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Exposure of aircrew to cosmic radiation. Calculation and experimental approach
International Congress Series 1225 (2002) 121 – 129
Exposure of aircrew to cosmic radiation. Calculation and experimental approach Frantisˇek Spurny´ * Department of Radiation Dosimetry, Nuclear Physics Institute, Academy of Sciences of the Czech Republic, Na Truhla´ ce 39/64, 180 86 Prague 8, Czech Republic
Abstract Exposure to cosmic radiation increases rapidly with the altitude. At the flight levels of commercial aircraft, it is of the order of several mSv per hour. Most aircrews are exposed regularly to the effective dose exceeding 1 mSv/year, the limit of exposure for nonprofessionals defined in the International Commission on Radiological Protection (ICRP) 60 recommendations, which were already incorporated to the Czech regulation since 1997. That is why this problem has been intensively studied in the author’s laboratory since the beginning, and the contribution reviews this effort. First, some experimental studies have been realised in 1991 – 1994. Later, a preliminary analysis of the exposure of the aircrew of companies operating in the Czech Republic has been realised based on the calculation by means of the transport code CARI (Civil Aeromedical Research Institute). Afterwards, the procedure of aircrew exposure estimation has been elaborated. The procedure was proposed to the Czech authorities to be used for all Czech companies concerned. It has been finally accepted, and the results concerning the exposure of aircrew during 1998 and 1999 are presented, analysed, and discussed. New experimental studies also followed. Mostly, active dosimeters were used to estimate low and high linear energy transfer component of the radiation field on board. The measurements on-board have been performed since 1999 in about 10 commercial flights. The results obtained were transformed to the effective dose and compared with the values of the same quantity calculated by means of the CARI. A reasonable agreement of both sets of data could be stated. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Radiation protection; Occupational exposure; High-energy radiation
*
Tel.: +420-2-8384-1772; fax: +420-2-8384-2788. E-mail address: [email protected] (F. Spurny´).
0531-5131/02 D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 3 1 - 5 1 3 1 ( 0 1 ) 0 0 5 2 1 - 0
F. Spurny´ / International Congress Series 1225 (2002) 121–129
122
1. Introduction The cosmic radiation is one of the contributors to the natural radiation environment. The level of exposure to it increases with altitude. At the air transport altitudes, it can reach 10 mSv/h. In 1990, the International Commission on Radiological Protection (ICRP) recommended that the radiation exposure due to cosmic rays at high altitudes be taken into account where appropriate, as part of occupational exposure to radiation [1]. Aircrew has become another group of workers for whom exposure to ionising radiation is an occupational hazard [2 –6]. There are significant differences in exposure conditions of aircraft crew and occupational exposures, generally, [6]:
the fraction of dose equivalent deposited at high linear energy transfer (LET) is much greater for aircrew, about 50%, compared to only a few percent for others; and, there are a little more than one half of females in aircrew, and they represent only a few percent of other occupationally exposed persons. Both these two factors increase the importance of correct estimation of aircrew exposure.
2. Cosmic radiation at subsonic aircraft altitude—aircrew exposure Cosmic radiation on the aircraft board is composed mostly of secondary particles created during the transport of cosmic rays in the Earth’s atmosphere [7]. These particles originate more than 90% from the galactic cosmic radiation. The exposure level is predictable, with the exception of very rare giant solar flares. If such event would occur, it had to be treated independently. Necessary approaches are not discussed in this contribution. At the altitudes of subsonic air transport (9– 12 km), the radiation field can be divided into the component with low linear energy transfer (LET) (mostly the electrons and the high energy protons) and the component with high LET (mostly neutrons with energies up to about 100 MeV). These components contribute to the dose equivalent, close to magnetic poles, roughly by one half. The importance of high LET component relatively decreases when going to equatorial regions. The exposure level varies only a little within an aircraft and depends on several parameters. One can estimate the exposure experimentally or by calculation. The values presented in this chapter were obtained from the code CARI, developed at the Civil Aeromedical Research Institute of the US Federal Aviation Administration in Oklahoma [3] and widely used for this purpose [2,4 –7]. The exposure level was expressed in the ambient dose equivalent H*(10). The variations of the dose equivalent rate with the geographic latitude and longitude at the flight altitude of 41 000 ft and the solar activity close to the minimum during the period 1958 –1998 are shown in Fig. 1. The combined influence of the solar activity and geomagnetic parameters is demonstrated in Fig. 2, where the relative values of H*(10) on some flight routes are presented for the solar activity variations during the periods 1958 – 1998 (heliocentric potential (HCP) between 400 and 1800 MV). One can see there that the
F. Spurny´ / International Congress Series 1225 (2002) 121–129
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Fig. 1. Geographic variations of the dose equivalent rate at the altitude 41 000 ft, at HCP 420 MV.
