L 5: Measurement of ionizing radiation Flashcards
Exposure definition
- Measurement of ionization in air produced by photons.
- Value of total charge of ions of one sign (+/-) produced in air when all electrons liberated are completed stopped in air.
- Cannot be used for photons above 3MeV energy
Unit of measurement of ionizing radiation exposure
Roentgen
1R=
Board Question
2.58 x 10 to the power -4c/Kg in air
Exposure formula
X=dQ/dm
dQ= charge
dm= mass
Exposure units per formula
C/Kg
Ionization can be measured only in
Air and only for photons
Ionization cannot be measured in
Electrons or protons
Free air ionization chamber
- Primary Std chamber for calibration of secondary chambers.
- A condition of electron equilibrium.
- ionization loss = ionization gain.
Accurate measurements require corrections of
- Air attenuation
- Recombination on ions
- Temp
- Pressure
- Humidity
- Ionization produced by scattered photons
- Non-uniform electric field
Upper limit of photon energy to measure Roentgen
3 MeV
Thimble Chamber
They are field instruments used in RT
They are still sensitive to air pressure, tempt and humidity
Stem Effect
In Thimble chamber
Effect of ionization in the body of the stem
Between the end of the chamber and the metal cap
Farmer Chamber
They are used to calibrate low energy beam (<2MeV) used in RT
Electrometer
It is a charge measuring device
Eg: Negative feedback operational amplifier
Secondary chambers
These chambers require calibration from std chambers
eg: Farmers Chamber. Farmer-type chamber
Parallel plate chamber
Extrapolation chamber
Used to measure surface dose or dose in the build up region where dose gradients are high
Polarity effect
Chamber readings changing with chane in polarity
ideally should be <0.5% for a well designed chamber
Biological T 1/2 life
t 1/2 and effective t1/2 will be given
