[ RadSafe ] Solar Flare Dosimetry

[Marcel Schouwenburg]

Source: http://www.npl.co.uk/publications/news/ionrad/issue15/#solar

 It is now well established that the average commercial airline crewmember
receives an annual radiation dose greater than the average worker in the
nuclear industry. This occupational exposure, around 3 millisieverts per
year, is in addition to the UK average background radiation dose (2.2
millisieverts from natural sources, or 2.6 millisieverts taking medical
exposures into account), resulting in aircrew typically receive around
twice the radiation exposure received by the general public.

NPL has been involved in measuring the cosmic radiation exposure of
aircrew since 2000, as part of a collaborative project with the Mullard
Space Science Laboratory, Virgin Atlantic Airways and the Civil Aviation
Authority. The outcome of this is a database of radiation doses for over
eight hundred flights, which have been used to test the performance of
computer programmes used by the airline industry to calculate the
radiation exposure of their aircrew. The results are reassuring: although
calculated doses for individual flights may be wayward by as much as 50%,
the longer-term averages are usually within 20%, which is acceptable for
radiation fields with a large neutron component such as cosmic rays at
aircraft altitudes.

At least, this is the case 99% of the time when the Sun is behaving
normally. However, there are occasions (once per year, on average), when
an X-ray burst from the Sun (known as a Solar Flare) is accompanied by a
massive burst of particle radiation that hits the Earth, raising the
radiation levels at altitude for several hours. Most of the time it is
thought that these radiation levels are hardly raised at all; however,
just occasionally, they can be elevated much further. Calculations
indicate that, had the current generation of airliners been flying in
February 1956, certain high-latitude long-haul flights may have received
an incredible five-millisievert dose in just a few hours, more than
doubling their average radiation exposure for that year!

In the past 60 years, only a handful of events have raised dose rates
sufficiently for a single flight to receive a dose in excess of 1
millisievert. However, the mechanisms behind their generation are poorly
understood and they are entirely unpredictable: it is possible that an
event on the scale of February 1956 may occur next month or next year, and
the lack of understanding means that events ten times larger cannot yet be
ruled out, however unlikely.

The lack of knowledge behind these events is reflected in the accuracy of
their predicted doses. For those few events where two or more estimates
are available, the calculations can disagree by as much as a factor of
two.

With this in mind, NPL is currently planning a 'Flare-watch' programme,
whereby a network of radiation detectors is established, providing
continuous monitoring of the radiation levels at aircraft altitudes. The
idea is not so much to provide a definitive value for a flare dose (the
dose rates will vary significantly with altitude and location), but to
provide computational modellers with experimental data to help refine
their calculations.

Monitoring is expected to begin in the latter half of 2005.

Contact:
Dr Graeme Taylor
Division: Quality of Life, Building 47
Telephone: +44 (0)20 8943 7087
Email: graeme.taylor op npl.co.uk
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Marcel Schouwenburg
RadSafe moderator & listowner