National Physical Laboratory

Measurement of cosmic ray and other high energy fields

Solar Flare
Solar flare captured by the SOHO satellite
on 4 November 2003.
Image courtesy of SOHO/EIT consortium
(ESA and NASA)

Radiation levels at aircraft cruising altitudes are twenty times higher than at sea level. The main source is galactic cosmic radiation, primarily generated by exploding stars within our own galaxy. These elevated dose rates mean that, on average, a typical airline pilot receives a larger annual radiation dose than someone working in the nuclear industry.
NPL has been studying aircrew exposure to cosmic rays for several years, in collaboration with academia, regulators and airlines.

NPL became involved in the field of cosmic ray dosimetry due to its expertise in the use of tissue equivalent proportional counters (TEPCs). Put simply, TEPCs are constructed from tissue equivalent materials (having compositions similar to muscle tissue); consequently, secondary charged particles generated by the incident radiation are similar to those that would be generated in actual tissue, both in terms of particle type and energy distribution. Furthermore, the gas filling of the counter is at a low pressure so that it behaves like a volume smaller than a single human cell. As a direct consequence of this, the energy deposited within the TEPC by any particle crossing the chamber is related to how densely ionising the particle is, and hence how damaging it is to human tissue. This mode of operation means that TEPCs are ideal for measuring the exposure from the highly energetic particles that constitute cosmic radiation at aircraft altitudes.

NPL has provided calibrations and analysis for TEPC measurements made on over 800 flights with a variety of airlines as part of a multi-institutional collaboration and has helped demonstrate that existing software packages used to calculate aircrew exposure work adequately.

One facet of aircrew exposure that is not so well defined originates from infrequent and unpredictable Solar Particle Events (SPEs). Often associated with solar flares, SPEs represent massive ejections of proton plasma from the surface layers of the Sun. If such a plasma field collides with the Earth, dose rates at aircraft altitudes can soar by many orders of magnitude, albeit only for a few hours. The energy distribution of the proton plasma varies from event to event, making them very difficult both to interpret and to estimate the associated radiation exposures. Very little in-flight measurement data exists for such SPEs, and consequently different estimates of their impact on aircrew exposure differ by a factor of two or more.

In order to provide more measurement data to help reduce the uncertainties of SPE dosimetry, NPL is currently engaged in a programme to fly a large number of simple, robust dosimeters with a leading airline. Hence, during any future SPE, measurement data acquired by dosimeters at differing locations and altitudes should provide vital information to validate the future development of software packages used for solar particle event dosimetry.


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Last Updated: 25 Mar 2010
Created: 17 Apr 2007


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