Resident Physics Lecture

1. Resident Physics Lecture http://www.radiology.mcg.edu/radiologyphysics/ • Christensen, Chapter 1 Radiation George David Associate Professor Department of Radiology Medical College of Georgia

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

3. Quicky Science Review

4. 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) 10-12 pico p (millionth millionth) Angstrom = A = 10-10 m

5. 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

6. What’s the Smallest Thing that is Sugar? • Divide, divide, divide • The smallest entity that is still sugar is the sugar molecule • Molecules made up of atoms

7. Interesting Fact You Already Knew • There are only 92 naturally occurring types of atomsHOWEVER • There are zillions of different types of molecules That’s way cool.

8. Ever Seen This?

9. Composition of the Atom + • Protons • Neutrons • Electrons -

10. Protons + • Positive charge • Live in nucleus

11. Neutrons • No charge (free?) • Live in nucleus • Ever-so-slightly more mass than proton

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

13. 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 + + - + - + - -

14. 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) HeliumAtomic Weight=4 HeliumAtomic Weight=3 + + - + - + - -

15. 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

16. 4 He 2 Atomic Symbol Atomic Weight(# protons + # neutrons) + + - Atomic #(# protons) -

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

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

19. Coulomb Force Between Charged Particles + - k q1 q2 F = ------------ r2 • Equation F = Coulomb force q’s = charges of the two objects k = constant r = distance between objects

20. Coulomb Equation Story k q1 q2 F = ------------ r2 • Force proportional to magnitude of charges • Higher charge => More force + + - + + + + - +

21. Coulomb Equation Story k q1 q2 F = ------------ r2 • Force falls off with square of distance • 2X as far: 1/4 force • 3X as far: 1/9 force + + + + + + - - + + + + + + + + + Small distance => Large force - Long distance => Small force

22. If like charges repel, why doesn’t the nucleus fly apart?

23. Theory of Relativity

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

25. Putting it in perspective. How is this possible?

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

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

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

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

30. Nuclear Stability • How can unstable nuclei become more stable? • Reduce energy • Ejecting energy (photon)OR • Ejecting mass Gamma Alpha Beta

31. Nuclear Stability • # protons vs # neutrons for stable isotopes Stable nuclei found in nature Reference line where # protons = # neutrons

32. 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

33. Some Definitions • Isotopes • 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

34. 131 125 127 I ISOTOPES: I I 53 53 53 131 I 132 133 ISOTONES: Xe Cs 53 54 55 131 I 131 131 53 Xe Cs ISOBARS: 54 55 99M 99 Tc Tc ISOMERS: ISOCREAM Examples

35. 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 - -

36. Electrons & Shells • Atom mostly empty space • If atom were baseball stadium, nucleus would be size of baseball • Nucleus contains almost all of atom’s mass • Electron shells determine element’s chemical properties

37. Shell Capacities

38. 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 + + + + - - -

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

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

41. * 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 Requiresenergyinput! K - + + + + - -

42. The Shell Game L • Gap in lower electron shell = Potential Energy • An atom with a gap in a lower shell is unhappy (unstable) • Electrons will attempt to fill gapin lower shells - K - + + + + -

43. The Shell Game L • 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 - K - + + + + - Energyreleased

44. 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

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

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

47. 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

48. 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

49. 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

50. 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