Dosimetry badges


There are three general groups of dosimetry badges:

  1. Film badges
  2. Thermoluminescent Detectors (TLDs)
  3. Electronic Dosimeters

Another way to measure radiactivity is with a Geiger Muller counter.


Film Badges

These use a silver-halide film (similar to that used in plain film radiography).

Advantages

  • Cheap
  • Can distinguish between different energies of photons
  • Can measure doses from different types of radiation
  • Provide a permanent record
  • Accurate for exposures > 100 millirem

Disadvantages

  • 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. They are changed and read-out every 3 months.

Advantages

  • Can be made very small for finger/eye doses
  • Can be reused

Disadvantages

  • 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

 

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Electronic Dosimeters

Most commonly used electronic dosimeter uses silicone diode detector. They can provide a direct electronic readout and don't need the processing that is needed by the other types of dosimetry badges. Require yearly battery replacement and checking.

Advantages

  • Very sensitive. Nearly 100x more sensitive than a TLD and can measure to nearest 1 µSv
  • Good for measuring pregnancy doses

Disadvantages

  • High initial cost

Geiger Muller Counter

This is a tube filled with an inert gas (usually helium, neon or argon) at low pressure. When radiation enters the tube, it ionises the gas and produces an electrical charge which is then measured and displayed electronically. The outer casing is the negative cathode and the inner wire is the positive anode.

Advantages

  • Instant readings

Disadvantages

  • Cannot differentiate between radiation types
  • Cannot measure high radiation rates due to "dead time" - insensitive period after each ionisation of the gas during which any further radiation will not result in a count.

Next page: Patient dosimetry


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