☢️Radiobiology Unit 2 – Fundamentals of Radiation Physics

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 remove electrons from atoms or molecules, creating ions (alpha, beta, gamma, X-rays)
• Non-ionizing radiation lacks the energy to ionize atoms or molecules but can cause excitation (radio waves, microwaves, visible light)
• Radioactivity is the spontaneous emission of radiation from an unstable atomic nucleus
• Half-life represents the time required for half of a given quantity of a radioactive substance to decay
• Absorbed dose quantifies the amount of energy deposited by ionizing radiation per unit mass of matter, measured in grays (Gy)
• Equivalent dose considers the biological effectiveness of different types of radiation, measured in sieverts (Sv)
• Linear energy transfer (LET) describes the average energy deposited per unit length of the radiation track

Types of Radiation and Their Properties

• Alpha radiation consists of heavy, positively charged particles (helium nuclei) with high LET and short range in matter
  • Easily stopped by a sheet of paper or skin but can cause significant damage if ingested or inhaled
• Beta radiation comprises high-energy electrons or positrons emitted from the nucleus during radioactive decay
  • Can penetrate deeper than alpha particles but can be stopped by a few millimeters of aluminum or plastic
• Gamma radiation is high-energy electromagnetic radiation originating from the nucleus during radioactive decay
  • Highly penetrating and requires dense materials like lead or concrete for effective shielding
• X-rays are similar to gamma rays but originate from electron transitions outside the nucleus
  • Widely used in medical imaging (radiography, CT scans) and radiation therapy
• Neutron radiation occurs when free neutrons are emitted during nuclear reactions or spontaneous fission
  • Can penetrate deeply into matter and cause significant biological damage due to their high LET
• Cosmic radiation originates from high-energy particles from space, including protons, alpha particles, and heavy ions
  • Contributes to natural background radiation and can be a concern for astronauts and high-altitude flights

Atomic Structure and Radioactive Decay

• Atoms consist of a dense, positively charged nucleus surrounded by negatively charged electrons
• The nucleus contains protons (positively charged) and neutrons (neutral), held together by strong nuclear forces
• Isotopes are variants of an element with the same number of protons but different numbers of neutrons
• Radioactive decay occurs when an unstable nucleus releases energy in the form of radiation to reach a more stable state
• Alpha decay involves the emission of an alpha particle (helium nucleus), decreasing the atomic number by 2 and mass number by 4
• Beta decay occurs when a neutron transforms into a proton, emitting an electron (beta minus) or when a proton transforms into a neutron, emitting a positron (beta plus)
• Gamma decay involves the emission of high-energy photons (gamma rays) from an excited nucleus without changing the atomic number or mass number
• Decay chains describe a series of radioactive decays until a stable isotope is reached (uranium-238 decay chain)

Radiation Interactions with Matter

• Photoelectric effect occurs when a photon transfers all its energy to an electron, ejecting it from the atom
  • Dominant interaction for low-energy photons and high-Z materials
• Compton scattering involves the inelastic scattering of a photon by a loosely bound electron, resulting in a lower-energy photon and a scattered electron
  • Predominant interaction for intermediate-energy photons and low-Z materials
• Pair production happens when a high-energy photon interacts with the electric field near a nucleus, creating an electron-positron pair
  • Requires photon energies exceeding 1.022 MeV (twice the rest mass energy of an electron)
• Ionization is the process by which radiation removes electrons from atoms or molecules, creating ion pairs
  • Primary mechanism for energy deposition by charged particles (alpha, beta)
• Excitation occurs when radiation transfers energy to an electron, raising it to a higher energy state without ionization
  • Can lead to the emission of characteristic X-rays or Auger electrons when the excited electron returns to its ground state
• Bremsstrahlung (braking radiation) is the emission of X-rays when a charged particle (usually an electron) is decelerated by the electric field of an atomic nucleus
  • Important in X-ray tubes and contributes to the continuous X-ray spectrum

