Section 8 Flashcards
What fundamental principle is in TLD
Reliance on trap states
Explain the molecular level of TLD
Excited e falls into trap state
Heat gives e energy to reach conduction band
E releases and produces light
What does the gas filled detector have and when does current/voltage flow?
2 electrodes in air
- current only flows if rad ionises gas
- response depends on supply voltage
What are the gas filled detectors?
Ionisation, thimble and GM
What are the features of ionisation chamber?
Plateau in graph - output indep of voltage
Low V - only primary ions collected
What is the current in the ionisation chamber like
Small current - amplify and measure
What are the chambers in the ionisation detector?
small chambers measure exposure of primary xray beam & large chamber to monitor scatter
Is current proportional to in ionisation?
Radiation exposure - cannot count photons
What is the voltage effect in ion.?
Unaffected by small variations in V
What are the 2 disadv for ion?
Low intrinsic efficiency
Do not provide permanent record
Electronic equilibrium
electrons leaving = # electrons entering
Thimble chamber
Small Free air chamber with a central electrode
The size of a Proportional chamber pulse is
proportional to the energy deposited by the radiation –> energy discrimination
Geiger Muller has a large
voltage and a single ionising event can produce an avalanche of ionisation
Saturation means
pulse size is independent of energy deposited –> energy discrimination
GM disadvantages
Dead time (not desired); does not say what the radiation is
GM advantages
Simple, rugged, fairly cheap
Diodes
Semiconductor device with 2 layer - n and p
N-type
Excess of negative charge
P-type
Excess of positive charge
Current is proportional
to dose rate (real time reading)
In diodes, Sensitivity changes with
repeated use due to radiation induced damage
MOSFET
Metal Oxide Silicon Field Effect Transistor
Advantages of MOSFET
instant readout, no angular dependence, uniform energy response (exc at low e)
