Survey Instruments (2) Flashcards
What are the two types of survey meters used in radiography?
- Ionization Chamber
2. Geiger Muller Detector
Describe an Ionization chamber survey meter
- gas filled chamber (air) containing two electrodes
- amplifier
- readout device
- battery to supply power
How does an Ionization chamber survey meter work?
relies on the ionization of the gas molecules to detect radiation.
Electric field in the chamber from the battery, when photons enter they interact with gas creating ion pairs, producing + and - ions that migrate to the opposing charge, creating a momentary drop in voltage allowing current to flow - flow is measured with an ammeter.
Facts about Ionization chambers
- low sensitivity
- measure high radiation fields
What is an Analog survey meter?
Two scales, four settings
top scale = high intensity radiation (1mR to 10R/h or 20uSv to 100mSv/h)
Settings:
- off position
- check position
- high scale
- low scale
What do survey meters measure?
Dose Rate
Describe the Geiger Muller Detector survey meter
- uses higher voltage
- gas = argon
- gas interacts with incoming ions
Facts about the Geiger Muller Detector
- measure low radiation doses
- can become saturated if there is to many ion pairs
How does a Geiger Muller Detector work?
Because of the high voltage Gas Multiplication occurs. # of electrons collected at anode depends on # of ion pairs produced
What is gas multiplication or amplification?
The production of electrons (negative ions) is sped up due to high voltage
Settings on a Geiger Muller Detector
off x100 = 0 to 1000mR/h x10 = 0 to 100mR/h x1 = 0 to 10mR/h x0.1 = 0 to 1mR/h
External vs Internal Detectors
external = detect non-waveform radiation (read in counts-per-minute) referred to as Pancake Detectors or Contamination Detectors
Survey Meter requirements
enforced by CNSC + Health Canada
- worn at all times in radiography
- dose dates 20uSv/h to 100mSv/h
- way to check battery
- calibrated every year
- readings accurate to +/- 20%
