Chapter 2: Radiation Physics

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Radiation Physics

Chapter 2

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Radiation Physics

• Define the key terms associated with radiation physics
• Review fundamental concepts of atomic and molecular structure
• Describe the process of radiation Identify x-ray machine components
• Describe how x-rays are produced

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Fundamental Concepts

• Matter- anything that occupies space and has mass. When matter is altered; energy results
• All matter is composed of atoms

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Atomic Structure

• The atom consists of 2 parts:
• Central nucleus
• Orbiting electrons

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• Nucleus consists of:
• Protons (positive electrical charge)
• Neutrons aka nucleons (no electrical charge)
• The number of protons and neutrons in the nucleus determines its atomic weight
• The number of protons inside the nucleus equals the number of electrons outside the nucleus and determines its atomic number
• Periodic table of elements arranges atoms according to their atomic number

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• Electrons are negatively charged particles that travel around the nucleus in well-defined paths known as orbits or shells
• An atom contains a maximum of 7 shells
• The shell that is closest to the nucleus is the K shell and the furthest is Q shell
• Each shell is located at a specific distance from the nucleus and represent different energy levels
• Electrons are maintained in their orbits by the electrostatic force, or binding energy
• K shell has the strongest binding energy only 2 can exist in the K shell

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Molecular Structure

• Atoms are capable of combining with each other to form molecules

• Molecule- two or more atoms joined by chemical bonds

• A molecule is a state of equilibrium when the number of protons equals the number of electrons

  

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Ionization

• Neutral atom- contains an equal number of protons and electrons

• Ion- an electrically unbalanced particle; an atom that gains or loses an electron

• Production of ions, or the process of converting an atom into ions

• An ion pair is formed when an electron is removed from an atom. The atom becomes the positive ion, and the ejected electron becomes the negative ion

  

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Ion Pair

• The ion pair reacts with other ions until electrically stable, neutral atoms are formed

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Radiation

• Emission and propagation of energy through space or a substance in the form of waves or particles

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Radioactivity

• Process by which certain unstable atoms of elements undergo spontaneous disintegration, or decay, in an effort to attain a more balanced nuclear state

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Ionizing Radiation

• Radiation that is capable of producing ions by removing or adding an electron to an atom
• 2 types:
• Particulate
• *Electromagnetic *

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Particulate Radiation

• Tiny particles of matter that possess mass and travel in straight lines at high speeds
• Transmit kinetic energy by means of their extremely fast-moving, small masses
• Examples:
• Alpha particles
• Beta particles
• Cathode rays
• Protons
• Neutrons

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Electromagnetic Radiation

• Propagation of wave-like energy (without mass) through space or matter. Oscillating electric and magnetic fields
• Examples:
• Cosmic rays (from the sun)
• Gamma rays
• X-rays
• Visible light
• Microwave
• Radar

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• Electromagnetic spectrum
• Most electromagnetic radiations are nonionizing; only very high energy radiations are capable of ionization. EMR are believed to move through space as both a particle and a wave
• Particle concept
• Photons or quanta
• Wave concept
• Velocity
• Wavelength
• Frequency
• Velocity- speed of the wave 186,000 miles/second
• Wavelength- distance between the crest of one wave and the crest of the next
• Wavelength determines the energy and penetrating power of the radiation
• Shorter the wavelength the greater potential to penetrate
• Measured in nanometers, or meters

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• Frequency- refers to the number of wavelengths that pass a given point in a certain amount of time
• Measured in Hertz
• Frequency and wavelength are inversely related

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X-Radiation

• High energy, ionizing electromagnetic radiation
• Weightless bundles of energy (photons) without an electrical charge that travel in waves with a specific frequency at a speed of light (360,000 miles per sound)

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Properties of X-Rays

• Appearance- X-rays are invisible
• Mass- X-rays have no mass or weight
• Charge- X-rays have no charge
• Speed- X-rays travel at the speed of light
• Wavelength- X-rays travel in waves and have short wavelengths with a high frequency
• Path of travel- X-rays travel in straight lines and can be deflected, or scattered
• Focusing capability- X-rays cannot be focused to a point and always diverge from a point
• Penetrating power- X-rays can penetrate liquids, solids, and gases
• The composition of the substance determines whether x-rays penetrate or pass through, or are absorbed
• Absorption- X-rays are absorbed by matter; the absorption depends on the atomic structure of matter and the wavelength of the x-ray
• Ionization capability- X-rays interact with materials they penetrate and cause ionization
• Fluorescence capability- X-rays can cause certain substances to fluoresce or emit radiation in longer wavelength
• Ex. Visible light, ultraviolet light
• Effect on receptor- X-rays can produce an image on a receptor
• Effect on living tissue- X-rays can cause biological changes in living cells