exposure levels are at solar minimum for the routes close to magnetic poles (New York, Montreal) up to twice higher than at a solar maximum. For the routes situated close to the equator, these variations are much lower, while for the route between Abu Dhabi and Bangkok, they do not exceed 10%. As far as the flight altitude influence is concerned, the exposure level increases more than twice between 9 and 12 km, and the increase is a little higher in the period of low solar activity. Integral values of the dose equivalent for a route depend also on the duration of a flight. It was estimated that the exposure rate levels are, at the HCP, about 500 MV, for short-haul flights (shorter than 3 h) typically around the 2– 3 mSv/h, while for long-haul flights, are roughly twice higher, mostly due to smaller relative contribution of ascent and descent times. Annual exposure for an aircrew depends also on the total annual flight time. Annual dose limits may differ in different countries, it is generally limited to less than 1000. It was estimated that 60% of total UK aircraft crew flying mostly short-haul routes would receive, annually in average, about 2 mSv, and 40% of them flying mostly long-haul routes, about 4 mSv [6]. Such values are higher than average values for most of the other occupationally exposed workers. For example, the average annual dose in the UK was 0.8 mSv in 1996 [6], the average values for Czech Republic in 1975 – 1989 were between 0.58 and 0.94 mSv for medical occupational exposures, and between 1.10 and 1.51 mSv for industrial application of ionising radiation, with descending tendency [8].
124
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Fig. 2. The influence of solar activity during some flights.
3. Preliminary studies in the Czech Republic First experimental studies of the aircrew exposure were realised in Czech Republic in the years 1991– 1994 and were already published [9]. The results agreed reasonably well with the results obtained by other authors and compiled by the author in Ref. [2]. The ICRP 60 recommendations were already incorporated to the Czech regulation since 1997 [10]. Since the end of that year, a preliminary analysis of the exposure of Czech air company’s aircrew members has been started with the goal to develop an ‘‘optimum’’ procedure of aircrew exposure level estimation. The analysis was based on the CARI code already mentioned [3]. It was found that the calculation based on optimal flight parameters, which varies considering the aircraft type, gives results that are satisfying to the general requirements of individual dosimetry [11,12]. The main principle of this modelling, agreed by air companies, was the choice of an optimum flight altitude (closer to higher level for each aircraft to be on the safe side of estimation) and constant time for taking-off and landing.
4. Individual dosimetry of Czech air company’s aircrew The procedure was further developed, and the code CARI has been proposed in the version 5E, which calculates already the exposure level in the effective dose E, the quantity used for the limitation. It was finally proposed to the Czech authorities to be used for companies concerned. It has been accepted and used since 1998. Fig. 3 shows the aircrew
F. Spurny´ / International Congress Series 1225 (2002) 121–129
125
Fig. 3. Effective dose relative distributions of Czech air companies in 1999.
effective dose distributions for all air companies concerned for the year 1999. Fig. 4 shows, for one of them, the comparison of these distributions for the years 1998 and 1999. The following main characteristics can be deduced from these figures:
average effective dose value for all companies were, in 1999, a little lower by 2 mSv; for two companies flying only short-haul routes in Europe and to its holiday regions, there is one broad maximum, while the distribution for the third one, flying also long-haul flights to the North America and the Far Orient is characterised by two maxims; these maxims are still better seen in the comparison of two years (Fig. 4).
5. On-board measurements during 1999 [13] Several route types were chosen to characterise the most possible exposure types for aircrew members of air companies: intraeuropean flights to Northern Europe and to holiday regions of Spain and/or Greece; North Atlantic routes (to New York and Toronto); and a flight to Southeast Asia (Bangkok). Measurements have been performed between March and November 1999 on 11 round trip flights. For each flight, all necessary parameters registered by means of the board computer have been received. The exposure for individual flight has also been calculated by means of the CARI 5E code and were compared with the measured data.
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Fig. 4. Comparison of effective dose relative distributions of CSA air company in the years 1998 and 1999.
