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AIRCREW DOSIMETRY

Ionizing radiation is not a threat to human body as long as it remains at low level. Indeed, everyone is exposed to a mean radiation dose of 2.4 mSv (milliSievert) each year from natural sources such as telluric radiation or the concentration of radon in our houses. When the altitude increases, the dose increases either and the cosmic radiation becomes the main radiation contributor. Indeed the earth atmosphere is a shield protecting us from high-energy particules coming from space. As it strikes the atmosphere, a given particule will hit an atom in the air, creating a cascade of many secondary particules of lower energy, down to the ground. As the height and density of the atmosphere above us is smaller at aircraft altitude, the risk to encounter high-energy secondary particules there is higher. As a consequence, the radiation dose is hundred times more intense there.

The primary particules are both galactic cosmic rays (GCR) and solar particules (mainly protons). The galactic component is permanent but modulated by the solar activity in the course of the 11-year solar cycle whereas the solar component is sporadic, and due to solar activity.

Recently, the European Commission included the exposure of aircraft crew to cosmic radiation as occupational exposure. The airline companies have now the legal requirement to check that the air crew members do not receive a dose higher than 100 mSv over 5 years with a maximum of 50 mSv for a given year. The pregnant air crew, as the foetus is more exposed, should not receive more than 1 mSv until the end of her pregnancy.

To calculate the dose received on a given flight, 3 important parameters should be taken into account:
  • The flight date. The solar cycle modulates the amount of particles striking the earth. This variation is translated into the so-called modulation parameter, which is an input to models such as CARI or EPCARD. These models compute the radiation rate for a given flight date, at any location in earth atmosphere, up to 80000ft. The last one is currently used for the SIEVERT system.
  • The flight plan. It gives essential information like the flight duration and the altitude at several locations (way points). The way points coordinates need to be known because the radiation dose also varies with latitude, and, to a smaller extent, with longitude. Indeed, the earth magnetic field is another shield against particules, which is more efficient at the equator than at the pole. The following figure illustrate the importance of latitude when computing the dose for a given flight. The dose rates are computed for January 2005, at the altitude of 39000 feet. Dark red regions are the most exposed, with a maximum of 9.42 μSv.h-1 whereas light blue ones are the safest, with only 1.9 μSv.h-1 .
  • The neutron monitor records. For two reasons: first the neutron monitor output is used to compute the modulation parameters (Kerguelen neutron monitor for the Sievert system). Second, in the case of Solar Particule Event, high-energy protons may reach the earth atmosphere and behave like the GCR. In such a case, the count rate of neutron monitors suddenly increases and high level count may last for a few hours. Such an event is called a Ground Level Enhancement (GLE). When the increase is not negligible, the specific radiation amount at aircraft altitude must be taken into account, for example with dedicated models such as SiGLE (see below).

To get a rough estimate of the dose one would receive flying from Paris to any other location in the world, one can check the following map. The flight plans are not taken into account in this case. The way points are deduced from the orthodromic path between Paris and other locations (shortest path between two points on a sphere). Check also other departure locations: New-York, Tokyo, Buenos Aires and Equata .


SiGLE semi-empirical model
The model SiGLE is based on measurements of doses received on board Concorde flights. Indeed, to prevent excess of dose, Concorde had dosimeters permanently in operation during its flights. The data are completed with measurements onboard subsonic flights and results of theoretical calculations. The model enables to calculate the dose received onboard a given flight, during every GLEs observed with neutron monitors. It has been applied to the GLEs observed since 1942 (see publications, 2003). Among the 70 GLEs observed in 64 years, only a few have been source of more than 1 mSv onboard a subsonic flight from Paris to San Francisco, one of the more exposed flights. Nevertheless, the most significant ones are taken into account in the Sievert system. The bar chart summarizes the contribution of GLEs to the total dose along this route for most of the GLEs since 1942. In the recent GLE history, the one that occured in january 2005 is the subject of a specific page.


was developed by [DGAC] and partners: [IRSN] , [l'Observatoire de Paris] , [IPEV]