Radiation Detection and Surveying

Question 1

Definition of specific activity?

Answer 1

The amount of radio-activity found in a gram of material

E.g., Radioactive material with long half life will have low specific activity

Question 2

What happens to energetically charged particles when they pass through a radiation detector?

Answer 2

As the charged particles pass through matter they slow down due to friction, loose energy and get attenuated.

Will kick off e-, so get a trail of ionised atoms

Question 3

What do Rad-dectors use to measure trails of ionised atoms?

Answer 3

Use high voltages to measure electric current

Question 4

What are the two things that happen when radiation interacts with matter?

Answer 4

1. Excitation

2. Ionisation

Question 5

What are the three ways gamma particles interact with matter?

Answer 5

• Photoelectric effect
• Compton scattering
• Pair production

These are attenuation mechanisms

Question 6

Explain how the photoelectric effect works (Gamma attenuation)?

Answer 6

Photon gives up all its energy to an inner shell electron.
Ejects the photon from the atom.

• Relatively low energy adsorbed by material
• Can detect energy adsorbed and work out energy of original photon

Question 7

Explain how the compton scattering works (Gamma attenuation)?

Answer 7

High photon energy which is partially given to the inner shell electron.
Photon is scattered with a reduced energy.

• Angle of re-emission changes the energy adsorbed by the atom so cannot work out original photon energy

Question 8

Explain how the pair production works (Gamma attenuation)?

Answer 8

Photon hits the nucleus, creates positron and electron

• Needs 1.02MeV from photon
• Both positron and electron lose energy

Question 9

If water is used as an attenuation medium, which attenuation mechanisms dominate?

Answer 9

• Photon energy needs to be ~30MeV
• Photoelectric effect dominates
• If energy increases will go into compton realm

Question 10

What are the three types of detectors? and what they detect?

Answer 10

1. Gas Counter (Geiger)
   - detect alpha, beta some gamma
   - no isotope ID
2. Scintillator
   - detect alpha, beta and gamma
   - moderate/bad info about type of radiation
3. Solid and state detectors
   - detect gamma (alpha and beta in lab only)
   - excellent resolution
   - can measure radioisotopes

Question 11

Advantages of Gas-filled detectors?

Answer 11

• Highly sensitive
• Detect low intensity radiation
• Simple electronic amplification
• Little need to reduce noise

Question 12

Disadvantages fo Gas-filled detectors?

Answer 12

• Cannot tell between alpha, beta or gamma
• No energy discrimination
• Entire gas volume ionised
• Dead time

Question 13

Why do you get dead time in a Gieger-Muller tube? (Gas-filled detector)

Answer 13

Due to time required for +ve argon ions to drift to the cathode and accept electrons.

Question 14

How does a gas-filled detector work?

Answer 14

Two stage ionisation process:

1. ions produced by radiation effects
2. ions additional produced in gas by Coulombic effects and collected at electrodes in the detector.

Question 15

How do scintillation detectors work?

Answer 15

• Detect radiation by induction of luminescence

- Absorb energy and emit visible EM radiation

Question 16

What happens inside a scintillator?

Answer 16

1. Excitation upon absorption of radiation
2. Emit light photons (de-excitation)
3. Transit of light to the photocathode in the photomultiplier tube.

Photomultiplier tube converts electronic detector outputs into useful info.

Question 17

How do solid-state semiconductors work?

Answer 17

• Chamber filled with solid-state material e.g., diamond

- Will act as insulator until photons interact and acts as conductor

Question 18

Pros and cons of solid-state semiconductors?

Answer 18

Pros:

• Fast response
• Less statistical fluctuation

Cons:

• Sensitive to heat, has to be cooled
• Photomultiplier output is week, have to amplify