National Physical Laboratory

ARMUG Frequently Asked Questions

NPL, in conjunction with ARMUG members, has compiled the following list of questions and answers on the measurement of radioactivity in air which we hope users will find useful. Members' feedback on the content would be gratefully received, including any suggestions for additional points.



What factors should be considered when selecting a monitor?

At the outset, the objective of the monitoring, and the environment in which it will take place, (especially with regard to the presence on radon isotopes) must be clearly understood. Check any available Type Test data for instruments under consideration – they must indicate that the instrument can perform adequately for your application. If the monitoring conditions are particularly demanding (for example, if concentrations of radon isotopes or solid particulates are likely to be elevated), a field trial is recommended. Also, check that the instrument is acceptable from the point of view of maintenance - spare parts must be readily available and the manufacturers or distributors must be adequately trained and able to provide back-up. Also, consider speaking to other users who have experiences with the same instrument – have they found it performs adequately?



What are the pros and cons of different detector types?

Solid state α/β-in-air monitors have the advantages that they can provide information on particle energies and enable the user to carry out α/β separation and α-spectrometry, but they are expensive, and their immunity to radiofrequencies is poor.

Geiger-Müller tubes are inexpensive and simple to operate, and can be used for the assay of β-emitters, but background count-rates are too high for measuring α-emitters. α/β discrimination is not possible with these instruments, neither can they provide particle energy information.



Where should a safe alarm limit be set to avoid false alarms?

Limits of 5 – 8 DAC.h for α-emitters, and 1 DAC.h for β-emitters, are suggested. Ensure that the likely Activity Median Aerodynamic Diameter (AMAD), and the ambient radon levels, are taken into account.



When should monitoring be carried out?

Monitoring is necessary:

  • during operations, to identify containment failures and to protect the workforce;

  • to ensure that radioactive discharges are both within authorised limits and are at the expected activity levels;

  • in the event of a potential incident to detect airborne 'plumes'.



Where should the sampling head be positioned?

This depends on the purpose of the measurement. When monitoring for containment failure, there should be at least one monitor positioned as close to the anticipated failure point as possible (and downstream of the air-flow from that point) and one monitor positioned remote but downstream from the point. When carrying out static sampling, monitors should be deployed across the whole work area and in positions where they are most likely to detect activity. When monitoring for dose control purposes, sampling should be done in operator’s breathing zone using a 'swan neck' or similar sampling head.



When should alarms be set, and when should retrospective measurements be done?

Alarms should be set for the higher risk operations, or when occupancy levels in the area are high.

Static monitors provide assurance when working at environmental levels, and are also useful for accident consequence investigations.



What sampling times should be adopted ?

This is an operational decision which should be made by the operations manager and the RPA.



Why are particular flow rates used?

To keep a reasonable pressure differential across a small filter and also to limit power usage. In any case the rates used are comparable to human breathing rates (which are typically 37 litres per minute).



What are the effects of long-term deposition on filter paper, and how should the filters be maintained?

Accumulation of dust may result in:

  • a reduction in flow rate and/or an increase in the pressure drop across the filter;
  • masking of previously-deposited activity;
  • an improvement in the counting geometry for freshly-deposited activity (when the filter is new, particulates tend to be trapped down holes in the paper, resulting in collimation of emitted particles – this is less likely when deep holes in the filter have been filled up with dust);
  • a change in the spectrum of deposited airborne contamination and radon daughters (this is dependent on the filter medium).

In the absence of an instrument differential pressure-drop monitor function, filters should be changed according to ambient dust levels (e.g. during facility demolition this may need to be done hourly, but in clean rooms maybe once a week would be sufficient). This decision lies with the local Responsible Person, taking advice from the RPA.



What are the influences of pipe fittings between the sampling point and detector?

Generally, pipes should be kept as short as possible. The influence of pipe diameter is that narrow pipes result in a large pressure drop and a high flow rate (resulting in impaction on bends), whereas wide pipes result in a low flow rate and deposition of particulates on horizontal surfaces. So it is advisable to avoid long horizontal pipe runs, narrow pipes and bends. Any bends in the system should have large radii to minimise impaction.



What methods are available for radon compensation?

A range of a-particle energies arise from radon isotopes. One simple spectrometric method is to set a wide window around each radon daughter peak and then correct for spillage of signal from high-energy peaks to lower-energy peaks. The method needs to be tailored to the local area. Multi-channel spectroscopy tends to enable better factors and requires less adjustment for the local area. If it is possible to wait for the radon daughters to decay away, this should be done.



