Radiation Protection Basics

Fundamentals of Radiation

• Definition of Radiation:

  • Radiation is defined as the emission of energy through space or a medium.
  • It is fundamentally energy in motion or in transit.
  • Radiation travels in two ways: as waves or in discrete bundles of energy known as photons.
  • It exists in various forms across the energy spectrum.
  • Measurement Units: Radiation is most commonly measured using the following energy units:
    • Electron volts ($eV$)
    • Kilo electron volts ($keV$)
    • Mega electron volts ($MeV$)

• Excitation:

  • Excitation is the process where some forms of radiation provide enough energy to an electron to raise it to a higher energy state, though not enough to remove it from the atom entirely.

Understanding Radioactivity

• Definition of Radioactivity:

  • Radioactivity is the spontaneous emission of radiation from the nucleus of an unstable atom.
  • The core motivation for this process is the unstable nucleus's drive to become stable.
  • To achieve stability, the nucleus must release energy, which it does by "spitting out" or emitting particles and rays.

• Primary Emissions:

  • Alpha particles.
  • Beta particles.
  • Gamma rays.

• Quantitative Definitions:

  • Radioactivity can be defined as the rate of decay of radioactive material.
  • It can also be defined as the rate of change in the total number of atoms.

Sources of Radiation: Natural and Manmade

• 1. Natural (Background) Sources:

  • Cosmic Radiation:
    • This radiation is produced by the sun and stars.
    • It reaches humans through interactions with the Earth’s atmosphere and magnetic field.
    • Dosage is dependent on geographical location and altitude: the higher the altitude (height above sea level), the higher the dose.
  • Terrestrial Radiation:
    • Found naturally in soil, rocks, water, air, and vegetation.
    • Primary radioactive elements found in the Earth include Uranium and Thorium.
    • Radon Gas:
      • A byproduct of Uranium decay.
      • It is tasteless, colorless, and odorless.
      • It diffuses through soil and rocks.
      • Radon accounts for the single greatest amount of natural radiation exposure to humans.
  • Internal Radiation:
    • Refers to radioactive materials residing inside the human body.
    • Examples include Potassium 40 ($K-40$), Uranium, and Thorium.
  • Average Dose:
    • The total U.S. average background dose is approximately $300\,mrem/year$, which includes the contribution from Radon.

• 2. Manmade Sources:

  • Medical Radiation:
    • Used for diagnostic X-rays, radiation therapy treatments, and Radiopharmaceuticals.
    • This category is responsible for the second greatest amount of radiation exposure for humans, trailing only Radon.
  • Consumer Products: Various everyday items contribute to manmade exposure.

Categories of Radiation: Ionizing vs. Non-Ionizing

• Non-Ionizing Radiation:

  • Definition: Radiation that lacks the energy required to remove electrons from atoms.
  • Mechanism: Because it cannot cause ionization, it does not strip electrons during interactions.
  • Examples: Light, lasers, heat, microwaves, and radar.

• Ionizing Radiation:

  • Definition: Radiation that possesses sufficient energy to remove electrons from atoms as it passes through matter.
  • Mechanism of Ionization:
    • When ionizing radiation enters tissue, it interacts with atoms and imparts enough energy to eject an electron from its orbital shell.
    • This creates an Ion Pair, consisting of a positively charged ion (the atom that lost the electron) and a free electron (the negatively charged particle).
    • Ionization is the fundamental mechanism used in radiation therapy to deliver a dose, deposit energy into tissues, and damage or destroy cancer cells.
  • Examples: X-ray photons, gamma rays, beta particles, protons, neutrons, and alpha particles.

Direct vs. Indirect Ionization

• Directly Ionizing Radiation:

  • Requirement: These particles must carry an electric charge.
  • Examples: Alpha particles, beta particles (electrons), and protons.
  • Mechanism: Their charge allows them to interact directly with orbital electrons through Coulomb interactions. They collide with and dislodge electrons, depositing energy directly along their travel path.

• Indirectly Ionizing Radiation:

  • Requirement: These particles or rays carry no electric charge.
  • Examples: X-rays, gamma rays, and neutrons.
  • Mechanism (Two-Step Process):
    1. Step 1: The uncharged photon or neutron interacts with matter to create a secondary charged particle (e.g., a photon creates an electron).
    2. Step 2: The secondary charged particle then interacts directly with atoms in the medium, causing ionization as it slows down and comes to rest.

Electromagnetic Radiation (EMR)

• Physical Nature:

  • Energy resulting from oscillating or fluctuating electric and magnetic fields.
  • Travels through a vacuum at the speed of light: approximately $3 \times 10^8\,m/s$.

