Chapter 2 Radiation Characteristics and Measurement

CHARACTERISTICS AND MEASUREMENT OF RADIATION

LEARNING OBJECTIVES

• Define the key terms.
• Draw and label a typical atom.
• Describe the process of ionization.
• Differentiate between radiation and radioactivity.
• List the properties shared by all energies of the electromagnetic spectrum.
• Explain the relationship between wavelength and frequency.
• List the properties of X-rays.
• Identify and describe the two processes by which kinetic energy is converted to electromagnetic energy within the dental X-ray tube.
• Differentiate between primary, secondary, and scatter radiations.
• List and describe the four possible interactions of dental X-rays with matter.
• Define the terms used to measure X-radiation.
• Match the Système Internationale (SI) units of X-radiation measurement to the corresponding traditional terms.
• Identify three sources of naturally occurring background radiation.

INTRODUCTION

• Patients tend to link dental X-rays with other types of radiation exposure.
• It’s important that dental hygienists understand:
  • What dental radiation is.
  • What it can do.
  • What it cannot do.
• Matter: Anything that occupies space and has mass.
• Energy: The ability to do work and overcome resistance.
• Forms of energy include: heat, light, electricity, and X-radiation.
  • Radiation is produced whenever the state of matter is altered by natural or artificial means.

ATOMIC STRUCTURE

• Each element is made up of atoms.
  • Atom: The smallest particle of an element that retains the properties of that element.
    • Atoms generally combine with other atoms to form molecules.
  • Molecule: The smallest particle of a substance that retains the properties of that substance.

ATOMIC STRUCTURE

• Atoms are composed of:
  1. Electrons: Negatively charged, constantly in motion.
  2. Protons: Positively charged, the number in the nucleus determines the atomic number.
  3. Neutrons: Have no charge.

ENERGY LEVELS

• Electrons revolve around the nucleus in shells.
• Normally, an atom is electrically neutral (equal number of protons and electrons).
• Maintains orbit due to positive attraction of protons known as binding energy.
  • Binding energy is strongest in the innermost K shell.

IONIZATION

• Occurs when an electron is removed from an electrically neutral atom.
  • Ion: A charged particle.
  • A positively charged ion and a negatively charged electron are called an ion pair.
  • Ionization: The process of formation of ion pairs.

IONIZATION

• When an atom is struck by an X-ray photon, an electron is dislodged.
  • As the electron travels, it dislodges other electrons from other atoms, creating additional ion pairs.
• This process is an attempt to regain electrical stability.

IONIZING RADIATION

• Any radiation that produces ions.
• Radiation: The emission and movement of energy through space in the form of:
  1. Electromagnetic radiation:
  • Includes X-rays and gamma rays; only a portion is ionizing type.
  1. Particulate radiation:
  • Includes alpha and beta particles, protons, electrons, and neutrons.
    • Characteristics of particulate radiation:
  1. Occupy space.
  2. Have mass and weight.
  3. Have an electrical charge (exception: neutrons).

RADIOACTIVITY

• The process whereby certain unstable elements undergo spontaneous disintegration (decay) to attain a stable nuclear state.
  • Accompanied by emissions of one or more types of radiation, leading to the formation of a new isotope.
  • Some isotopes are stable, while others are unstable or radioactive.
  • Radioactive isotopes emit nuclear radiation as rapid-moving particles or high-energy electromagnetic waves.
  • Particle emissions change the atom from one isotope to another, which may continue until the atom becomes a stable isotope.
  • This process is termed decay.

ELECTROMAGNETIC RADIATION

• Movement of wavelike energy through space as a combination of electric and magnetic fields.
• Arranged according to energies in the electromagnetic spectrum.
• Move through space as both a particle and a wave (quantum and wave theories).

WAVE THEORY

• Electromagnetic radiation is propagated in the form of waves that exhibit:
  • Wavelength: Distance between wave crests.
  • Frequency: Number of waves that pass a given point per unit of time.
  • Velocity: Speed of the wave.
• The energy of radiation is determined by its wavelength:
  • Shorter wavelengths have more energy and are more penetrating.
  • Wavelength measured in meters or Angstrom (Å) units, where 1 Å = 1/250,000,000 inches.

WAVE THEORY WAVELENGTH

• Wavelength is defined as the distance between two similar points, e.g., the distance between the crests of a wave.
• Frequency: Measured in Hertz (Hz); the higher the frequency, the more penetrating the radiation.
• Velocity: In a vacuum, all electromagnetic radiations travel at the speed of light, which is approximately 186,000 miles/second or $3 imes 10^8 ext{ m/s}$.

LONG AND SHORT WAVELENGTHS

• Long wavelengths:

  • Low frequency.
  • Low energy.
  • Less penetrating power.
  • Often referred to as soft radiation.

• Short wavelengths:

  • High frequency.
  • High energy.
  • More penetrating power.
  • Referred to as hard radiation.

