time, distance, and shielding are key factors in reducing external radiation exposure
  • Minimizing time spent near a radiation source reduces total exposure
  • Increasing distance from a source reduces dose rate according to the inverse square law
  • Using appropriate shielding materials (lead, concrete, water) attenuates radiation intensity
• Contamination control involves preventing the spread of radioactive materials and decontaminating affected areas
  • Personal protective equipment (gloves, respirators) minimizes intake of radioactive substances
  • Proper ventilation and filtration systems reduce airborne contamination
• Waste management practices ensure the safe handling, storage, and disposal of radioactive waste
  • Classification based on activity, half-life, and radionuclide content (low-level, intermediate-level, high-level)
  • Containment and isolation techniques prevent release into the environment
• Emergency preparedness and response plans are essential for mitigating the consequences of radiological incidents
  • Includes evacuation procedures, sheltering guidelines, and medical countermeasures (potassium iodide)

Dosimetry Techniques and Equipment

• Personal dosimeters measure individual radiation exposure and are worn by workers in radiation environments
  • Film badges contain radiation-sensitive film that darkens upon exposure
  • Thermoluminescent dosimeters (TLDs) use crystals that emit light when heated after radiation exposure
  • Optically stimulated luminescence (OSL) dosimeters use similar principles but with optical stimulation
  • Electronic personal dosimeters (EPDs) provide real-time dose and dose rate measurements
• Area monitoring involves the use of fixed or portable instruments to assess radiation levels in a specific location
  • Geiger-Müller (GM) counters detect ionizing radiation through gas ionization and are useful for contamination surveys
  • Ionization chambers measure exposure rate (roentgens per hour) and are used for environmental monitoring
  • Scintillation detectors use light-emitting materials (sodium iodide) to detect gamma radiation and are highly sensitive
• Bioassay techniques measure the amount of radioactive material in the body through direct or indirect methods
  • Whole-body counting detects gamma-emitting radionuclides using external detectors
  • Urinalysis and fecal analysis measure the excretion of radioactive substances and estimate intake
• Retrospective dosimetry estimates past radiation exposures using biological markers or physical materials
  • Chromosome aberration analysis assesses the frequency of dicentric chromosomes in lymphocytes
  • Electron paramagnetic resonance (EPR) measures radiation-induced free radicals in tooth enamel or bone
• Computational dosimetry uses mathematical models and simulation techniques to estimate radiation dose
  • Monte Carlo methods simulate radiation transport and interactions in complex geometries
  • Anthropomorphic phantoms represent human anatomy and are used for dose calculations and calibration

Regulatory Framework and Safety Standards

• International Commission on Radiological Protection (ICRP) provides recommendations and guidance on radiation protection principles
  • Publishes reports on dose limits, risk factors, and protection strategies
• International Atomic Energy Agency (IAEA) develops safety standards and promotes the peaceful use of nuclear technology
  • Establishes requirements for radiation protection, waste management, and emergency preparedness
• National regulatory authorities implement and enforce radiation safety regulations in their respective countries
  • U.S. Nuclear Regulatory Commission (NRC) regulates commercial nuclear facilities and radioactive materials
  • U.S. Environmental Protection Agency (EPA) sets standards for public exposure and environmental radiation protection
• Occupational safety standards specify requirements for radiation worker training, monitoring, and record-keeping
  • OSHA Ionizing Radiation Standard (29 CFR 1910.1096) applies to general industry
  • NRC Standards for Protection Against Radiation (10 CFR Part 20) applies to licensees
• Environmental radiation protection standards limit public exposure and radioactive releases into air and water
  • Clean Air Act regulations (40 CFR Part 61) restrict radionuclide emissions from nuclear facilities
  • Safe Drinking Water Act regulations (40 CFR Part 141) set maximum contaminant levels for radionuclides in public water systems
• Transportation regulations govern the packaging, labeling, and shipping of radioactive materials
  • U.S. Department of Transportation (DOT) Hazardous Materials Regulations (49 CFR Parts 100-185)
  • International Air Transport Association (IATA) Dangerous Goods Regulations for air shipments

Practical Applications and Case Studies

• Medical applications of radiation include diagnostic imaging and radiation therapy
  • X-ray radiography, CT scans, and nuclear medicine techniques (PET, SPECT) use ionizing radiation for diagnosis
  • External beam radiation therapy (EBRT) delivers high doses to tumor sites using linear accelerators
  • Brachytherapy involves the placement of sealed radioactive sources directly into or near tumor tissue
• Nuclear power plants generate electricity through controlled nuclear fission reactions
  • Pressurized water reactors (PWRs) and boiling water reactors (BWRs) are common designs
  • Radiation protection focuses on containment, shielding, and monitoring of reactor systems and spent fuel
• Industrial radiography uses high-energy gamma rays or x-rays to inspect materials for defects
  • Requires strict safety protocols, including proper shielding, access control, and emergency response plans
• Radiation sterilization is used to eliminate microorganisms from medical devices, food, and other products
  • Commonly uses cobalt-60 or electron beam irradiation
  • Dose levels are carefully controlled to ensure product safety and integrity
• Fukushima Daiichi nuclear disaster (2011) demonstrates the importance of emergency preparedness and response
  • Earthquake and tsunami damaged reactor cooling systems, leading to fuel melting and radioactive releases
  • Evacuation, sheltering, and food and water restrictions were implemented to protect public health
  • Long-term environmental monitoring and remediation efforts are ongoing
• Goiânia accident (1987) highlights the risks of orphan radioactive sources and the need for public education
  • Abandoned cesium-137 source was scavenged from a derelict radiotherapy clinic in Brazil
  • Contamination spread among local residents who were unaware of the hazards
  • Resulted in four deaths, 250 contaminated individuals, and extensive environmental contamination
• Chernobyl disaster (1986) underscores the importance of reactor safety and international cooperation
  • Flawed reactor design and operator errors led to a steam explosion and fire in Unit 4 of the Chernobyl Nuclear Power Plant
  • Radioactive plume spread across Europe, contaminating large areas and exposing millions to increased radiation levels
  • International response included radiation monitoring, agricultural countermeasures, and long-term health studies