The measurements of the low LET component were performed with Reuter Stokes argon-filled high-pressure steel ionisation chamber RSS 112, NB 3201 scintillation counter, and two types of individual electronic dosimeters, both based on a Si-diode. To characterise the contribution of high LET (neutron) component, we used bubble damage neutron detectors (BDNDs) with a 100 keV neutron energy threshold and superheated drop detectors (SDD) with three energy thresholds (0.1, 1.0, and 6.0 MeV). Bubble damage spectrometer as well as BDND available from the Bubble Technology Industries, Chalk River, was used to determine the dose equivalent on the basis of an estimation of neutron spectra. It is composed of six sets of detectors with six ‘‘thresholds’’ in neutron energy: 0.01, 0.10, 0.60, 1.0, 2.5, and 10 MeV. Low LET radiation-measuring instruments were calibrated with 60Co photons, and high LET radiation measuring equipment with an AmBe neutron source. For both components, the response has been primarily expressed in terms of the ambient dose equivalent of the Table 1 Relative contributions of low and high LET components to the total dose equivalent Destination from Prague
Ratio of values high to low LET component
Contribution of high LET component to the total
Northern America Northern Europe Southern Europe Abu Dhabi Abu Dhabi – Bangkok
1.15a 1.09 0.86 0.514 0.439
0.535a 0.521 0.462 0.338 0.291
a
Uncertainty of values is estimated to about ± 15% (1r).
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Table 2 Comparison of experimental and calculated total exposure levels on board of aircraft Flight route
H*(10) measured, mSv
E, calculated by CARI 5E, mSv
Prague – Oslo – Prague Prague – Helsinki – Prague Prague – Madrid – Prague Prague Stockholm – Prague Prague – New York (Newark) New York (Newark) – Prague Prague – St. Petersburg – Prague Prague – Moscow – Prague Prague – Montreal – Toronto Toronto – Prague Prague – Prevesa – Prague Prague – Valencia – Prague Prague – Abu Dhabi – Bangkok Bangkok – Abu Dhabi – Prague
14.8 21.2 18.5 15.0 42.4 33.6 20.7 17.7 42.6 45.0 15.5 17.6 26.8 29.5
12.1 17.7 16.2 15.8 38.0 32.2 18.2 17.5 37.4 36.9 14.6 17.9 26.9 28.2
reference radiation. The results obtained with all methods characterising low and/or high LET component on-board have been combined and corrected, taking into account the response in CERN high-energy reference field (concrete shielding) [14]. First, we have analysed to what extent our experimental results would reflect some of general tendencies. The results of such confrontation are presented in Table 1. In Table 1, one can see that the tendencies expected are well confirmed by our experimental results, i.e., the relative importance of both component of radiation field at northern parts is close to being the same, the importance of high LET component clearly diminishes when going to the south. Experimentally determined values of H*(10) and the theoretical values of the effective dose E are compared in Table 2. The calculation has been performed for actual flight altitude profile and geomagnetic parameters of the route, taking into account the actual value of heliocentric potential (solar activity) in a month. One can see that both sets of values do not differ much. The experimental values of H*(10) are a little higher, an average of (9 ± 6)%, than the theoretical E-values. The values of E should be higher than the values of H*(10) in the fields on-board, about 20% in the case of ‘‘northern routes’’, and a little less for the routes close to the equator [15]. It seems that the code CARI 5E underestimates a little the actual exposure level.
6. Conclusions (1) It is believed that both the experimental and calculated values presented in this contribution represent a reliable [12] estimation (1r about ± 20%) of the aircrew exposure levels. (2) Of course, our knowledge of the exposure level of aircrew must be improved. As far as the calculation is concerned, a new version of the code (CARI 6) appeared at the
128
F. Spurny´ / International Congress Series 1225 (2002) 121–129
beginning of year 2000. We have tested it already and found that the values obtained are at subsonic flight altitudes about 10% higher than those calculated with CARI 5E. In such case, the agreement of experimental and calculated data would still be better. Other codes were also developed and will be tested in the frame of new EURADOS working group activities and new EC research project [16,17]. (3) Moreover, more complex experimental measurements on-board complemented by the calibration in high-energy laboratory reference fields must continue. New measurements are in progress in several laboratories, particularly in the frame of EC projects supporting these studies [16]. Further measurements would be more concentrated to the equatorial regions, and it would also be very important to succeed with the measurements during an important solar flare.