PROPERTIES OF X-RAYS

• X-rays possess the following properties:
  • Invisible to the naked eye.
  • Travel in straight lines.
  • Travel at the speed of light in a vacuum.
  • Have no mass or weight.
  • Have no charge.
  • Interact with matter causing ionization.
  • Can penetrate opaque tissues and structures.
  • Affect photographic film emulsion creating a latent image.
  • Can affect biological tissues.
  • Cannot be sensed by human perception.

RADIOPACITY AND RADIOLUCENCY

• The ability to penetrate materials or tissues depends on:
  • The wavelength of the X-ray.
  • The thickness and density of the object.
  • The composition of the object determines whether X-rays will penetrate and pass through it or be absorbed.

PRODUCTION OF X-RAYS

• Generated inside an X-ray tube when high-speed electrons are abruptly stopped or slowed down.
• Kinetic energy: Associated with bodies in motion; converted to electromagnetic energy during the X-ray production process leading to:
  1. General (Bremsstrahlung) radiation.
  2. Characteristic radiation.

GENERAL (BREMSSTRAHLUNG) RADIATION

• Also known as “breaking radiation.”
• Produced when high-speed electrons are stopped or slowed down by the tungsten atoms of the target.
  • Involves interaction with the nucleus of an atom.
  • Majority of X-rays produced in dental X-ray machines come from general radiation.

CHARACTERISTIC RADIATION

• Produced when high-speed electrons collide with an orbiting K-shell electron of the tungsten target.
  • Dislodges the K-shell electron from the atom.
  • Another electron from the outer shell fills the void, resulting in emitted X-ray energy.
  • This radiation can only be produced when the X-ray machine operates at or below 70 KVP.

DESCRIPTION OF X-RAY FORMS

• Primary radiation:

  • Generated at the target of the tubehead.
  • Useful beam, specifically X-rays generated for making a radiographic image.

• Secondary radiation:

  • Less penetrating than primary radiation.
  • Not useful for the radiographic image, leading to lowered contrast; formed as a result of the primary beam interacting with matter.

• Scatter radiation:

  • A form of secondary radiation.
  • X-rays deflected in all directions due to interaction with matter.
  • Not useful for radiographic images; results in additional exposure to both patient and operator.

FOUR POSSIBLE INTERACTIONS OF DENTAL X-RAYS WITH MATTER

1. No interaction:
   • Approximately 9% of X-rays pass through a patient’s tissues without interacting.
2. Coherent scattering (Thompson scattering):
   • Accounts for around 8% of interactions.
3. Photoelectric effect:
   • Responsible for about 30% of interactions.
4. Compton scattering:
   • Accounts for approximately 60% of interactions.

COHERENT SCATTERING

• Occurs when a low-energy X-ray passes near an atom's outer electron.
  • Causes the electron to vibrate at the same frequency as the incoming X-ray, resulting in the creation of a new X-ray without energy loss.
  • Typically occurs in a forward direction.

PHOTOELECTRIC EFFECT

• Characterized by an all-or-nothing energy loss; the X-ray photon transfers all its energy to an orbital electron, which is ejected from its orbit.
  • This phenomenon results in an ion pair consisting of a photoelectron and a positive ion atom.

COMPTON SCATTERING

• Similar to the photoelectric effect, a photon interacts with an orbital electron and ejects it with only a portion of its energy.
  • A new, weaker X-ray photon is formed and scattered in a new direction, resulting in secondary radiation.

UNITS OF RADIATION

• The International Commission on Radiation Units and Measurements (ICRU) established standards for radiation units and quantities.
• Système International (SI): modern version, SI units are different than traditional units.

X-RAY PROTECTION MEASUREMENT

• Exposure: Measurement of ionization in air produced by gamma or X-rays (C/kg or (R)).
  • Not a measure of dose; an exposure only becomes a dose when the radiation is absorbed in tissues.
• Absorbed dose: Amount of energy deposited in any form of matter (including gamma, X-rays, and/or alpha, beta particles).
  • Measured in gray or rad units and can pertain to soft tissues, teeth, or other materials.
• Dose equivalent: Compares biological effects of various types of radiation, measured in sievert or rem units.
• Effective dose equivalent: Compares the risk of radiation exposure producing a biological response; measured in microsievert (μSv).

TYPES OF RADIATION BASED ON PENETRATION POWER

• Radiation types and their comparative penetrating powers:
  • Alpha ($ext{α}$)
  • Beta ($ext{β}$)
  • Gamma ($ext{γ}$): Has the most penetrating power of common types of radiation.
  • X-rays ($ext{X}$)
  • Neutrons ($ext{n}$)
  • Neutrinos ($ext{ν}$): Notably, neutrinos have the most penetrating power of all.

BACKGROUND RADIATION

• Man-made radiation constitutes approximately 48% of total radiation exposure to the US population, including:
  • Dental X-rays.
  • Medical procedures, nuclear medicine, and fluoroscopy.
• Consumer products and activities account for about 2% of total radiation exposure, e.g., building materials, cigarette smoking, fossil fuels.
• Naturally occurring background sources are always present in the environment, including:
  • Cosmic radiations from outer space.
  • Terrestrial radiations from the Earth and its environments.
  • Background radiations from naturally occurring radionuclides through inhalation and ingestion.