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. 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 nucleus
Exist only in designated shell locations
Weighs 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
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 + + - + - + - -

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. Why Doesn’t the Nucleus Fly Apart with all those + Charges?
??????????????

24. “c”, the speed of light, is a really big number.
E = mc2
Energy & mass are equivalent. A little bit of mass is equivalent to a whole lot of energy.
“c”, the speed of light, is a really big number.

25. What do two cars & three trains weigh?
Total Mass
Car weighs 2 oz.
Train weights 3 oz.
What do two cars & three trains weigh?

26. Total Mass But not in the nucleus!!! + + + +
mass of the total = mass of the parts + + + +
But not in the nucleus!!!

27. < Mass Defect mass of the total < mass of the parts+ - + -
<
Where did the mass go?

28. Where did the mass go? Binding energy of the nucleus of course
Mass Defect + - + -
<
Where did the mass go? Binding energy of the nucleus of course

29. And this is what we call radioactivity.
Nuclear Stability
How can unstable nuclei become more stable?
Reduce energy
Ejecting energy (photon) OR
Ejecting mass
And this is what we call radioactivity.
Gamma
Beta
Alpha

30. Nuclear Stability Displays # protons & # neutrons for stable isotopes
Stable nuclei found in nature
Reference line where # protons = # neutrons

31. Nuclear Stability Related to neutron to proton ratio
Too high or too low = unstable
Decay tends to try to “center” the ratio
Ratio close to 1:1 for low atomic #’s
Ratio climbs with atomic #
1.6:1 for lead
High atomic #’s tend to be less stable

32. Some Definitions Isotopes Isotones Isobars Isomers Isobergs
Nuclides with same # protons
Isotones
Nuclides with same # neutrons
Isobars
Nuclides with same atomic weight
Isomers
Nuclides with same # protons & neutrons but different energy states
Isobergs
Not really

33. Examples ISOTOPES: I ISOTONES: Cs Xe I ISOBARS: Xe Cs I ISOMERS: Tc
125
127
131 53
ISOTONES: Cs
133 55 Xe
132 54 I
131 53
ISOBARS: Xe
131 54 Cs 55 I 53
ISOMERS: Tc
99M 99
ISOCREAM

34. Orbital Electrons X 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 - -

35. 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

36. 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

37. 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
K shell has highest binding force
higher atomic # materials (higher Z) result in more binding energy
more positive charge in nucleus L - K + + + + - - -

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

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

40. The Shell Game Electrons can move from shell to shell*
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 - + + + + - -

41. The Shell Game
Electron in higher shell & gap in lower shell = Potential Energy
An atom with a gap in a lower shell is unhappy (unstable)
Electrons will attempt to drop to lower shells to fill the gap K L + -

42. The Shell Game
For atom to move to lower energy shell, atom must release energy
Equal to difference between binding energy of shells
Form of energy release
characteristic x-rays L - K - + + + + -
Energy released

43. 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

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

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

46. 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

47. 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

48. 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

49. 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