The SIEVERT system: taking into account

1
The SIEVERT system taking into account GCR and
SPE effects aboard aircraft
N. Fuller a, P. Lantos a and J.F.
Bottollier-Depois b
a Paris Observatory, 92190 Meudon, FRANCE b IRSN,
B.P. 17, 92262 Fontenay-aux-Roses, FRANCE
Recently, the European Commission (EU directive
96/29/EURATOM) included the exposure of aircraft
crew to cosmic radiation as occupational
exposure. It was following the recommendations of
the International Commission on Radiological
Protection (ICRP, 1991) concerning the exposure
to enhanced or elevated levels of radiation from
natural sources. The effective dose should not be
higher than 100 mSv over 5 years with a maximum
of 50 mSv for a given year (specific rules apply
to pregnant air crew). The radiation doses
onboard aircraft are due to two sources Galactic
Cosmic Rays (GCR) and Solar Proton Events (SPE).
The doses are the result of the numerous
secondary particles created in the atmosphere by
high energy primary particles. The galactic
component is permanent but modulated by the solar
activity in the course of the 11-year solar
cycle. The modulation parameter is an input of
models such as EPCARD (Schraube, 1999) which
computes the dose for GCR at any point in space
up to an altitude of 80,000 feet. The SPE, when
detected at ground level by neutron monitors
(GLE), may enhance significantly the doses
received onboard aircraft. A specific
semi-empirical model named SiGLE was developed
(Lantos Fuller, 2003) to take into account
these events. Using EPCARD and SiGLE, the
computerized system for flight assessment of
exposure to cosmic radiation in air transport, or
SIEVERT (Bottollier-Depois, 2003), is proposed
to airline companies for assisting them in the
application of this new legal requirement. This
dose assessment tool was developed by the French
General Directorate of Civil Aviation (DGAC) and
partners the Institute for Radiation Protection
and Nuclear Safety (IRSN) and Paris Observatory.
This professional service is accessible to
airlines but also to a larger public via the
internet site www.sievert-system.org, which
allows any passenger to get an estimate of the
dose received during a given flight.
The SIEVERT principle Airspace is divided into
cells. Each one is 1000 feet in altitude, 10 in
longitude and 2 in latitude. Altogether they
form a map of 265,000 cells to each of these
cells is assigned an effective dose rate value.
The time spent by the plane on each cell and the
corresponding dose are calculated their
accumulative total gives the dose received during
the flight.
The IRSN updates the map of the dose rates every
month by taking solar activity into account. A
map of the hourly dose at a typical subsonic
altitude is given as an example for January 2005.
In the case of a GLE, a specific map is created
(see below). In addition, regular radiation
measurements, from dosimeters installed on the
ground and on aircraft, are used to confirm and,
if necessary, to correct the obtained values.
The company prepares a file of completed or
scheduled flights, and sends it to the SIEVERT
Internet address. The system then completes the
file by adding the effective dose that
corresponds to each flight. Doses are calculated
according to flight characteristics, using the
dosimetric input data validated by the IRSN. It
is asked to airlines to described a flight using
way points. If the information is minimal (like
information available on flight ticket), the dose
value is assessed using a standard route profile.
The data, at this stage, are anonymous. Airlines
are in charge to add up the doses received during
flights carried out by each member of the flight
personnel.
Doses obtained from measurements by IRSN between
1996 and 1998. The circles contain the average
dose equivalent rate on the flight in µSv/h, and
the radiation mean quality factor. The total dose
equivalent (mean rate x time) is given for a
round-trip flight.
Dose calculation principle with SIEVERT
Validation Results show that a monthly
cartography based on the average intensity
measured with a neutron monitor is sufficient to
achieve a precision of about 20 on effective
dose calculation, for each flight. This study
also pointed the importance of using the detailed
flight plan of each flight to achieve sufficient
precision. Indeed, on a subsonic route like
Paris-Washington, two flights, operated on the
same month, with the same aircraft, on the same
route and direction could exhibit a relative
variation of more than 50 .
Principles of data exchange between SIEVERT and
airline companies.
The SiGLE principle The semi-empirical model
SiGLE combines few available measurements
obtained on board Concorde during GLEs in 1989
and 2000 and on board a subsonic flight during a
GLE in 2001, with calculations based on particle
transport codes for GLE 42 on 29 September 1989,
to compute an estimate of the dose D(t) received
during GLEs .
From the Air France and British Airways Concorde
measurements, a linear relationship C(g) between
ground based neutron monitor GLE time profiles
and dose rates at 60000 in altitude is derived
for different particle rigidity spectral
exponents (noted ?). The rigidity spectrum
exponent is deduced from the ratio between two
neutron monitors (Webber Quenby, 1959 Lantos,
2005) when an complete calculation is not
available (recent GLEs for example).
