The production of ionisation in air is the distinguishing feature of ionising radiation and has been the basis of its measurement since the early days of radiation dosimetry. The physical processes involved are well understood and good consistency can be achieved with fully characterised ionisation chambers, allowing their use throughout the world as national primary standards of air kerma.

NPL has developed, established and maintained several primary standard ionisation chambers since the 1930s to enable accurate dosimetry over a wide range of radiation energies and to support radiological protection and radiotherapy treatments.

NPL 50 kV free air chamber

Different types of Ionometric Primary Standards

The quantity realised by the primary standard ionisation chambers at NPL is air kerma, converted from the obsolete quantity exposure. For X-ray generating potentials below 420 kV, NPL has established 'free air' chambers as the national primary standards - one operating in the range from about 10 kV to 50 kV (low energy) and another operating in the range from about 40 kV to 420 kV (medium energy). The medium energy primary standard free air chamber has a unique modular design that allows it to be dismantled and transported to other National Measurement Institutes (NMIs) enabling the direct comparison of national primary standards.

Graphite-walled cavity ionisation chambers were originally developed as the primary standard for use in 2 MV X-ray beams for therapy and protection level qualities. The use of these chambers was extended to 1 MV X-rays to accommodate the changing needs of the radiation protection industry. These chambers have more recently been used in ⁶⁰Co γ-ray beams for therapy and protection level calibrations and ¹³⁷Cs γ-ray beams for protection level calibrations.

A primary standard extrapolation ionisation chamber (realising absorbed dose to tissue) was developed for the calibration of planar ophthalmic applicators emitting rays, used for the treatment of certain conditions of the eye. Absorbed dose from all ophthalmic applicators, planar and curved, is now measured using alanine, traceable to the absorbed dose primary standard calorimeter

A spherical graphite-walled cavity ionisation chamber was established in 2003 as the world's first primary standard of Reference Air Kerma Rate for the measurement of high dose rate (HDR) brachytherapy sources. A calibrated secondary standard protection level ion chamber is used to characterise low dose rate (LDR) brachytherapy sources.

Secondary standards and measurement services

NPL has designed and licensed the manufacture of photon secondary standard therapy and protection level ionisation chambers for use in hospital radiotherapy departments and industry throughout the world. The International Atomic Energy Agency (IAEA) and World Health Organization (WHO) adopted the NPL-designed therapy level secondary standard for distribution to developing countries. NPL offers several measurement services for the calibration of secondary standard ionisation chambers for environmental, diagnostic, protection and therapy level (teletherapy and brachytherapy) use.

Therapy level ionisation chambers calibrated at NPL in terms of air kerma are used in hospitals to measure absorbed dose to water in medium energy X-rays using national or international codes of practice to convert to the desired quantity. High-energy photon and electron ionisation chambers are calibrated in phantom directly in terms of absorbed dose to water.

Brachytherapy calibration of a farmer type chamber

A diagnostic chamber on the carriage of the diagnostic X-ray facility

Future developments

NPL is always striving toward higher measurement accuracy with lower uncertainty to enhance patient care and further radiation protection.

A set of spherical graphite-walled ionisation chambers has been constructed to replace the existing primary standard cavity chambers, for use at protection and therapy levels and with HDR brachytherapy sources.

There is rapid growth in the use of proton and light ion beams for radiotherapy, but ionometry is less well understood compared to photon and electron beams. Research in this area continues with the aim of bringing our understanding to the same level as for the more widely used radiations.