5. IONISING RADIATION: IONISATION CHAMBERS

Question 1

1. Why can electricity not flow through air?

Answer 1

• there are no free electrons or ions to carry the current

Question 2

2. What is an Electric Current?

Answer 2

• it is simply the flow of electrons or ions

Question 3

3. What would allow for electricity to flow through air?

Answer 3

• it some of the atoms on the air would become ionised
• free electrons in the air would be produced
• an electric current can flow

Question 4

4. What happens during a flash of lightning?

Answer 4

• a very high potential gradient exists between the cloud
  and the ground
• this potential gradient is sufficient enough to ionise the
  air
• this allows current to flow

Question 5

5. Provide labels for the numbered spaces?

Answer 5

1. The Ionisation Chamber is filled
   with air
2. Electrons
3. Protons

Question 6

6. Explain what is happening in this diagram.

Answer 6

• the radiation ionises the air in the chamber
• this leads to the movement of negative charges to
  positive potentials (charges)
• this allows us to measure the current
• this results in the amount of radiation being revealed

Question 7

7. What happens in an Ionising Chamber?

Answer 7

• ionising radiation frees electrons into the air
• the electrons then fill the chamber
• this allows for a current to flow
• this current is measured by a very sensitive ammeter

Question 8

8. What is applied across the metal plates in an Ionisation Chamber?

Answer 8

• a potential
• this chamber may also be sealed or open to the
  atmosphere

Question 9

9. What happens when the Ionising Chamber is exposed to ionising radiation?

Answer 9

• positive and negative ions are produced
• the positively charged ions are attracted to the
  negative plate
• the negatively charged ions are attracted to the
  positive plate
• this allows for a current to flow through the chamber

Question 10

10. What kind of currents are often measured in an Ionisation Chamber?

Answer 10

• currents of the order 10⎺⁹
• this corresponds to 6 x 10⁹ electrons per second
• this can be quiet difficult to measure

Question 11

11. What are Ionisation Chambers used to measure?

Answer 11

• they are used to measure the Ionising Radiation
  Output
  OF Therapeutic and Diagnostic Ionising Radiation
  Generators
• they are also used in making accurate measurements if
  Patient Radiation Dose

Question 12

12. What is this?

Answer 12

• a Geiger Muller Tube

Question 13

13. What is the G-M tube and when was it invented?

Answer 13

• in 1929
• it is a very sensitive form of Ionisation chamber

Question 14

14. How sensitive is the G-M Tube?

Answer 14

• it can detect single ionising particles with enter the
  tube
• it detects Alpha, Beta and Gamma Rays

Question 15

15. What is the construction of the G-M Tube?

Answer 15

• it is very similar to an Ionisation Chamber
• it has a central wire electrode
• this is inside the hallow metal tube

Question 16

16. How does a G-M Tube differ from an Ionisation Chamber?

Answer 16

• it is filled with a gas
• such as Argon or Neon
• instead of Air

Question 17

17. At what pressure is the gas inside the G-M Tube?

Answer 17

• it is at a pressure of about 1/5 of atmospheric pressure

Question 18

18. What produces free electrons within the G-M Tube?

Answer 18

• Incident Ionising Radiation
• these free electrons will be attracted toward the
  Central Electrode
• the central electrode has a positive potential
• this potential is larger than the one in an Ionising
  Chamber

Question 19

19.What accelerates the electrons?

Answer 19

• the potential
• this accelerates the electrons towards the central
  anode
• this allows the electrons to gain sufficient energy
• this causes further ionisation
• this results in a chain reaction

Question 20

20. What causes photons to be emitted in a G-M Tube?

Answer 20

• all the electrons produced will hit the central anode
• this can produce Visible-Light or Ultraviolet radiation

Question 21

21. How many electrons can the original incident Radiation produce in the Ionising Chamber?

Answer 21

• about 10⁵ electrons
• this is measured as a pulse of current lasting about 1μs

Question 22

22. What kind of medicine do we use a G-M tube in?

Answer 22

• Nuclear Medicine
• detecting Isotope Emissions
• not so much in Diagnostic Radiology

Question 23

23. Which travels faster in the tube: electrons or positive ions?

Answer 23

• Electrons
• the tube takes quiet a long time to recover from the
  Recorded Pulse
• this recovery time is known as “Dead Time” for the
  tube

Question 24

24. What is Dead Time of the tube important for?

Answer 24

• it is important in the practical uses of the G-M tubes
• it limits the number of events that can be recorded
  each second

Question 25

25. What is a G-M tube used for?

Answer 25

• it is used for the detection and measurement of
  Gamma Rays

IF AN ALPHA PARTICLE REACTS WITH MATTER:
- it would produce gamma rays
- the G-M tube would be able to detect these

Question 26

26. What are the 2 main difficulties in Diagnostic Radiology for the use of the G-M Tube?

Answer 26

1. The response time is several seconds
2. There is a strong energy dependence at low photon
   energies