Instruments

Question 1

Gas Filled Detectors

Answer 1

Ionization Chamber
Proportional Counters
G-M Counter

Question 2

Ionization Region

Answer 2

Pulse size depends on energy of incident radiation, not applied voltage
Operates under the Bragg-Gray Principle to determine absorbed dose
Used for high energy alpha, beta, and gamma radiation

Question 3

Proportional Region

Answer 3

Pulse height depends on energy of incident radiation and has a linear relationship with the voltage applied
Depends on Townsend Avalanche effect
Used to detect less energetic particles
Typically uses P-10 Gas

Question 4

P-10 Gas

Answer 4

90% Argon, 10% methane, fill gas

Question 5

Gas Ionization Regions

Answer 5

Recombination
Ionization
Proportional
Limited Proportional
Geiger-Mueller
Continuous Discharge

Question 6

Limited Proportional Region

Answer 6

Avalanche effect begins to increase, causing less discrimination of the incident radiation energy

Question 7

Geiger-Mueller Region

Answer 7

Avalanche effect reaches maximum, charge collected is constant (log scale), and cannot distinguish between incident radiation energy

Question 8

Energy required to create and electron-ion pair

Answer 8

35 eV

Question 9

Scintillation Detectors

Answer 9

Scintillation material absorbs radiation energy and emits visible light
The visible light is focused into a photocathode
The photocathode is a metal surface that undergoes the photoeletric effect and ejects photoelectrons
Photoelectrons enter the photomultiplier tube
Dynodes in PMT are held at increasing voltage potentials that exponentially increase the number of photoelectrons
Size of current photons depends on number of photoelectrons

Question 10

Semiconductor Detectors

Answer 10

Small band gap between conduction band and valence band in semiconductor materials
Consist of P-type and N-type semiconductors pressed together
Region in between the semiconductors is called the Depletion Region
Reverse bias applied by connecting P to - and N to +
Radiation enters depletion region creating an electron-hole pair
Electrons move through the circuit creating a current pulse
# of electron-hole pairs depends on energy of incident radiation
Current pulse depends on number of electron-hole pairs

Question 11

Ionization Chamber

Answer 11

Radiation interacts with a fill gas and produces ion pairs along its track
A voltage is applied creating an electric field in the gas
Free electrons migrate toward the electrode and positive ions migrate to the negatively charged wall
Once collected at the electrode, a steady current is produced
Current is measured corresponding to number of ion-pairs formed that corresponds to energy of incident radiation

Question 12

Proportional Counter

Answer 12

Ionization chamber operated at a higher voltage to encourage the avalanche effect.
Allows the detection of lower energy radiation
Can distinguish between particles of different energies
Pulse from avalanche arriving at the electrode is linearly proportional to the number of ORIGINAL ion pairs and gas multiplication factor

Question 13

Geiger-Mueller Counter

Answer 13

Operated in pulse mode
Each pulse has the same amplitude
Gamma rays will interact with the wall, liberating electrons that will then ionize the gas
Gas should have low electron affinity like proportional counters

Question 14

Organic Scintillators

Answer 14

Based on excitation at the molecular level

Molecule de-excites by emitting prompt fluorescent light.

Question 15

Inorganic Scintillators

Answer 15

Crystal lattice structure that creates electron-hole pairs when excited. When the electron returns to the valence band it emits light.