Which reference sources should be used for instrument calibration?

This subject is covered in detail in the Appendix to NPL Measurement Good Practice Guide No. 82: "The examination and testing of equipment for monitoring airborne radioactive particulate in the workplace".1

Key points are:

  • The reference source should be the same diameter as the filter paper being monitored and should be counted in the same geometry.
  • Ideally, the energy of the emitted particle in the reference source should be the same as in the sample, although even here there may be differences in detection efficiency for short-range emitters (e.g. low-energy β-emitters) due to what is effectively collimation of the radiation when the activity is 'buried' in the filter (see also A9 above).
  • If a reference source of the same radionuclide is not available, then one with similar radiation characteristics to the sample can still be used.
  • The best way to derive a calibration is by radiochemical analysis of a measured filter – this is particularly important where there is obviously surface dust or dirt on the filter.
  • Reference sources should comply with the requirements of ISO 8769, which defines sources as Class 1 and Class 2 standards, or as Working sources.
  • Several types of sources are available, namely:
    • Anodised Aluminium
    • Electroplated and vacuum-deposited sources
    • Activity incorporated in silver foils
    • Sources with a protective seal
  • The less the material between the radioactivity and the source surface, the better the source will perform in terms of the ratio of emission rate to activity and, probably more importantly, in terms of the angular distribution. Radiation from sources with thick covering layers in relation to the range will have few emissions at angles well away from the normal. Variations in source construction can lead to a variation in recorded count rate from the air sampler per unit emission from the source surface. This can cause confusion during testing.



What are the main sources of measurement uncertainty?

These include:

  • representativeness of sampling with respect to air actually inhaled by workers;
  • incorrect positioning of sampling head;
  • radon discrimination;
  • low activities;
  • clogged or damaged filters;
  • sources of relatively minor uncertainty (e.g. instrument calibration).

NPL can provide advice on the estimation of measurement uncertainties.



Where can Type Test data be found?

Manufacturers, suppliers and other organisations may provide these. They are usually based on recommendations from, for example, IEC.2 Some have been adopted as British and European standards.3,4



Which standards and specifications must be complied with?

Ultimately, it is Regulation 19 ('Monitoring of designated areas') of The Ionising Radiations Regulations 19995 which must be complied with. Documents such as NPL Measurement Good Practice Guides1,6-8 are useful in providing guidance in how to meet the requirements of this regulation.



What are other possible sources of information?

These include:

  • reputable instrument calibration laboratories;
  • instrument manufacturers;
  • professional bodies (e.g. the Ionising Radiation Metrology Forum (IRMF) and the Society for Radiological Protection (SRP));
  • others users (e.g. via user groups).



Is there a glossary of terms (e.g. DAC, DAC.h) available?

Yes – there is a glossary in NPL Measurement Good Practice Guide No. 82: '‘The examination and testing of equipment for monitoring airborne radioactive particulate in the workplace'.1


  1. National Physical Laboratory, Measurement Good Practice Guide No.82: The examination and testing of equipment for monitoring airborne radioactive particulate in the workplace, NPL, 2006.

  2. International Electrotechnical Commission, Radiation Protection Instrumentation - Monitoring Equipment - Radioactive Aerosols in the Environment, IEC 61172, 1992.

  3. British Standard, Equipment for continuous monitoring of radioactivity in gaseous effluents - Part 1: General requirements, BS EN 60761-1, 2004.

  4. British Standard, Equipment for continuous monitoring of radioactivity in gaseous effluents - Part 2: Specific requirements for radioactive aerosol monitors including transuranic aerosols, BS EN 60761-2, 2004.

  5. The Ionising Radiations Regulations, 1999, HMSO, 1999.

  6. National Physical Laboratory, Measurement Good Practice Guide No.14: The examination, testing and calibration of portable radiation protection instruments, NPL, 1999.

  7. National Physical Laboratory, Measurement Good Practice Guide No.29: The examination, testing and calibration of installed radiation protection instruments, NPL, 2001.

  8. National Physical Laboratory, Measurement Good Practice Guide No.30: Practical radiation monitoring, NPL, 2002.

Please note that more information on NPL Measurement Good Practice Guides (some being available as free downloads) can accessed by clicking this link

Peter Burgess, NPL
Julian Dean, NPL
John Simpson, Consultant

March 2008

For further information, please contact Julian Dean

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Last Updated: 9 May 2012
Created: 17 Apr 2007


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