RA 220 – RADIATION PHYSICS

1. RA 220 – RADIATION PHYSICS X-RAY PRODUCTION AND EMISSION

2. TARGET INTERACTION • X-rays produced by conversion of KINETIC energy to ELECTROMAGNETIC energy and HEAT • Three target interactions: • Heat production • Brems radiation • Characteristic radiation

3. HEAT PRODUCTION • More than 99% of energy is converted to heat • Outer shell electrons are “excited” – raised to a higher energy level • Infrared radiation emitted as electrons return to normal

4. HEAT PRODUCTION • Heat production increases DIRECTLY with increasing MA and with increasing KVP • Efficiency of x-ray production is INDEPENDENT of MA but varies DIRECTLY with KVP • 60 KVP - .5% x-ray • 100 KVP – 1% x-ray • 20 MV – 70% x-ray

5. BREMSSTRAHLUNG RADIATION • Interaction between projectile electron and nucleus of target atom • Electron enters atom and passes the nucleus • Electron attracted to positive nucleus • Electron slows down and changes course • Kinetic energy is lost in the form of an x-ray photon

6. BREMSSTRAHLUNG RADIATION

7. BREMSSTRAHLUNG RADIATION • Energy of Brems radiation is HETEROGENOUS – many different energies • Energy of Brems photon is equal to the difference between the kinetic energy of the entering and exiting electron • Low energy photon occurs with a small interaction • Higher energy photon occurs with a greater interaction • Highest energy photon occurs with a collision with the nucleus

8. BREMSSTRAHLUNG RADIATION Incoming Electron = 100 After interaction = 20 Photon = ? Incoming Electron = 100 After interaction = 70 Photon = ? nucleus nucleus

9. CHARACTERISTIC RADIATION • Interaction between a projectile electron and an inner orbital electron in target atom • Electron with sufficient energy collides with inner orbital electron (k-shell) and ejects it from its orbit (photoelectron) • Atom is ionized and is left with a void in the shell • Electron from another shell moves in to fill void • X-ray energy is released

10. CHARACTERISTIC RADIATION

11. CHARACTERISTIC RADIATION • Energy of the emitted photon is equal to the difference between the binding energies of the two target electrons involved • Energy is CHARACTERISTIC of the shell involved • The photon energy will always be the same when the same shell is involved

12. CHARACTERISTIC RADIATION

13. CHARACTERISTIC RADIATION • Tungsten has electron shells K to P • Energy example #1: Energy example #2: Eject K shell electron Eject L shell electron L to K = 59.0 keV M to L = 9.3 keV M to K = 67.2 N to L = 11.5 keV N to K = 69.1 O to L = 12.0 keV O to K = 69.4 P to L = 12 keV P to K = 69.5 • Effective energy=69 keV Effective energy = 12 keV

14. CHARACTERISTIC RADIATION • In order to produce characteristic radiation the energy of the incoming electron must be greater than the binding energy of the electron shell involved • A K-shell interaction requires an electron with greater than 69.5 KV energy to dislodge it

15. CHARACTERISTIC RADIATION • NOTE: only K-shell interactions produce useful beam energies!

16. X-RAY EMISSION SPECTRUM • Graphic representation of x-ray beam • CONTINUOUS – represents Brems radiation • DISCRETE – represents Characteristic radiation

17. X-RAY EMISSION SPECTRUM

18. X-RAY EMISSION SPECTRUM • Brems x-ray spectrum • Brems rays have a range of energies • If a beam is created at 80 KVp, it will have brems radiation up to what energy level? • Characteristic x-ray spectrum • Characteristic rays have a single energy • If a beam is created at 80 KVp it will have characteristic radiation at what energy level?

19. FACTORS AFFECTING X-RAY EMISSION SPECTRUM • MA/MAS • KVP • FILTRATION • TUBE TARGET MATERIAL • VOLTAGE WAVEFORM

20. MA/MAS • MA or MAS changes the total quantity of radiation of all energies in the beam – proportional change • Amplitude of the spectrum is changed, but shape remains the same

21. KVP • Kvp affects both quantity and quality of beam • Changing KVp changes both the amplitude and the shape of spectrum • Changing KVp changes the minimum wavelength – peak energy photon produced

22. FILTRATION • Filtration DECREASES beam quantity and INCREASES average energy • Adding filtration causes spectrum to decrease in amplitude and shift to the left • Less radiation is in the beam, but the average energy is higher

23. TUBE TARGET MATERIAL • Atomic number of target affects quantity and quality of beam • As Z# increases efficiency of production increases • Both brems energy and characteristic energy increase

24. VOLTAGE WAVEFORM • 3-Phase and high frequency are more efficient in producing photons • Reducing voltage ripple causes an increase in the quantity and quality of radiation • Spectrum amplitude increases and shifts to the right

25. X-RAY QUANTITY • Number of photons in the beam (intensity, exposure) • Measured in Roentgens (C/kg) – number of ion pairs produced in air • Also expressed as EXPOSURE RATE– R/min, mR/s, mR/mAs • Total exposure = Rate X Time

26. FACTORS AFFECTING X-RAY QUANTITY • MA – MAS • Directly proportional • KVP • Proportional to KVP2 • DISTANCE • Inverse square law • FILTRATION • Added filtration reduces beam quantity • Fewer photons in the beam

27. X-RAY QUALITY • Ability to penetrate matter • Energy of the beam – wavelength/frequency • Measured by HALF VALUE LAYER • The thickness of filtration it takes to reduce the beam quantity to one half its original value • Higher HVL = higher quality • Table 9-3 p.157

28. FACTORS AFFECTING X-RAY QUALITY • KVP • Increase KVP = Increase quality • FILTRATION • Increase filtration = Increase quality • Removes the lower energy photons • Types of filters • Inherent • Added • Compensating