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Resident Physics Lecture
• Christensen,
Chapter 1
Radiation
George David
Associate Professor
Department of Radiology
Medical College of Georgia
Physics Can Be Fun
Whoops, I
think I just lost
an electron
Atom #1
Are you sure?
Atom #2
Yeah, I’m
positive
groan
Atom #1
Atom #2
George David
Associate Professor
Department of Radiology
Medical College of Georgia
Quicky
Science
Review
Abbreviations
Memorize this.
That’s an order!
109
106
103
10-1
10-2
10-3
10-6
10-9
10-12
giga
mega
kilo
deci
centi
milli
micro
nano
pico
G
M
K
d
c
m
m
n
p
(billion)
(million)
(thousand)
(tenth)
(hundredth)
(thousandth)
(millionth)
(billionth)
(millionth millionth)
Angstrom = A = 10-10 m
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
What’s the Smallest Thing
that is Sugar?
• Divide, divide, divide
• The smallest entity
that is still sugar is the
sugar molecule
But What’s in that Sugar
Molecule?
• Different color balls?
• No! Atoms
Interesting Fact You Already Knew
• There are zillions of different
types of molecules
• There are only 92 naturally
occurring types of atoms
That’s way
cool.
Ever Seen This?
Composition of the Atom
• Protons
+
• Neutrons
• Electrons
-
Protons
• Positive charge
• Live in nucleus
+
Neutrons
• No charge (free?)
• Live in nucleus
• Ever-so-slightly
more mass than
proton
• Better than
oldtrons?
Electrons
• Negative charge
• Found
surrounding the
nucleus
– Exist only in designated
shell locations
• Weighs only
1/1836th as
much as proton
+
Atomic Number
• # protons
• Defines element & its properties
– Color
– State
• Helium is helium because it has 2
protons
– # neutrons does not affect chemistry
-
+
+
-
+
+
-
Atomic Weight
• # protons + # neutrons
– # nucleons
• A specific element often found
with multiple atomic weights
– Always the same # protons
– Different # neutrons
Helium
Atomic Weight=4
-
+
Helium
Atomic Weight=3
+
-
+
+
-
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
Particle
Proton
Neutron
Electron
Amu
1.00728
1.00867
.000549
Atomic Symbol
Atomic Weight
(# protons + # neutrons)
4
2
Atomic #
(# protons)
He
-
+
+
-
How Many Electrons?
• In a neutral atom (not negative
or positive)
# electrons = # protons
-
+
+
-
Charge Theory
Unlike
charges
attract
-
+
Like charges
repel
+
+
Coulomb Forces
• Equation
+
k q1 q2
F = -----------r2
F = Coulomb force
q’s = charges of the two objects
k = constant
r = distance between objects
Coulomb Equation Story
k q1 q2
F = -----------r2
• Force proportional to the
magnitude of the charges
-
+
+
+
-
+
+
+
+
+
Coulomb Equation Story
• Force falls off with the square of
k q1 q2
distance
F = -----------– Twice as far: one quarter the force
r2
– Three times as far: one ninth the force
+
+
+
-
+
-
+
+
+
+
+
+
+
+
+
-
+
+
+
+
+
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
-
~ +
~ +
+ ~
X-
-
Electrons & Shells
• Atom mostly empty space
– If atom were a baseball stadium nucleus would be size of baseball
• Nucleus contains almost all of atom’s
mass
• Electron shells determine element’s
chemical properties
Shell Capacities
Shell Electron
Capacity
(2x2)
1 (k)
2
2 (l)
8
3 (m)
18
4 (n)
32
5 (o)
50
6 (p)
72
7 (q)
98
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
L
K
~ +
~ +
+ ~
-
-
Electron Shells
• electrons attempt to reside in
lowest available energy shell
L
K
~ +
~ +
+ ~
-
-
Electron Shells
• electrons attempt to reside in
lowest available energy shell
L
K
~ +
~ +
+ ~
-
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
K
~ +
~ +
+ ~
-
-
-
Requires
energy
input!
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 gap
L
K
BUT
• to move to a lower energy
shell requires the release of
energy equal to the difference
between shells
– characteristic x-rays
~ +
~ +
+ ~
-
Energy
released
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
Electromagnetic Radiation
• Examples
–
–
–
–
–
x-rays
radio waves
microwaves
visible light
radiant heat
Electromagnetic Radiation
•EM sometimes act like
particles, sometimes like waves
•Particle concept explains
•radiation interactions with matter
•Wave concept explains
•refraction
•diffraction
•polarization
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 = Planck’s Constant X Frequency
E = hn
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
Wavelengths and EM
Highest wavelength = lowest frequency
Radio
Infrared
Visible light
Ultraviolet
Soft x-rays
Diagnostic x-rays
Therapeutic x-rays & gammas
Lowest wavelength = highest frequency
Velocity = Wavelength X Frequency
c=l X n
Energy vs. Wavelength as Equations
Energy = Planck’s 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