Measurement and Detection of Radiation

• Geiger-Müller (GM) counters detect ionizing radiation by measuring the electrical pulses created by ion pairs in a gas-filled tube
  • Widely used for general-purpose radiation monitoring but do not provide energy information
• Scintillation detectors use materials that emit light when exposed to ionizing radiation, which is then converted into an electrical signal by a photomultiplier tube
  • Commonly used in gamma cameras, PET scanners, and radiation portal monitors
• Semiconductor detectors (silicon, germanium) operate by measuring the electrical charges created by radiation interactions in a semiconductor material
  • Offer excellent energy resolution and are used in high-precision spectroscopy and imaging applications
• Thermoluminescent dosimeters (TLDs) store radiation energy in crystal defects and release it as light when heated
  • Used for personal dosimetry and environmental monitoring
• Optically stimulated luminescence (OSL) dosimeters work similarly to TLDs but use light to stimulate the release of stored energy
  • Increasingly used in personal dosimetry due to their high sensitivity and ease of readout
• Radiation dose is measured using various quantities, including absorbed dose (Gy), equivalent dose (Sv), and effective dose (Sv)
  • Absorbed dose quantifies energy deposition per unit mass, while equivalent and effective doses consider biological effects and tissue sensitivities

Biological Effects of Radiation

• Radiation can cause direct damage to biomolecules (DNA, proteins, lipids) through ionization and excitation
• Indirect damage occurs when radiation interacts with water molecules, creating reactive oxygen species (ROS) that can damage biomolecules
• DNA damage includes base modifications, single-strand breaks (SSBs), and double-strand breaks (DSBs)
  • DSBs are considered the most critical lesions for cell survival and genetic integrity
• Radiation-induced cell death can occur through apoptosis (programmed cell death), necrosis (uncontrolled cell death), or mitotic catastrophe (cell death during mitosis)
• Stochastic effects are probabilistic and have no threshold dose (cancer, genetic mutations)
  • Risk increases with dose, but severity is independent of dose
• Deterministic effects have a threshold dose below which the effect is not observed (skin erythema, cataracts, infertility)
  • Severity increases with dose above the threshold
• Radiosensitivity varies among cell types, with rapidly dividing cells (bone marrow, intestinal epithelium) being more sensitive than slowly dividing or non-dividing cells (neurons, muscle cells)
• Acute radiation syndrome (ARS) can occur after whole-body exposure to high doses of radiation, affecting the hematopoietic, gastrointestinal, and neurovascular systems

Radiation Protection Principles

• Justification: The benefits of a radiation exposure should outweigh the risks
  • Medical procedures should have a clear clinical indication and expected benefit for the patient
• Optimization: Radiation exposure should be kept as low as reasonably achievable (ALARA principle)
  • Achieved through proper shielding, distance, and minimizing exposure time
• Dose limitation: Exposure to individuals should not exceed established dose limits for occupational and public settings
  • Occupational limit: 50 mSv/year; Public limit: 1 mSv/year (excluding medical and natural background radiation)
• Time: Reducing the time spent near a radiation source decreases the total dose received
  • Minimize time in radiation areas and use remote handling tools when possible
• Distance: Increasing the distance from a radiation source reduces the dose rate according to the inverse square law
  • Maintain a safe distance from sources and use distance as a protective measure
• Shielding: Appropriate shielding materials can attenuate or block radiation, reducing the dose to individuals
  • Use lead aprons, thyroid collars, and leaded glass in medical settings; concrete, lead, or water for industrial and research applications
• Contamination control: Prevent the spread of radioactive materials and promptly decontaminate affected areas
  • Use proper protective equipment (gloves, coveralls, respirators) and monitor for contamination

Applications in Medicine and Research

• Diagnostic radiology uses X-rays to visualize internal structures (radiography, fluoroscopy, computed tomography)
  • Provides valuable information for diagnosing fractures, infections, tumors, and other pathologies
• Nuclear medicine employs radioactive tracers to assess physiological functions and diagnose diseases
  • Examples include thyroid scans (iodine-131), bone scans (technetium-99m), and cardiac stress tests (thallium-201)
• Radiation therapy uses high-energy radiation (X-rays, gamma rays, electrons) to treat cancer by damaging tumor cells
  • External beam radiation therapy (EBRT) delivers radiation from an external source, while brachytherapy involves placing radioactive sources directly in or near the tumor
• Positron emission tomography (PET) uses positron-emitting radionuclides to image metabolic processes and diagnose diseases
  • Commonly used in oncology, neurology, and cardiology to assess tumor metabolism, brain function, and myocardial perfusion
• Radiation is used in various research applications, including studying biological processes, developing new materials, and analyzing chemical compounds
  • Examples include radiolabeling molecules for drug development, using X-ray crystallography to determine protein structures, and employing radiation to induce mutations for plant breeding
• Food irradiation uses ionizing radiation to preserve food and reduce the risk of foodborne illnesses
  • Extends shelf life, controls insect infestation, and reduces microbial contamination in foods like fruits, vegetables, and meat products