There are three general groups of dosimetry badges:
- Film badges
- Thermoluminescent Detectors (TLDs)
- Electronic Dosimeters
If an employee is provided with a dose badge they are required to wear them under IRR 17. They measure staff doses to ensure that dose limits are complied with and to deterine who should be classified.
These use a silver-halide film (similar to that used in plain film radiography). They are an old technology and have been largely replaced with TLDs.
- Can distinguish between different energies of photons
- Can measure doses from different types of radiation
- Provide a permanent record
- Accurate for exposures > 100 millirem
- Film fogging over time
- Prolonged exposures can adversely affect the film
- Not accurate to exposures < 20 millirem
- Must be developed and read by a processor, which is time consuming
- Must be changed every 1 month due to fogging over time
Thermoluminescent Detectors (TLDs)
This is the most commonly used dosimeter. To read absorbed radiation the TLD is heated and visible light is released from the crystal in proportion to absorbed radiation. This is then measured to calculate the amount of radiation the dosimeter has been exposed to. Calcium fluoride and lithium fluoride are commonly used. The TLD must be used in its casing as this applies filters to correct for deep and superficial absorption through the skin. Calibration post-read-out is still required to correct for differential absorption. The rate of changing the TLDs varies between institutions. Some institutions may use area monitoring instead of individual monitoring if the expected doses are low.
- Can be made very small for finger/eye doses
- Can be reused
- Cannot distinguish between different types of radiation
- More expensive than film badges
- Once read out, record is lost i.e. can’t provide permanent record
Most commonly used electronic dosimeter uses silicone diode detector. They can provide a direct electronic readout and live/real time readouts and don’t need the processing that is required for the other types of dosimetry badges. Require yearly battery replacement and checking.
- Very sensitive. Nearly 100x more sensitive than a TLD and can measure to nearest 1 µSv
- Good for measuring pregnancy doses
- High initial cost