Radiation Physics

Chapter 2

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

Fundamental Concepts

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

• All matter is composed of atoms

Atomic Structure

• The atom consists of 2 parts:

• Central nucleus

• Orbiting electrons

• 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

• 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

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

Ion Pair

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

Radiation

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

Radioactivity

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

Ionizing Radiation

• Radiation that is capable of producing ions by removing or adding an electron to an atom

• 2 types:

• Particulate

• *Electromagnetic *

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

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

• 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

• 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

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)

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

The X-Ray Machine

• Control panel

• Extension arm

• Tubehead

X-Ray Tube

• Cathode- Negative electrode supplies electrons necessary to generate x-rays

• Tungsten filament

• Molybdenum cup

• Anode- Positive electrode, converts electrons into x-ray photons

• Tungsten target

• Copper stem

• Electrons move from the cathode to the anode

• Cat -> Nap

• Cat- cathode

• N- negative

• A- anode

• P- positive

X-Ray Generating Apparatus

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

• 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

• 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

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

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%

• General radiation

• Also know as Bremsstrahlung radiation

• 70%

X-Radiation

• Primary

• The type of radiation that exits the tubehead

• Secondary

• Scatter

• Leakage

• Radiation that exits the PID (defect)

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

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

• Photoelectric effect

• 30%

• Compton Scatter

• 62%

• Coherent Scatter

• 8%

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

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

Coherent Scatter

• When an x-ray photon is scattered with no loss of energy, no ionization

• The scatter is termed coherent or unmodified scatter

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)