Chapter 2: Radiation Physics

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

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)