8. X-RAYS: INTERACTIONS OF HIGH ENERGY ELECTRONS WITH MATTER

Question 1

1. Which kinds of electrons are accelerated towards the anode?

Answer 1

• electrons that are released from the filament of the X-
  Ray Tube
• they are released by Thermionic Emission

Question 2

2. How is Thermionic Emission accomplished?

Answer 2

• it is accomplished using a potential difference
• this potential difference is found between the cathode
  and the anode

Question 3

3. What is another term for: “Potential Difference”?

Answer 3

• Voltage

Question 4

4. What determines the number of electrons released in each exposure?

Answer 4

• the selected mA (current)

Question 5

5. How do we find out the total number of X-Rays produced during an exposure?

Answer 5

• this is set by the milliamperes per second (mAs)
• this is the product of exposure time (TE)
  in seconds
  AND the electron current (mA)

Question 6

6. What determines the Kinetic Energy acquired by the electrons?

Answer 6

• the selected kV
• this is the Voltage
  (Potential Difference)

Question 7

7. What happens to the electrons as they reach the anode?

Answer 7

• the interact with the atoms of the anode

Question 8

8. What are the 2 Parameters of an X-Ray beam?

Answer 8

1. QUALITY OF THE X-RAY BEAM
   - this is determined by the energy of the electrons
   - the kV x p
2. QUANTITY OF THE X-RAY BEAMS
   • this is the amount
   • it is the number of electrons
   • the mAs

Question 9

9.What are the 3 types of interactions that will occur between the electrons and the atoms of the Anode?

Answer 9

1. Incoming electrons will interact with the outer-shell
   electrons of the target atoms
2. Incoming electrons will interact with the inner-shell
   electrons of the target atoms
3. Incoming electrons will interact with the nuclei of the
   target atoms

Question 10

10. How many interactions do incoming electrons usually undertake with the target atoms?

Answer 10

• around 1000 interactions
• then they give up all their kinetic energy
• these interactions will occur within 0.5mm of the
  surface of the target

Question 11

11. What is Tungsten’s Atomic Number?

Answer 11

• 74
• this means that there are 74 electrons in each atom of
  the target material
• all these electrons are available for interactions
• most of these electrons are found on the outer orbitals
  of each of the Tungsten atoms

NB:
- the incoming electrons will require relatively small
amounts of energy to interact with the shell-bound
electrons

Question 12

12. What happens when incoming electrons interact with the electrons in Tungsten atoms?

Answer 12

• small amounts of energy are released as
  electromagnetic radiations
• the energy amounts are too small to produce X-Rays
• this means that virtually all the energy is release as
  heat

Question 13

13. What results in the electromagnetic radiations being released during interactions between electrons?

Answer 13

• Electrostatic Repulsion

Question 14

14. What percentage of energy produced at the target of the X-Ray tube occurs in the form of heat?

Answer 14

• 99%

Question 15

15. Which 2 interactive processes between incoming electrons and target atoms produce X-Rays?

Answer 15

1. Characteristic Radiation
2. Bremsstrahlung Radiation

Question 16

16. Provide the descriptions for parts of this diagram numbered 1-6.

Answer 16

1. INCOMING FAST ELECTRONS
   
   • these collide with the electron from the inner orbital
     of the atom
2. EJECTED ELECTRON
3. VACANT ELECTRON ORBITAL POSITION
4. ELECTRON FROM ANOTHER ORBITAL FILLS THE
   VACANCY
   
   • this is usually from a higher orbital
5. ENERGY IS RELEASED AS A CHARACTERISTIC X-RAY
6. THE FURTHER AWAY FROM THE NUCLEUS THE
   ORBITAL IS:
   
   • the higher the orbital energy will be

Question 17

17. What kinds of shells are K and L Orbital Shells?

Answer 17

• inner shells

Question 18

18. What do the two orbital shells in Tungsten allow for?

Answer 18

• the ejection of a electron
• this leads to the emission of a photon
• this photon has Electromagnetic Radiation

Question 19

19. When can these Electromagnetic Radiated Photons be perceived as X-Rays in nature?

Answer 19

• when they have the sufficient energy to escape from
  the X-Ray tube

Question 20

20. What are 2 types of Characteristic Radiation reactions?

Answer 20

1. EXCITATION
2. IONISATION

Question 21

21. What is Excitation?

Answer 21

• this is when sufficient energy is given to the orbital
  bound electron
• this electron will now be raised to the next orbital

Question 22

22. What is Ionisation?

Answer 22

• this is when the binding energy of an orbital electron is
  overcome
• the electron is released from the atom

Question 23

23. What can be said about the removal of the electron from its orbital shell during Excitation?

Answer 23

• the electron has only been removed temporarily
• the electron remains within the structure of the atom
• the electron ultimately returns back to its original
  position

Question 24

24. What does Excitation result in?

Answer 24

• it results in a photon being given out
• this photon has the same amount of energy as the
  energy gained by the electron when it raised itself to a
  different orbital shell

Question 25

25. What is the released electron known as when it comes to Ionisation?

Answer 25

- it is known as a Delta Ray - it carries kinetic energy with it - this energy is donated during the interaction

Question 26

26. What will the Delta Ray do during Ionisation?

Answer 26

- it will interact with other atoms - this will continue until it has lost all of its acquired energy

Question 27

27. When does Ionisation occur?

Answer 27

- it only occurs if the kinetic energy that carried by the incoming electron is equal to or greater than the specific binding energy of the orbital bound electron

Question 28

28. What can be said about the electron energies when it comes to Tungsten?

Answer 28

- the K-Orbital Binding energy is 69.5 keV - the L-Orbital Binding energy is 10.2 keV

Question 29

29. What happens when the K-Shell Electron is ionised?

Answer 29

- it will leave the atom with a vacancy in the K-shell orbital - this kind of vacancy cannot be sustained in nature IMMEDIATELY: -an electron from an outer orbital shell will drop into the K-Shell

Question 30

30. What happens once the outer orbital electron has dropped into the K-Shell Orbital vacancy?

Answer 30

- an X-Ray will be released - its energy value will be equal to: the energy value of the donating shell MINUS the energy value of the receiving shell

Question 31

31. What happens if an electron moves from the L-Shell to the K-Shell?

Answer 31

- this is known as a K-Alpha emission

Question 32

32. What happens if an electron moves from the M-Shell to the K-Shell?

Answer 32

- this is known as a K-Betta Emission

Question 33

33. What will the removal of an inner shell electron always result in?

Answer 33

- it will always lead to the production of X-Rays - these X-Rays fall into well defined energy bands - these energy bands are characteristic for each atom - these radiations are characteristic of the material in which they are produced