As early as 1990, the International

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1 Institute of Radiation Protection How large is the exposure of pilots, aircrew and other frequent flyers to cosmic radiation? Hans Schraube Current topics As early as 1990, the International Commission for Radiation Protection (ICRP) estimated that pilots and other air crew had an exposure level from cosmic radiation that was similar to, or on average higher even, than that of people who worked with artificial radiation in medicine and technology. The ICRP made recommendations for maximum yearly doses which passed into European Law in 1996, and were subsequently taken over by the European member states. Since August 2003, under the German Radiation Protection Regulations of July 2001, the radiation dose that aircrew receive from cosmic radiation in the course of their work must be determined if the annual dose could potentially be more than 1 msv (millisievert). A number of European institutes started programmes in direct response to the ICRP recommendations aimed at the theoretical and experimental assessment of natural radiation exposure in aircraft. The Institute of Radiation Protection, which had already carried out relevant research work in the seventies, also took part in these investigations, eventually developing the software program EPCARD (European Program Package for the Calculation of Aviation Route Doses) with support from the European Commission and scientists at the University of Siegen. Using this program, it is possible to calculate the dose from natural cosmic radiation along any flight route and flight profile. The Luftfahrtbundesamt (LBA, Federal Office for Aviation) granted official approval for the programme in December Why does one need an extensive software program for the determination of radiation doses in aircraft and cannot simply measure them during a flight? 1

2 This is because legislation requires the determination of the effective dose. This quantity includes both physical and radiobiological information, and serves for the estimation of the radiation risk. From the universe, predominantly from the galactic space, energetic charged particles - mainly protons - enter our solar system. When these primary particles penetrate into the atmosphere of the earth, they cause an avalanche of secondary particles: neutrons, pions, mesons, electrons, photons and additional protons. The different particles of this modified cosmic radiation differ considerably in their biological effectiveness. For instance, neutrons have a much larger biological effectiveness per unit of absorbed dose than photons. Calculation of the effective dose must also take into account the fact that organs differ greatly in their radiation sensitivity. Therefore, the effective dose is not a directly measurable quantity. Its determination depends on knowledge of the energy spectra of the different types of particles. In the energy range important for dosimetry, the information can be gained by so-called Monte-Carlo calculations. (Monte-Carlo programs simulate physical processes according to given conditions, in which some parameters, for instance the initial energy of a particle, are selected by a random number chosen according to physical input data.) For neutrons, the relevant interval lies between electron volts and 500 mega electron volts. In addition, the radiation exposure depends not only on the geographic location and the flight altitude but is also subject to temporal variations. The energetic primary and secondary particles mentioned above react, according to their energy and charge, considerably with the earth s atmosphere. They lose energy and are finally absorbed in the atmosphere or at the earth s surface. The lower an aircraft flies, the weaker the cosmic-radiation dose rate. A dependence on the geographic latitude is caused by the earth s magnetic field. It is nearly constant in time. It can be overcome by charged particles most easily at the poles, because particles there pass approximately parallel to the field lines. At the geomagnetic equator the particles must have an energy above 15 GeV (15 billions electron volts) in order to reach the earth s atmosphere vertical to the field lines. Since the much more abundant particles of lower energy are turned away from the earth, the radiation exposure at the equator is much lower than at the poles. Finally, the intensity of the cosmic radiation also depends on the solar activity. The sun emits a huge stream of matter, the so-called solar wind, which ranges about hundred astronomical units (1 a.u. = distance sun-earth, ca. 150 million kilometers), which the charged primary particles have to overcome. The intensity of the solar wind fluctuates with the solar activity, which can be inferred from the number of sun spots, with a period of 11 and 22 years, respectively. The shielding effect of the atmosphere was calculated by means of a Monte- Carlo program, where the best models of NASA for the incoming galactic radiation and for the solar modulation were used. The description of the particle interactions in the MC program FLUKA that was used (developed by INFN and CERN 1 ) considers all physical 1 INFN = Istituto Nazionale di Fisica Nucleare, Italy CERN = European Laboratory for High-energy Physics, Geneva, Switzerland 2

