Basics of X-Ray Production: Nuclear Physics and Electron Shells

1. Resident Physics Lectures • Christensen, Chapter 2C Production of X-Rays George David Associate Professor Department of Radiology Medical College of Georgia

2. The Atomic Nucleus + • Protons + Charges # protons = atomic # (Z) • Neutrons • No charge • Mass about the same as proton Atomic Weight(mass)= # protons + # neutrons + + ~ ~ ~

3. Single Phase Three Phase kVp = kilovolts peak * kVp • peak kilovoltage applied across x-ray tube • voltage applied across x-ray tube pulses and varies • single phase • three phase kVp

4. + keV = kilo-electron volt • energy of an electron • Kinetic energy • Higher energy electron moves faster • Electrons can be manipulated by electric fields • Accelerated • Steered

5. Orbital Electrons • Electrons - charges 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 - + ~ + ~ + ~ - -

6. L K - + ~ + ~ + ~ - - Binding Energy • energy required to remove orbital electron from atom • 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

7. Electron Shells (cont.) • Electrons can only reside in a shell • electron has exactly the energy associated with its shell • electrons attempt to reside in lowest available energy shell L K - + ~ + ~ + ~ - - -

8. The Shell Game * • Electrons can move from shell to shell • to move to higher energy shell requires energy input equal to difference between shells L Requiresenergyinput! K - + ~ - + ~ + ~ - -

9. L K - + ~ + ~ + ~ Energyreleased - The Shell Game (cont.) • to move to a lower energy shell requires the release of energy equal to the difference between shells • characteristic x-rays - -

10. X-Ray Production(cont.) • X-Rays are produced in the x-ray tube by two distinct processes • Characteristic radiation • Bremsstrahlung

11. L K - + ~ + ~ + ~ - Characteristic Radiation • Occurs whenever electrons drop into lower shell • Inner shell has lower energy state • Energy difference between shells emitted as characteristic x-ray • 0-28% of total x-ray beam energy - -

12. L K - + ~ + ~ + ~ - Characteristic Radiation + • High speed electron from cathode slams into target knocking out inner shell orbital electron • orbital electron removed from atom • electrons from higher energy shells cascade down to fill vacancies • Characteristic x-rays emitted. - -

13. L K - + ~ + ~ + ~ - - - Characteristic Radiation • Consists only of discrete x-ray energies corresponding to energy difference between electron shells of target • Specific energies are characteristic of target material • for tungsten 59 keV corresponds to the difference in energy between K and L shells

14. + ~ + ~ + ~ Characteristic Radiation (cont.) • threshold energy required for incident electron (from cathode) to eject orbital electron = electron’s binding energy L K - -

15. L K - + ~ + ~ + ~ - - Bremsstrahlung • interaction of moving electron with nucleus of target atoms • Positive nucleus causes moving electron to change speed / direction • Kinetic energy lost • Emitted in form of Bremsstrahlung x-ray -

16. L K - + ~ + ~ + ~ - - - Bremsstrahlung (cont.) • Bremsstrahlung means braking radiation • Moving electrons have many Bremsstrahlung reactions • small amount of energy lost with each

17. Bremsstrahlung (cont.) • Energy lost by moving electron is random & depends on • distance from nucleus • charge (Z) of nucleus • Bremsstrahlung Energy Spectrum 0 - peak kilovoltage (kVp) applied to x-ray tube • most x-ray photons low energy • lowest energy photons don’t escape tube • easily filtered by tube enclosures or added filtration

18. Beam Intensity • Product of • # photons in beam • energy per photon • Units • Roentgens (R) per unit time • Measure of ionization rate of air • Depends on • kVp • mA • target material • filtration • waveform

19. Intensity & Target Material • higher target atomic # results in greater x-ray production efficiency • higher positive charge of nucleus causes more Bremsstrahlung • discrete energies of characteristic radiation determined by anode material • Energy differences between shells • molybdenum used in mammo • characteristic radiation of 17 & 19 keV

20. Intensity & Technique • beam intensity proportional to mA • beam Intensity ~ proportional to kVp2 filament voltage source + high voltage source