National Physical Laboratory

Measurement of dose rate

radiation 2

Dose rate is the dose absorbed in unit time and indicates the amount of radioactive dose received by a person within a certain period of time.

The dose rate is often given in thousandths of sieverts per hour.

Depending on the calibration procedures used by the manufacturer, detectors provide information about the strength of the radioactive source.

Operational dose quantities

It is not possible to directly, and physically measure the amount of energy deposited (i.e. doses) in organs or people. Therefore alternative operational dose quantities are used by international convention to provide best and accepted estimates.

It is Good Practice to measure doses to personnel with a personal dosimeter, but an alternative method is to estimate the effective dose with a hand held monitoring instrument.

The convention in radiation protection for area monitoring is to use:

  • Ambient dose equivalent - for control of effective dose to people, e.g. area monitoring

Ambient does equivalent H*(d) is the normal monitoring quantity for X, gamma and neutron radiation where d is the depth at which the dose rate applies.

International convention in radiation protection is to use the ambient dose equivalent at 10 mm depth i.e. H*(10).

The ambient dose gives a conservative estimate of the effective dose a person would receive when staying at the point of the monitoring instrument.

  • Directional dose equivalent – for control of doses to skin and lens of eye.

Directional dose equivalent H′(d) is intended for use with less penetrating radiation. Its main use is for skin dose at a depth of 0.07 mm, but it is also used for dose to the eye lens at 3 mm.

Note: the monitor is expected to be pointed at something which is the cause of the exposure.

Operational quantities that should be measured using portable instruments during dose rate surveys are therefore either:

  • ambient dose equivalent at 10 mm depth, H*(10) - SI unit: sievert (Sv)
  • ambient dose equivalent rate, H*(10) h-1 - SI unit: sievert per hour (Sv h-1)
  • directional dose equivalent at 0.07 mm depth, H′(0.07) - SI unit: sievert (Sv)
  • directional dose equivalent rate, H′(0.07) h-1 - SI unit: sievert per hour (Sv h-1)
Some instruments may use different quantities such as fluence rate, air kerma rate, absorbed dose rate in air. Estimation of dose is a specialist task when using instruments not scaled in dose equivalent or dose equivalent rate.

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