Download Radiation

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
Memorize this.
That’s an order!
109
106
103
10-1
10-2
10-3
10-6
10-9
10-12
Abbreviations
giga
mega
kilo
deci
centi
milli
micro
nano
pico
G (billion)
M (million)
K (thousand)
d (tenth)
c (hundredth)
m (thousandth)
m (millionth)
n (billionth)
p (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  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
What’s in that Sugar Molecule?
 Different color balls?
 No! Atoms
Interesting Fact You Already Knew
 There are only 92 naturally occurring types of
atoms
HOWEVER
 There are zillions of different types of molecules
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 outside
-
nucleus
 Exist only in designated
shell locations
 Weighs 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
 Specific elements often found with multiple atomic
weights (isotopes)
 Always the same # protons
 Different # neutrons
 For a particular element, some isotopes may be stable,
others unstable (radioactive)
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

Atomic # = 6
Particle
Proton
Neutron
Electron
Amu
1.00728
1.00867
.000549
Atomic Symbol
Atomic Weight
(# protons + # neutrons)
Atomic #
(# protons)
4
He
2
-
+ +
-
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 distance
k q1 q2
 Twice as far: one quarter the force
F = -----------r2
 Three times as far: one ninth the force
-
+
+
+
+
+
+
+
+
-
+
+
+
-
+
+
+
+
Orbital Electrons
 Electrons
“-” charge
very small mass compared to 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
 Definition
 Energy required to remove
orbital electron from atom
 Ionization
 Negative electrons attracted to
positive nucleus
 more binding energy for shells
closer to nucleus
-
L
K
-
+
+
+
+
 K shell has highest binding force
 higher atomic # materials (higher
Z) result in more binding energy
 more positive charge in nucleus
-
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
 Excitation
Deposit
energy here
L
Requires
energy
input!
K
-
+
+
-
+
+
The Shell Game
 Electron in higher shell & gap in lower
shell = Potential Energy
L
K
-
+
 An atom with a gap in a lower shell is
unhappy (unstable)
+
-
 Electrons will attempt to drop to lower
shells to fill the gap
+
+
The Shell Game
 For atom to move to lower energy
L
-
shell, atom must release energy
K
 Equal to difference between binding
energy of shells
 Form of energy release
-
+
+
+
-
 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)
•energy gained by electron accelerated by 1
volt potential
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