National Physical Laboratory

Measurement of dose quantities using microdosimetry

Microdosimetry is a useful tool in the measurement of radiation levels in the workplace and has one distinct advantage over other techniques: it records a spectrum that can be analysed to give separate photon and neutron dose rates and dose equivalent rates in a single measurement.

Principles of the microdosimetric measurement system:

Hawk tissue equivalent
proportional counter (TEPC)

  • The tissue equivalent proportional counter (TEPC) is designed to mimic the response of human tissue to irradiation.
  • It uses a low pressure gas (with an atomic composition similar to that of muscle tissue) to fill a cavity roughly 12 cm in diameter.
  • The cavity lies at the centre of a sphere of conducting plastic (also with an atomic composition similar to that of muscle tissue).
  • This similarity of composition means that the TEPC satisfies the Bragg-Gray criterion of cavity-ionisation theory, and allows the absorbed dose to be measured precisely.
  • The TEPC tends to ‘sample’ a charged particle’s track rather than stop it completely and, in the majority of cases, the energy deposited in the gas is related to the particle’s linear energy transfer (LET) in tissue.
  • This relation to LET enables microdosimetry to distinguish between recoil electrons, protons, alpha particles and heavy ions. This, in turn, enables the determination of neutron and gamma ray absorbed doses, quality factors and dose equivalents.

Instruments available for hire:

The original NPL microdosimetric measurement system relies on three MCA cards housed in a PC, together with the associated modular units of electronics required to process the detector signals, and remains the best-characterised, highest-resolution, but most labour-intensive, system.

NPL now possesses a self-contained battery powered microdosimetric measurement system that is as easy to use as a neutron area survey meter. Benefiting from miniaturised, low power electronics, this detector, which fits in a suitcase, trades off spectral resolution to gain portability and ease of use. Logging data every minute, this TEPC can be hired out to customers who can perform mixed field radiation surveys in a number of workplace areas, with its LCD display giving the user an approximate indication of the dose rates in situ.

The instrument needs to be returned to NPL for detailed analysis; all the customer need do is provide timing information for the TEPC when used in several locations.

Measurement times for both systems vary from a few minutes for high dose rates (up to ~50 mSv h-1) to (ideally) several hours for low dose rates (< 1 µSv h-1).


Field Measurement Quantities

  • All with a single measurement.
  • Dose equivalent values are calculated using the Q(L) relationship of ICRP Publication 60.

Instrument Calibration:

  • The instruments are nominally self calibrating, but the calibration is checked by 241Am-Be irradiations.

Correction Technique:

  • Although TEPCs have poor efficiencies for intermediate energy neutrons, corrections can be applied, derived from the shape of the spectrum. (See NPL Report CIRM 26, Neutron Response Characteristics of the NPL Tissue-Equivalent Proportional Counter. Taylor, G.C., (March 1999)).

TEPC Measurements
Comparison of TEPC measurements


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Last Updated: 25 Mar 2010
Created: 17 Apr 2007


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