National Physical Laboratory

What is the right instrument?

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1. For X, γ and β dose equivalent rate measurement? 

Instruments that can be used include:

 

1. Geiger Muller detector

There are three varieties of Geiger Muller (GM) detector used for the measurement of ambient and directional dose equivalent rate from X, γ and β sources:

View the three varieties:

steel wall, energy compensated 

typical minimum useful energy = 50 keV X, γ radiation

no useful β response

thin end window, energy compensated 

typical minimum useful energy = 10 keV X, γ radiation

no useful β response

thin end window, uncompensated

response to X, γ, and β sources.

2. Ionisation chambers

  • Popular instruments for routine monitoring.
  • Capable of measuring both ambient (slide closed) and directional (slide open) dose equivalent rate with sufficient accuracy from any energy of X, γ or β radiation. Provided that the radiation is incident generally from the front.
  • Will operate correctly in pulsed fields.
  • Ion chamber sensitivity is generally low and detectors are large.

Advantages of the GM detector over the ionisation chamber:

  • Audio output, which makes it easier to find hot spots etc.
  • Generally higher sensitivity. Where the aim is to measure dose rates at a few µSv h-1 and below, ion chambers lack sensitivity, are slow to respond and have a high level of statistical fluctuation. In contrast, energy compensated 2 inch pancake GM detectors have a sensitivity of 5 counts per second per µSv per hour, which allows relatively quick measurement down to 1 µSv h-1.
  • Compact detector, which allows measurement closer to sources.

Disadvantages from above are:

  • Slightly restricted polar response.
  • Difficulties in interpreting readings in pulsed fields.

3. Plastic scintillator detectors

  • Reproduce the ambient dose equivalent fairly accurately  (because the scintillator has a mean atomic number close to that of tissue).
  • Generally sensitive and capable of making useful measurements below 1 µSv h-1. 
  • Normally respond correctly to pulsed sources, but this should be confirmed.
  • Not useful for measuring b dose equivalent rate.

4. Sodium iodide based scintillation detectors (Isotope Identifiers)

  • Relatively new class of instrument.
  • Main function is the identification of the radionuclides responsible for unexpected gamma radiation on the outside of vehicles and packages.
  • Many also have an auxiliary dose rate function.
  • Do not have an intrinsically good ambient dose equivalent rate response, but with modern electronics, the pulse spectrum can be processed using an algorithm to generate a reasonable dosimetric response over a useful energy range, typically 60 or 80 keV upwards.
  • Have the merit of high sensitivity allowing the measurement of dose rate down to close to background levels reasonably easily. The main constraint on their use is the relatively high minimum useful energy.

5. Dose rate measuring proportional counter

  • Rare in the UK.
  • Combine the best characteristics of the ionisation chamber (good energy and polar responses) with a detector that amplifies the signal and overcomes problems of measuring very low signal levels.

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