3 Current topics Fig. 1: Mean sunspot number (lower curve, left scale) and correlated monthly averaged counts per hour devided by 100 (upper curve, right scale) from neutron monitor at Climax, Colorado, USA, in the Rocky Mountains, 3400 m height. The colored area represents the solar activity predicted by the NASA Marshall Space Center. Solar cycles are monitored since 1755, algebraic signs specify the respective polarity of the solar magnetic field. interaction processes and employs data sets from experiments at highenergy accelerators. As a result, an MC calculation gives a complete description of the cosmic radiation fields in the earth atmosphere by fluence densities of the most important primary and secondary particles under all possible conditions of the magnetic shielding and solar modulation. This serves as an input data base for EPCARD. The influence of the solar activity on the radiation dose can be described qualitatively in the following approximate way. First, the solar activity is correlated with the number of sun spots, which were found by Galileo around 1612 (lower curve in Fig.1). The maximum of the sunspot number corresponds also to the maximum of the magnetic field of the sun, which again corresponds to maximum shielding against the highly energetic galactic particles and therefore minimum radiation dose. The highly energetic radiation components which reach the earth s surface (upper curve in Fig. 1) are registered in so-called neutron monitors, which are located at 25 selected geographical places. With a simplified model of the magnetic field of the sun, a correlation can be given between the reading of the neutron monitors and the deceleration potential of the solar wind, which the cosmic particles have to overcome. With this correlation, one can calculate how the energies of the particles at the entrance into the magnetic sphere and the atmosphere of the earth depend on the solar activity. How do these physical effects influence the radiation dose in usual flight altitudes? 3

Figure 2: Diagram serving for a rough estimation of the ambient dose-equivalent rate for flight altitudes between 5 and 15 km near the poles and equator for the period of high and low sun activity,

4 Figure 2: Diagram serving for a rough estimation of the ambient dose-equivalent rate for flight altitudes between 5 and 15 km near the poles and equator for the period of high and low sun activity, respectively. Figure 3: Comparison of flight duration (continuous curve, right scale) and effective dose (left scale) for flights from Munich and Frankfurt (*) to selected destinations by the shortest route, arranged by increasing flight duration. The doses were calculated using EPCARDv3.2 for January 2002 for the following conditions: ascent and descent 30 minutes each, assumed flight altitude 37,000 ft (approx. 11 km). 4

5 Figure 2 can serve for a rough estimation. Here the ambient dose-equivalent rate is shown as a function of flight altitude. This is the quantity which would be indicated by an ideal dosemeter. It can be seen that, for instance, at 10 km altitude (about ft) the dose rate near the equator is about 1.5 µsv/h (micro Sievert per hour) and that the solar activity is of less influence. Near the poles, however, the dose rate can be twice as high when the sun is in a period of relatively high activity. From Figure 3, a first estimation of the flight dose can be derived, which will vary considerably according to the flight route and the flight profile. It can be clearly seen that, on flights over the equator (e.g., from Munich to Sao Paulo), much less dose is accumulated at comparable duration of flight than on northern Atlantic routes (e.g., to San Francisco). For comparison, if one stays at our latitudes at sea level, the effective dose from natural cosmic radiation amounts to about 0.3 msv per year, compared with a total external natural radiation exposure of about 1 msv. For the evaluation of the radiation risk, the composition of the radiation field plays an important role. Different types of particles are differently absorbed by the organs of the body and create a variety of secondary particles. Therefore, in EPCARD all radiation components (which contribute to the exposure) are separately calculated. In Figure 4, the relative contributions to the effective doses are plotted for the flight routes indicated in Figure 3. The contributions of neutrons and protons are nearly equal, both together contributing about 65% to 80% to the total dose, depending on the flight route. However, the ICRP recently proposed to reduce the radiation weighting factor of protons from 5 to 2, which would lower the contribution of protons accordingly. For the calculation of effective doses, and also for interpretation of the readings of dose measurement devices, one has to know the energy distribution of all components of the cosmic radiation. As already mentioned, these are results from MC calculations. Especially for neutrons, they were already calculated in middle seventies in the USA, but the results were questionable and ignored for dose calculations. For experimental verification, a neutron spectrometer was brought by to two high mountains of very different magnetic cutoff rigidity: Mt. Zugspitze at about 4 GV and Mt. Chacaltaya at about 13 GV. The energy that primary cosmic particles must have to overcome the earth s magnetic field is determined by this quantity, which lies between 0 GV at the magnetic poles and 17 GV at the geomagnetic equator. Figure 5 shows the experimental result. Because of the energy-dependent weighting of neutrons, only the component above 0.1 MeV contributes to the effective dose. Important, however, is the relative maximum of the spectrum at about 100 MeV, whose existence and height was experimentally confirmed for the first time. The Institute of Radiation Protection was concerned very early with the experimental determination of radiation dose in aircraft. In the years 1990 to 1993, with help of the German airline Lufthansa, a series of measurement flights were carried out. As mentioned above, a temporally and regionally limited number of flights are not sufficient for a complete description of the radiation field in the earth s atmosphere. However, the measurement data obtained then can now be used in a significant way to verify the global calculations of the radiation field experimentally. Current topics 5

Figure 4: Relative contribution of the cosmic ray components to effective dose for the flight routes indicated in Figure 3 (alphabetically arranged). Doses are calculated for 1998, i.e. the time period of the lowest sun activity, and therefore, the highest cosmic radiation exposure.

