CONDENSER IONIZATION CHAMBER:

• Charge placed on central wire
• Connected to electrometer (expensive battery)
• Charge them up, then bring into room, irradiate, reconnect to electrometer and then get reading
• A fully charge chamber reads 0
• Looses charge as it is exposed to radiation (gains electrons thus the charge on the wire in the chamber decreases)
• A fully discharged chamber will mean that you would have been exposed to at least the full scale reading of the chamber
• Must reconnect to electrometer to get reading
• Used for photon beams

POCKET IONIZATION CHAMBER (personal)

Condensor type chamber

• Connected to a charging base
• Can see reading through window in one end of chamber
• You must zero the chamber before it is used – this is done by adjusting the power controller on the charging base
• Chamber is usually worn
• Advantages:
• Be used for short periods
• Give immediate readings
• Disadvantages:
• Charge can leak giving a false reading
• Trauma to the chamber will change the reading

BALDWIN – FARMER SUBSTANDARD DOSIMETER:

1955 Farmer designed a dosimeter that worked for all energies used in radiation therapy

Chamber is connected by a coaxial cable to an electrometer

Electrometer – is a instrument that measures charge

• Stem leakeage only .4% (4 MV x-rays)
• Can read the dose while exposure is taking place
• We use this in clinic

PARRALLEL PLATE CHAMBER (pancake chamber):

• Inside a block of plastic
• Sensitive volume is circular in shape
• Top electrode window is very thin (.01 to .03 mm)
• Thin window allows for measurements just about on the skin surface (skin dose)
• For readings at greater depths just add phanthom material to the top of the chamber

CUTIE PIE:

• Portable survey meter
• Used for fields larger than 1mR/hr
• Has 2 knobs for sensitivity (1 is for sensitivity and the other controls exposure rate or summation) which means you can either get the total dose or the dose rate

GEIGER COUNTER:

• Portable survey meter used to measure less than 100 mR/h
• Gas filled chamber
• After a single ionizing event takes place all the gas becomes ionized
• This gas must return to its original state to take another reading (quenching agent is added to gas to speed this process up – this down time is called resolving time)
• Used to measure : low radiation fields , contamination, lost sources

TLD’S:

Thermoluminescent Dosimeter

Most common substance used is Lithium Flouride (LiF)

Note: We characterize 3 types of material by the size of the forbidden band:

At room temperature:

1. it is a conductor if there is no forbidden band
2. If the forbidden band is 1ev then it is a semiconductor
3. If the forbidden band is 10ev then its and insulator

HOW THE TLD WORKS:

• Electrons are in the valance shell
• It is then irradiated and the electrons move to the conducting band (the electrons are now in a exited state and want to stabalize themselves by moving back to the valence band)
• Electron while trying to move back to valence band get caught in energy traps in the forbidden band  (for TLD’s we want this trap to be long lived)

 

TO FREE THE ELELCTRON FROM THE TRAP:

• We HEAT the TLD
• This heating removes the trap and the electron can now go back to the valence shell
• When it gets back to the valence shell it gives off a photon
• WE count the number of photons and relate this to a dose thus we now have our reading  (we associate the amount of light given off to a dose)

 

Before we can use a TLD for the first time after they come from the manufacturer we must calibrate each ONE:

• We anneal them
• Annealing procedures
  • Kahn : 1 hour at 400 degrees Celcius and then 24 hours at 80 degrees Celcius
  • Stanton : 1 hour at 400 degrees Celcius and then 2 hours at 100 degrees Celcius

OSL OPTICALLY STIMULATING LUMINESCENCE:

• Principle that OSL’s work on is the same as TLD’s
• Crystals trap and store energy from the exposure to ionizing radiation
• We then shine a green laser on the crystal and measure the intensity of blue light emitted
• We can do repeated readings
• Less expensive and we can get instantaneous readings
• Are made from Aluminum Oxide that is dopes with Carbon  Al2O3

DIODES:

• Made from semiconductors (crystalline silicon)
• Diode as an appropriate buildup cap built onto it
• In the radiation field the overall significant effect in the diode is the production of many electron hole pairs along the track of the ionizing particle
• works on reverse bias  ( decreases current flow)
• These hole pairs generate electrons.  We record the electrons with the electrometer and then relate this to a dose
• Main Advantage : With a diode in air we only need 3ev to produce and ion pair as aoppesed to 30ev needed in our gas filled chambers
• Diodes are 10 times more sensitive for the same dose of radiation as oppsed to gas filled chambers
• One problem is everytime we expose the diode to radiation it is damaged (the crystal lattice structure in diode is damaged)  / when the diode is new it is given a large initial dose of radiation to get it through the initial damage phase