D(t) A(z,g ) x L(lG) x C(g ) x I(t)
The measurement on board a Czech Airlines flight
from Prague-to New York (Spurný Dachev, 2001),
during the GLE numbered 60, on 15 April 2001, as
well as plots based on theoretical calculations
by OBrien et al. (1998), are used to derive the
attenuation factor A(z,g) between dose rate at
60000 feet in altitude and dose rate at the
aeroplane altitude, noted z. Because the
available flights of Concorde were restricted to
routes between New York (geomagnetic latitude
?G  50.7N) and Paris (?G  51.1N) or London
(?G  53.7N), the computed doses are valid for
the North Atlantic path.
L(?G) function, giving the variation of the dose
rate with the geomagnetic latitude at subsonic
altitudes, is estimated using results of dose
rate calculation during GLE 42 (OBrien Sauer,
2000) for Greenwich meridian. Then, from the
results on North Atlantic path, the dose rates
are deduced for other geomagnetic latitudes.
Dose equivalent rate coefficient L in function
of the geomagnetic latitude for subsonic altitude
35 000 feet. The reference latitude corresponds
to North Atlantic routes. Lower axis gives
corresponding vertical cut-off rigidity for
northern hemisphere and European sector (epoch
1995).
The reference monitor for the model is Kerguelen
Islands, in South Indian Ocean. It is located at
Port-aux-Français (?G  57.5S and vertical
cut-off rigidity of 1.1 GV) . It is operated by
the French Institute for Polar Research (IPEV)
Logarithm of the attenuation of dose equivalent
rate in function of altitude for different values
of the rigidity spectrum exponent g. Attenuation
for galactic cosmic rays is indicated with dashed
line and attenuation for GLEs with average
rigidity spectrum exponent g - 4.7 is
indicated with a dotted line.
Application to past and last GLE The bar plot
below summarises the effective doses received for
two routes during 31 GLEs (over 67 observed until
2004), the others giving negligible radiation
effect. To each GLE correspond four bars. The
first (in black) is the contribution to effective
dose of the GLE itself for Paris-New York flight
on-board Concorde. The second (in white) is the
total effective dose taking into account GCR
contribution too, calculated for the month of the
event. The two last bars are the same but for
Paris-San Francisco subsonic flight. All
calculations correspond to the worse case in
terms of departure time. It should be noted that
the lower protection at supersonic altitude is
counterbalanced by the flight durations which are
quite different 11 h 24 m for subsonic flights
instead of 3½ h for Concorde. This explains the
rather small difference observed between black
bars for a given GLE. According to these results,
over the 67 GLEs observed since 1942, only 18 are
to be included in operational dose calculations,
if we consider that the GLEs below 30 µSv could
be neglected (this limit is representative of the
lower limit of the effective dose received from
GCR during a typical intercontinental journey).
The GLE 68 of the 20 January 2005 showed a very
important North-South anisotropy above 65 in
geomagnetic latitude. It was measured at an
intensity of 178.4 with Kerguelen NM, 3308
with Terre Adélie NM and 2091 with McMurdo NM
(with 5 minute counts). In the North hemisphere,
at about the same geomagnetic latitudes, the
intensity is only 277 for Inuvik (Canada) NM,
114  for Thule (Greenland) NM and 112  for
Barentsburg (Spitzberg) NM. It thus appears as
one of the strongest GLEs observed during the
last fifty years. The following table gives doses
received from GLE 68 and from galactic cosmic
rays for a few typical flights. The flights are
based on actual flight plans and doses are
calculated with the SiGLE model. The doses
received from galactic cosmic rays (GCR) are
calculated with CARI 6 software.
Conclusion SIEVERT provides a correct application
of the regulation for at least three reasons.
First, the results obtained are close enough to
reality to avoid under-estimating the doses
received by the personnel. Second, the radiation
dose assessment mode is the same for all
airlines. Third, if checks become required in the
future, retrospective dose calculations might
always be performed. A pioneering aspect of
SIEVERT lies in the fact that it takes both
potential radiation sources into account, GCR and
SPE, using two efficient tools, EPCARD and SiGLE,
which have been tested and validated. The system
is used in routine at a national level since
2000. About 70,000 flights per month are
proceeded by the overall French airlines.
The flights from Paris to San Francisco and the
flight from Tokyo to Paris along polar route are
specifically corrected from the anisotropy
mentioned above (SiGLE gives respectively 96.9
µSv and 88.3 µSv without correction). The world
map gives an example, at subsonic altitude, of
the hourly dose computed with SiGLE at the time
of the maximum of the GLE (GLE GCR).
History of significant GLEs in term of dose since
1942 for a supersonic and a subsonic flight (see
text for full description)