Gas Detectors

Question 1

Describe the Ionisation Region

Answer 1

• Recombination voltage surpassed
• Voltage too low for gas amplification/secondary ionisation to occur
• Voltage increases doesn’t cause increase in ion pairs collected
• Low strength electric field = no gas multiplication
• No dead time = effective at high dose rates

Question 2

Describe the Proportional Region

Answer 2

• Increase in voltage provides electrons with enough energy to ionise additional atoms (secondary ionisation)
• No. of secondary ions is proportional to primary ions
• Voltage kept constant so gas multiplication remains constant
• Capable of spectroscopy (particle identification/energy measurement)

Question 3

Describe the Geiger-Mueller Region

Answer 3

• High strength electric field (no signal amplifiers required)
• Can detect all charged radiation and gamma rays
• High dead time so less effective at high dose rates
• Can’t do spectroscopy as signal independent of original ionisation type/energy

Question 4

Example of Scintillator & Semi-conductor

Answer 4

Scintillator = Thallium activated Sodium Iodide, NaI(Th)
Semi-conductor = High purity Germanium detector, HPG

Question 5

Explain how a Scintillator works

Answer 5

General Overview: Radiation reacts with a scintillator and produces flashes of varying intensity proportional to the energy of the radiation.

• Incident photon deposits energy into the scintillator to cause region of excitation
• De-excitement releases light photon which passes through photocathode
• Photo converted to a photoelectron via the photoelectric effect
• Electrons are accelerated towards diodes which release additional electrons
• Electrons multiplied which produces electrical signal at anode

Question 6

Advantages & Disadvantages of Scintillators

Answer 6

Advantages:
- Higher energy resolution that proportional counters (high density & Z)
- High efficiency/counting rates
- Inexpensive (simple technology)
- Very short response time (10-9 organic, 10-6 inorganic)

Disadvantages:
- Requires housing in a airtight container to protect from moisture
- Low energy gamma response
- Bulky and Fragile

Question 7

Explain how a Semi-conductor works

Answer 7

• Ionising radiation enters detector and interacts with Ge crystal
• Incident photon ionises the atoms of the semi-conductors
• Electrons transfer to conduction band leaving a hole behind in a valance band
• No. of electron-hole pairs os proportional to the energy of the radiation
• Electrons and holes move apart to create electronic pulse which is used to calculate radiation intensity

Question 8

Advantages & Disadvantages of Semi-Conductors

Answer 8

Advantages:
- Higher energy resolution than scintillators (FWHK = 1.8 vs 50kev)
- High efficiency than scintillators (higher Z)
- Lower energy required to make e-h pair (2.9 vs 3.6ev)
- Larger crystals (cm’s vs mm’s)

Disadvantages:
- Requires cooldown to liquid nitrogen temps
- Low energy gamma response
- Expensive (high quality electronics)
- Suffer from radiation damage

Question 9

Purpose of the grid in a gridded ionisation chamber

Answer 9

Screen anode (electron collector) from the movement of ionising particles in the main chamber.
Allows pulse to be developed quickly as more electrons pass between grid/anode.

Question 10

Advantage and disadvantage of organic scintillators.

Answer 10

Adv:
-Durability

Dis:
- Can be grown in large sizes
- Anisotropic response results in poor energy resolution

Question 10

Advantage and disadvantage of organic scintillators.

Answer 10

Advantages:
-Durability

Disadvantages:
- Can be grown in large sizes
- Anisotropic response results in poor energy resolution