1. Resident Physics Lecture
Christensen, Chapter 1
Radiation
George David
Associate Professor
Department of Radiology
Medical College of Georgia

2. Whoops, I think I just lost an electron
Physics Can Be Fun
Atom #1
Atom #2
Whoops, I think I just lost an electron
Are you sure?
Atom #1
Atom #2
Yeah, I’m positive
groan
George David
Associate Professor
Department of Radiology
Medical College of Georgia

3. Quicky Science Review

4. Memorize this. That’s an order!
Abbreviations
Memorize this. That’s 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) pico p (millionth millionth)
Angstrom = A = m

5. Energy Aside Kinetic Energy Potential 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

6. What’s the Smallest Thing that is Sugar?
Divide, divide, divide
The smallest entity that is still sugar is the sugar molecule

7. But What’s in that Sugar Molecule?
Different color balls?
No! Atoms

8. 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.

9. Ever Seen This?

10. Composition of the Atom
Protons
Neutrons
Electrons + -

11. Protons
Positive charge
Live in nucleus +

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

13. Electrons Negative charge Found outside the nucleus
Exist only in designated shell locations
Weighs only 1/1836th as much as proton - - +

14. Atomic Number Helium Also Helium # protons
Defines element & its properties
Color
State
Helium is helium because it has 2 protons
# neutrons does not affect chemistry
Helium
Also Helium + + - + - + - -

15. Atomic Weight # protons + # neutrons
# nucleons
A specific element 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 + + - + - + - -

16. Atomic Mass Unit (amu) Particle Amu Proton 1.00728 Neutron 1.00867
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
Amu
Proton
Neutron
Electron

17. Atomic Weight (# protons + # neutrons)
Atomic Symbol
Atomic Weight (# protons + # neutrons) He 2 4 + + -
Atomic # (# protons) -

18. How Many Electrons?
In a neutral atom (not negative or positive) # electrons = # protons + + - -

19. Unlike charges attract
Charge Theory
Unlike charges attract
Like charges repel + + + -

20. Coulomb Forces + - k q1 q2 F = ------------ Equation r2
F = Coulomb force
q’s = charges of the two objects
k = constant
r = distance between objects

21. Coulomb Equation Story
k q1 q2
F = r2
Force proportional to the magnitude of the charges + + - + + + + - +

22. 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 + + - - + -

23. Orbital Electrons X 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 - -

24. 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

25. 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

26. 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 + + + + - - -

27. Electron Shells
electrons attempt to reside in lowest available energy shell L K + + + + - - -

28. Electron Shells
electrons attempt to reside in lowest available energy shell L K - + + + + - -

29. The Shell Game Electrons can move from shell to shell*
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
Requires energy input! K - + + + + - -

30. 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 BUT
to move to a lower energy shell requires the release of energy equal to the difference between shells
characteristic x-rays L - K - + + + + -
Energy released

31. 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

32. Electromagnetic Radiation
Examples
x-rays
radio waves
microwaves
visible light
radiant heat

33. Electromagnetic Radiation
EM sometimes act like particles, sometimes like waves
Particle concept explains
radiation interactions with matter
Wave concept explains
refraction
diffraction
polarization

34. 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

35. Velocity = Wavelength X Frequency
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

36. Wavelengths and EM Low energy High energy
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

37. 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