Resident Physics Lecture

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Resident Physics Lecture
http//www.radiology.mcg.edu/radiologyphysics/

• Christensen, Chapter 1
• Radiation

George David Associate Professor Department of
Radiology Medical College of Georgia
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Physics Can Be Fun
George David Associate Professor Department of
Radiology Medical College of Georgia
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Quicky Science Review
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Abbreviations
Memorize this. Thats an order!

• 109 giga G (billion)
• 106 mega M (million)
• 103 kilo K (thousand)
• 10-1 deci d (tenth)
• 10-2 centi c (hundredth)
• 10-3 milli m (thousandth)
• 10-6 micro m (millionth)
• 10-9 nano n (billionth)
• 10-12 pico p (millionth
  millionth)

Angstrom A 10-10 m
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Energy Aside

• Kinetic Energy
• Energy of an object by virtue of its speed
• K.E. (1/2) X mv2
• m is mass
• v velocity
• Potential Energy
• Energy of an object by virtue of its position

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Whats the Smallest Thing that is Sugar?

• Divide, divide, divide
• The smallest entity that is still sugar is the
  sugar molecule

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But Whats in that Sugar Molecule?

• Different color balls?
• No! Atoms

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Interesting Fact You Already Knew

• There are zillions of different types of
  molecules
• There are only 92 naturally occurring types of
  atoms

Thats way cool.
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Ever Seen This?
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Composition of the Atom

• Protons
• Neutrons
• Electrons

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Protons

• Positive charge
• Live in nucleus

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Neutrons

• No charge (free?)
• Live in nucleus
• Ever-so-slightly more mass than proton
• Better than oldtrons?

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Electrons

• Negative charge
• Found surrounding the nucleus
• Exist only in designated shell locations
• Weighs only 1/1836th as much as proton

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Atomic Number

• protons
• Defines element its properties
• Color
• State
• Helium is helium because it has 2 protons
• neutrons does not affect chemistry

Helium
Also Helium




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Atomic Weight

• protons neutrons
• nucleons
• A specific element often found with multiple
  atomic weights
• Always the same protons
• Different neutrons

HeliumAtomic Weight4
HeliumAtomic Weight3




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Atomic Mass Unit (amu)

• Nominally
• 1 amu the weight of a proton or neutron
• Officially
• 1 amu 1/12 the weight of a carbon-12 atom
• 6 protons
• 6 neutrons

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Atomic Symbol
Atomic Weight( protons neutrons)


Atomic ( protons)
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How Many Electrons?

• In a neutral atom (not negative or positive)
  electrons protons

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Charge Theory

• Unlike charges attract

Like charges repel



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Coulomb Forces
k q1 q2 F ------------ r2

• Equation

F Coulomb force qs charges of the two
objects k constant r distance between objects
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Coulomb Equation Story
k q1 q2 F ------------ r2

• Force proportional to the magnitude of the charges

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Coulomb Equation Story

• Force falls off with the square of distance
• Twice as far one quarter the force
• Three times as far one ninth the force

k q1 q2 F ------------ r2
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Orbital Electrons

• Electrons
• - charge
• very small mass compared with protons / neutrons
• Electrons reside only at certain energy levels or
  Shells
• Designations start at K shell
• K shell closest to nucleus
• L shell next closest
• Shells proceed up from K, L, M, N, etc.
• Except for K shell, all shells contain sub-shells

L
K
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X
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Electrons Shells

• Atom mostly empty space
• If atom were a baseball stadium nucleus would be
  size of baseball
• Nucleus contains almost all of atoms mass
• Electron shells determine elements chemical
  properties

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Shell Capacities
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Binding Energy

• Negative electrons attracted to positive nucleus
• more binding energy for shells closer to nucleus
• K shell has highest binding force
• higher atomic materials (higher Z) result in
  more binding energy
• more positive charge in nucleus
• energy required to remove orbital electron from
  atom

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Electron Shells

• electrons attempt to reside in lowest available
  energy shell

L
K
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Electron Shells

• electrons attempt to reside in lowest available
  energy shell

L
K
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The Shell Game

• Electrons can move from shell to shell
• to move to higher energy shell requires energy
  input equal to difference between the binding
  energy of the two shells

L
Requiresenergyinput!
K
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The Shell Game

• An atom with a gap in a lower shell is unhappy
  (unstable)
• Electrons will attempt to drop to lower shells to
  fill the gapBUT
• to move to a lower energy shell requires the
  release of energy equal to the difference
  between shells
• characteristic x-rays

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Energyreleased
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Electromagnetic Radiation

• Transport of energy through space
• Properties of EM are combination of
• electric fields
• magnetic fields
• X-rays are one form of electromagnetic radiation
• No transport medium required

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Electromagnetic Radiation

• Examples
• x-rays
• radio waves
• microwaves
• visible light
• radiant heat

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Electromagnetic Radiation

• EM sometimes act like particles, sometimes like
  waves
• Particle concept explains
• radiation interactions with matter
• Wave concept explains
• refraction
• diffraction
• polarization

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Particle concept (cont)

• X-rays are discrete bundles of energy
• quantum or photon
• Photon Energy proportional to frequency
• higher frequency higher energy
• energy measured in electron volts (eV)
• amount of energy gained by an electron
  accelerated by potential of 1 volt

Energy Plancks Constant X Frequency
E hn
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Wave Properties of EM

• Wavelength
• distance between successive waves
• Frequency
• number of waves passing a particular point per
  unit time
• Velocity (c) of light / x-rays
• 186,000 miles/second OR
• 3 X 108 meters/second
• Wavelength frequency
• inversely proportional

Velocity Wavelength X Frequency c l X n
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Wavelengths and EM
Highest wavelength lowest frequency
Radio Infrared Visible light Ultraviolet Soft
x-rays Diagnostic x-rays Therapeutic x-rays
gammas
Low energy
High energy
Lowest wavelength highest frequency
Velocity Wavelength X Frequency c l X n
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Energy vs. Wavelength as Equations
Energy Plancks Constant X Frequency E
hn but Frequency Speed of Light / Wavelength n
c / l so E hc / l Energy (keV) 12.4 /
Wavelength (in Angstroms) E 12.4 / l