National Physical Laboratory

Ionising Radiation

Radioactivity and ionising radiation measurements are fundamental to a significantly large and expanding group of activities. The ranges of activity and absorbed dose covered are very wide.

Lowest levels: environmental/occupational

At the lowest dose and activity levels, are those found in the environment from natural sources such as the ground and cosmic rays. There are also low levels of radiation around us from smoke detectors, and at a lower level from radioactive waste from hospitals, industry and nuclear power. Cosmic rays (and particularly neutrons arising from cosmic rays) can interfere seriously with the operation of modern integrated circuits.

In all these cases NPL has a fundamental role in the provision of the accurate, traceable measurements required to ensure that the doses received by people are as low as reasonably practicable (ALARP), and so that equipment is not damaged by radiation.

Diagnostic levels

At higher doses and activity levels are those found for example in diagnostic use, such as with X-rays or PET scanners. The radiation dose delivered to patients (and in the case of PET measurements or nuclear medicine, to the people around them), must follow the ALARP principle and accurate measurements of dose and activity are fundamental to this.

Therapy levels

There are many different treatment modalities now for radiotherapy. NPL maintains the primary standard of absorbed dose in the most critical of areas; it is important in radiotherapy that the doses delivered are high enough to a tumour to have a proper therapeutic effect, but low enough to the surrounding healthy tissues that the patient's health is not compromised as a result. NPL plays a significant role in the development of new measurement instruments, and calibration and audit tools and services, ensuring that othe required quality of these measurements is maintained and developed.

Industrial levels

Many industrial processes require irradiation of materials to high doses. These range from the crosslinking of polymers (wire insulation, the production of heat-shrinkable tube, the curing of paints), or the modification of the electronic properties of semiconductors, to highly-critical applications such as the irradiation of medical devices or food for the purposes of sterilisation. New types of medical device may require new irradiation approaches and hence new dosimetry techniques, and NPL is involved in the development of these techniques and in the worldwide dissemination of best practice in dosimetry throughout the irradiation industry.

The irradiation industry, and especially the nuclear industry, uses, transports, and ultimately disposes of, high levels of radioactivity. Traceable measurements of all levels of activity from very low to very high are required for all these processes are required in order that the requirements of efficiency, safety and security are met.


In all these areas, mathematical modelling goes hand-in-hand with measurement: In many cases, the measurement may only be made indirectly and the actual quantity required is calculated using a validated model, or in other cases a correction factor is applied which is itself calculated using a model. NPL uses several different approaches, from finite-element heat-flow calculations to detailed Monte Carlo radiation transport codes, some of which are run on the NPL Grid in order that many months’ worth of calculations for one computer can be carried out in one day.

Read more about Ionising Radiation Quantities and Units

Ionising Radiation science areas

  • The NPL Neutron Metrology Group (NMG) operates world-leading facilities for measuring neutron source emission rates and providing extensive accelerator and source based calibration fields.
  • NPL provides world-class research, calibrations, specialised measurement services and training for the dosimetry of ionising radiation in cancer therapy, diagnostics and industrial sterilisation for medicine and pharmaceutics.
  • NPL's radioactivity team (part of the Radioactivity and Neutrons Group) combines wide-ranging expertise with state-of-the-art equipment and laboratory facilities.

Measurement needs

Please tell us about any measurement needs you may have. Some examples of these might be:

  • Issues relating to product inspection
  • Issues relating to process control
  • Training requirements
  • Obtaining specialist calibration or testing support
  • Problem solving on measurement issues
  • Demonstrating or assessing measurement system or product performance

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