What is Exposure and what is it used to calculate
The amount of electrical charge produced per mass of air.
Used to calculate dose.
The 3 operating regions of gas filled detectors and their voltages
- Ionization region (low voltage)
- Proportional region (medium voltage)
- Geiger Muller region (high voltage)
Effect of temperature and pressure on a sensitive volume to the atmosphere
- constant volume, not constant mass
- temperature increases = mass decreases
- pressure increases = mass increases
- mass increases = more interactions of ionizing radiation = more ion pairs released = more ionization current measured
5 key regions of gas filled detectors
- Recombination region
- ion chamber region
- proportional chamber region
- Geiger counter region
- electrical breakdown
Recombination region
- voltage is not high enough to collect all ions before they recombine
- current increases with voltage
- no useful detectors operate here
Ion chamber region
- all ions are collected
- no (or little) increase of current with voltage
- ion chambers operate in this region
Proportional Counter region
- electrons are accelerated and gain enough energy to knock out further electrons
- current is amplified but still proportional to radiation energy released
individual ionizing events and their energy can be measured - proportional counters operate in this region
Geiger Muller Counter region
- entire gas volume is ionized
- amplification of current is large and resulting pulse is easily counted
- Geiger Muller Counters operate in this region
Electrical Breakdown region
- electrical breakdown or sparking occurs
- destroys counter
- no counters are able to operate in this region
Ionization Chambers use
-Used for an accurate measurement of linear accelerators + x-ray equipment + strength of radioactive sources + radiation levels in occupied areas
Proportional counters use
Used to measure a mix of radiation types
Geiger Muller Counters use
Used for radiation protection and dosimetry
Ionization Chamber Process
- ionizing radiation enters chamber
- ionizes air into ion pairs
- electrons and positive ions collected in electric field
- electrons = positive electrode, positive ions = negative electrode
- ionization current is measured
Scintillation Detector
A crystal of sodium iodide. A photon releases light in the crystal, light is detected and amplified with a photomultiplier tube.
Scintillation Detector process
- scintillator is used (crustal which emits light when a photon passes through it)
- Scintillator is made light tight
- light flashes are viewed using a photomultiplier tube
photomultiplier tube has a photocathode that emits an electron when struck by a photon an then a series of amplifiers
Photomultiplier Tube Process
- light knocks electrons out of photocathode
- electrons are accelerated to dynodes
- electrons strike first dynode, more low energy electrons are mitted and are then accelerated towards next dynode
- cascade occurs with ever increasing number of electrons being produced at each stage
- electrons give a final current pulse
Thermoluminescent Dosemeter
Non conducting crystals where all the electrons are trapped in bonds, ionizing radiation releases some of these bonds.
Thermoluminescent Dosemeter process
- trapped electrons can be released by heat and fall back into valance bond with the emission of light
- intensity of light emission is measured