6 Figure 4: Relative contribution of the cosmic ray components to effective dose for the flight routes indicated in Figure 3 (alphabetically arranged). Doses are calculated for 1998, i.e. the time period of the lowest sun activity, and therefore, the highest cosmic radiation exposure. In Figure 6 the results of 19 measurement flights are given, which were carried out in the years in a period of high solar activity and therefore of relatively low dose rate. The values of the effective dose along the routes were found to be between 5 and 65 µsv. The associated ambient dose equivalents are expected to be 10 to 20% lower. The calculated values are 11% higher than the respective measurement values on average (Fig.7). A reason for this, among others, is that the calculations are performed for the radiation field outside an aircraft and, therefore, absorption effects of the aircraft construction and the fuel are not taken into account. Thus, a slightly conservative estimation of the actual radiation exposure results. The effect can be even a little higher, because the contribution of the protons to the experimental result could not completely be taken into account. This observation is in accordance with investigations of the INFN, where for different computational simulated types of aircrafts the dose was determined at different places within the aircraft. For aviation companies, a full version of our computer program EPCARD is available which is able to calculate for each flight the accumulated dose to assign to their pilots and other aircrew according the legal national regulations. The German Luftfahrtbundesamt (LBA) granted official approval for this version and published this act in Nachrichten für Luftfahrer (NfL II-107/03 from ). A simplified private use version can be found under the address Smaller air companies, especially, have the possibility, which was approved by LBA, 6

Figure 5: Neutron spectra determined experimentally in Schneefernerhaus (Mt. Zugspitze, 2660 m height) and Mt.

Energy region ranges between 1 mev and 10 GeV.

7 Figure 5: Neutron spectra determined experimentally in Schneefernerhaus (Mt. Zugspitze, 2660 m height) and Mt. Chacaltaya (Bolivia, 5240 m height). The same areas express the same fluence rate. Energy region ranges between 1 mev and 10 GeV. Current topics Figure 6: Comparison of EPCARD calculated route doses with in-flight measurements carried out in the years The non-measurable effective dose, E, the quantity ambient dose-equivalent, H*(10), and the measured values of H*(10), are plotted as bars (left scale), the ratios E/H* as points (right scale), respectively. Airport codes, arranged according to the abscissa order from left to right side, are explained in the table. 7

8 Figure 7: Ratios of calculated and measured route doses, H*(10)/H*exp, for in-flight measurements in Figure 6. to show proof that for their pilots the dose values accumulated in a year do not exceed 1 msv. The official approved version of EPCARD is already in use by the most important German air companies, e.g. Deutsche Lufthansa, LH-Cargo, LH- CityLine, Condor, Germania and LTU. For the French air companies, the partner institute IRSN of the Institute of Radiation Protection determines the flight doses also with the help of EPCARD on a preliminary test base. Negotiations are on the way with other European air companies. Selected publications Roesler, S., Heinrich, W. and Schraube, H.: Monte Carlo calculation of the radiation field at aircraft altitudes. Radiat. Prot. Dosim. 98, 4 (2002) Schraube, H., Leuthold, G., Heinrich, W., Roesler, S., Mares, V. and Schraube, G.: EPCARD European program package for the calculation of aviation route doses, User s manual. -National Research Center, Neuherberg, Germany (2002). ISSN Report 08/02 Mares, V., Roesler, S. and Schraube, H.: Averaged particle dose conversion factors in air crew dosimetry. Radiat. Prot. Dosim. 110, 1-4 (2004)

Theoretical Assessment of Aircrew Exposure to Galactic Cosmic Radiation Using the FLUKA Monte Carlo Code

Theoretical Assessment of Aircrew Exposure to Galactic Cosmic Radiation Using the FLUKA Monte Carlo Code R. Ashkenazi 1, 2, J. Koch 1 and I. Orion 2 1 Radiation Safety Division, Soreq Nuclear Research