• Wave-Particle Duality:

  • EMR exhibits wavelike behavior but also acts like particles called photons (individual packets of energy).
  • This duality allows EMR to interact with matter in various ways depending on the context.

• The Electromagnetic Spectrum:

  • Spans a wide range of energies.
  • Ranges from low-energy, long-wavelength radiation (radio waves) to high-energy, short-wavelength radiation (X-rays and gamma rays).
  • Relationships:
    • Increase in wavelength = Decrease in frequency and energy.
    • Decrease in wavelength = Increase in frequency and energy.
    • X-rays and gamma rays are on the far right of the spectrum (short wavelength, high frequency, high energy).

• Discovery:

  • Wilhelm Röntgen discovered radiation on November 8, 1895.

Comparison of X-ray and Gamma Ray Photons

• X-ray Characteristics:

  • Arise from orbital shells of an atom.
  • Unaffected by gravity, electric fields, or magnetic fields.
  • No mass and no charge.
  • Travel in straight but diverging lines.
  • Travel at the speed of light ($3 \times 10^8\,m/s$) in a vacuum.
  • Cannot be focused by a lens like visible light.
  • Gives up energy to matter slowly through atomic collisions.
  • Attenuation:
    • The reduction of energy in an X-ray beam as it travels through matter.
    • Causes: Absorption (X-ray taken up by tissue) and Scattering (X-ray changes direction and loses energy).
    • Relationship: Attenuation is exponential, not linear. The rate of attenuation slows as fewer photons remain deeper in the material.

• Gamma Ray Characteristics:

  • Nearly identical behavior to X-rays.
  • Share all characteristics: no mass, no charge, speed of light, straight/diverging travel, cannot be focused, and exponential attenuation.
  • Key Distinction (Origin):
    • X-rays are produced outside the nucleus via orbital electron interactions.
    • Gamma rays are emitted from within the nucleus during radioactive decay.
  • Historical Note: Early megavoltage therapy used Cobalt-60, a radioactive isotope that emits gamma radiation naturally during decay. These units preceded high-energy linear accelerators.

Particulate Radiation

• General Features:

  • Consists of subatomic particles moving at extremely high speeds.
  • These particles typically have mass and most carry a charge.
  • Most are directly ionizing (except neutrons).

• Alpha Particles:

  • Composed of $2$ protons and $2$ neutrons (identical to a helium nucleus).
  • Charge: $+2$.
  • Ejected during alpha decay.
  • Behavior: Due to large mass and strong positive charge, they create a very dense path of ionization. They lose energy quickly and have low penetration (approximately $0.7\,mm$ in tissue).

• Neutrons:

  • Have mass but no electric charge.
  • Unique because they are particle radiation but are indirectly ionizing.

• Beta Particles (Electrons/Positrons):

  • High-energy particles emitted from the nucleus (distinct from orbital electrons).
  • Beta Negative ($\beta^-$): Negative charge, similar to an electron.
  • Beta Positive ($\beta^+$): Positive charge, similar to a positron.
  • Orbital Electrons: Negative charge particles located in outer shells.

• Protons: Particles with a positive charge.

• Heavy Charged Particles: Particles more massive than protons/neutrons, such as carbon or helium atoms.

Linear Energy Transfer (LET)

• Definition:

  • LET describes the rate at which energy is deposited as radiation passes through tissue.
  • It defines the "quality" of ionizing radiation.
  • LET depends on the radiation's mass and electric charge.

• High LET Radiation:

  • Examples: Alpha particles, neutrons, protons, and heavy recoil nuclei.
  • Features: Typically have large mass and/or charge (neutrons are the exception due to mass).
  • Effect: Deposits energy quickly in a dense path. This corresponds to high biological damage over a short distance.

• Low LET Radiation:

  • Examples: X-rays, gamma rays, and beta particles (electrons).
  • Features: Generally have no mass and no charge (except beta particles, which are low mass).
  • Effect: Interacts sparsely in matter, often traveling farther in a zigzag pattern. Causes less biological damage compared to high LET radiation.

Summary Table of Radiation Properties

• Alpha Particle:
  • Type: Particle Radiation
  • Ionization: Direct
  • LET: High
• Proton:
  • Type: Particle Radiation
  • Ionization: Direct
  • LET: High
• Beta (Positive/Negative):
  • Type: Particulate Radiation
  • Ionization: Direct
  • LET: Low
• Neutron:
  • Type: Particulate Radiation
  • Ionization: Indirect (Neutral charge)
  • LET: High (Significant mass)
• X-ray Photons:
  • Type: Electromagnetic Radiation (EMR)
  • Ionization: Indirect
  • LET: Low
• Gamma Ray Photons:
  • Type: Electromagnetic Radiation (EMR)
  • Ionization: Indirect
  • LET: Low