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The X-Ray Machine

• Control panel
• Extension arm
• Tubehead

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X-Ray Tube

• Cathode- Negative electrode supplies electrons necessary to generate x-rays
• Tungsten filament
• Molybdenum cup

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• Anode- Positive electrode, converts electrons into x-ray photons
• Tungsten target
• Copper stem

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• Electrons move from the cathode to the anode
• Cat -> Nap
• Cat- cathode
• N- negative
• A- anode
• P- positive

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X-Ray Generating Apparatus

• Electricity- used to produce x-rays; electrical energy flows through a conductor, this flow is known as electric current

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• Amperage- measurement of the number of electrons moving through a conductor; current is measured in *amperes *or milliamperes (mA)
• Voltage- measurement of electrical force that cause electrons to move from a negative pole to a positive pole; measured in volts, or kilovolts (kV)
• Circuit- path of electrical current
• Low-voltage circuit or filament circuit- 3-5 volts, controlled by mA
• Regulates the flow of electrical current to the cathode filament and works with step-down transformer
• High-voltage circuit- 65,000-100,000 volts, controlled by kV
• Force behind the electrons that move from the cathode to anode this is hooked up to the step-up transformer

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• Transformer- Increases or decrease the voltage in an electrical circuit
• Step-down transformer
• The step-down transformer decreases the voltage in the low-voltage circuit
• Step-up transformer
• The step-up transformer increases the voltage in the high-voltage circuit
• Auto transformer

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Production of X-Radiation

• 1.) When the filament circuit is activated, the filament heats up, and thermionic emission occurs
• Thermionic emission occurs in the negative cathode inside the x-ray tubehead
• 2.) When the exposure button is activated, the electrons are accelerated from the cathode to the anode
• 3.) The electrons strike the tungsten target, and their kinetic energy is converted to x-ray and heat
• Less than 1% converted to x-rays
• Remaining 99% lost as heat

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Types of X-Rays Produced

• Electrons strike the tungsten target in the X-ray tube
• The kinetic energy of the electrons is converted into x-ray photons though one of two mechanisms:
• Characteristic radiation
• 30%

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• General radiation
• Also know as Bremsstrahlung radiation
• 70%

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X-Radiation

• Primary
• The type of radiation that exits the tubehead
• Secondary
• Scatter
• Leakage
• Radiation that exits the PID (defect)

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Interactions of X-Radiation

• When the x-ray photons exit the PID and strike the patient one of the following events may occur…
• X-rays can pass through the patient without any interaction
• X-ray photons can be completely absorbed by the patient
• X-ray photons can be scattered

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• No interaction- When an X-ray photon passes through an atom unchanged, no interaction, no loss of energy

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• Photoelectric effect

• 30%

  

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• Compton Scatter
• 62%

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• Coherent Scatter
• 8%

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Photoelectric Effect

• When an x-ray photon collides with an inner shell electron giving up all of its energy to eject the electron
• The photon is absorbed and ceases to exist
• The ejected electron is referred to as photoelectron and has a negative charge
• Ionization occurs

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Compton Scatter

• When an x-ray photon collides with an outer shell electron and ejects the electron from its orbit
• The photon is scattered in a different direction at a lower energy
• The ejected electron is referred to as a Compton Scatter and has a negative charge
• Ionization occurs

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Coherent Scatter

• When an x-ray photon is scattered with no loss of energy, no ionization
• The scatter is termed coherent or unmodified scatter

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Summary

• An atom consists of a central nucleus composed of protons, neutrons, and orbiting electrons
• Most atoms exist in a neutral state and contain equal numbers of protons and neutrons
• When unequal numbers of protons and electrons exist, the atom is electrically unbalanced and is termed an ion
• The production of ions is termed ionization; an ion pair (a positive ion and a negative ion) is produced. The atom is the positive ion, and the ejected electron is the negative ion
• Ionizing radiation is capable of producing ions and can be classified as particulate or electromagnetic
• Electromagnetic radiations (e.g., x-rays) exhibit characteristics of both particles and waves and are arranged according to their energies
• The energy of an electromagnetic radiation depends on wavelength and frequency
• A low-energy radiation has a low frequency and a long wavelength; a high-energy radiation has a high frequency and a short wavelength
• X-rays are weightless, neutral bundles of energy (photons) that travel in waves with a specific frequency at the speed of light
• X-rays are generated in an x-ray tube located in the x-ray tubehead
• The x-ray tube consists of a leaded-glass housing, a negative cathode, and a positive anode. Electrons are produced in the cathode and accelerated toward the anode; the anode converts the electrons into x-rays
• After x-rays exit the tubehead, several interactions are possible: The x-rays may pass through the patient (no interaction), may be completely absorbed by the patient (photoelectric effect), or may be scattered (Compton scatter and coherent scatter)

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