Acknowledgements These studies were supported through the grant of the Grant Agency of the Czech Republic No. 202/99/0151. We are also much obliged to CSA Air for the support and the interest in studies presented in this contribution. References [1] 1990 Recommendations of the International Commission on Radiological Protection, Ann. ICRP vol. 21 ICRP Publication 60, 1991, No. 1. [2] Exposure of aircrew to cosmic radiation, in: I.R. McAulay, et al (Eds.), EURADOS Report 1996-01, ISBN 92-827-7994-7, Luxembourg, 1996. [3] W. Friedberg, W. Snyder, D.N. Faulkner, Radiation exposure of air carrier crew members II, US FAA Report DOT/FAA/AM-92-2, 1992. [4] International Conference on Cosmic Radiation and Aircrew Exposure. European Communities, Radiat. Prot. Institute of Ireland, Assoc. European Airlines; July 1998; Proceedings published as a special volume of Radiat. Prot. Dosim. 86, 1999. [5] Recommendations for the Implementation of the Title VIII of the European Basic Safety Standards Directive Concerning Significant Increase in Exposure due to Natural Radiation Sources. EC, DG XI, ISBN 92-8275336-0, EC 1997. [6] D.T. Bartlett, Radiation protection concepts and quantities for the occupational exposure to cosmic radiation, Radiat. Prot. Dosim. 86 (4) (1999) 263 – 268. [7] K. O’Brien, W. Friedberg, H.H. Sauer, D.F. Smart, Atmospheric cosmic rays and solar energetic particles at aircraft altitudes, Environ. Int. 22 (Suppl. 1) (1996) S9 – S44. [8] Z. Prouza, F. Spurny´, V. Klener, I. Fojtikova´, P. Fojtik, J. Podskubkova´, Occupational radiation exposures in the Czech and Slovak Republic, Radiat. Prot. Dosim. 54 (1994) 333 – 336. [9] F. Spurny´, Experimental approach to the exposure of aircrew to cosmic radiation, Radiat. Prot. Dosim. 70 (1997) 409 – 412. [10] Directive of the State Office for Nuclear Safety concerning the Radiation Protection (in Czech). Sbı´rka za´kon. 184/1997, Sbı´rka za´kon R a´ stka 66, emitted the 19th August 1997. [11] F. Spurny´, A. Malusˇek, Exposure level of CSA a.s. aircrew. I. Contribution of different routes in 1997 (in ´ JF AV R 447/98; Prague, November 1998. Czech), Report ODZ U [12] International Commission on Radiological Protection 75: General Principles for the Radiation Protection of Workers, Pergamon, Oxford, 1997. [13] F. Spurny´, et al., Experimental determination of the air crew members of the CSA air company—Comparison with theoretical calculations (in Czech), Report DRD NPI AS CR 466/99, Prague, June 1999.
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[14] M. Ho¨fert, G.R. Stevenson, The CERN-CEC High Energy Reference Field Facility. Presented at the American Nuclear Society Meeting—8th Inter. Conf. on Radiation Shielding, Arlington, Texas. [15] D.T. Bartlett, Aspects of the exposure of aircraft crew to radiation, Radiat. Prot. Dosim. 81 (1999) 243 – 245. [16] EC Contract No.F15P-CT99-00239. Co-ordinator O’Sullivan D (DIAS Dublin), 2000 – 2003. [17] EURADOS WG 5: ‘‘Aircrew exposure’’, chairman Ulrich J. Schrewe, since 2001.
Exposure of aircrew to cosmic radiation. Calculation and experimental approach Frantisˇek Spurny´ * Department of Radiation Dosimetry, Nuclear Physics Institute, Academy of Sciences of the Czech Republic, Na Truhla´ ce 39/64, 180 86 Prague 8, Czech Republic
Abstract Exposure to cosmic radiation increases rapidly with the altitude. At the flight levels of commercial aircraft, it is of the order of several mSv per hour. Most aircrews are exposed regularly to the effective dose exceeding 1 mSv/year, the limit of exposure for nonprofessionals defined in the International Commission on Radiological Protection (ICRP) 60 recommendations, which were already incorporated to the Czech regulation since 1997. That is why this problem has been intensively studied in the author’s laboratory since the beginning, and the contribution reviews this effort. First, some experimental studies have been realised in 1991 – 1994. Later, a preliminary analysis of the exposure of the aircrew of companies operating in the Czech Republic has been realised based on the calculation by means of the transport code CARI (Civil Aeromedical Research Institute). Afterwards, the procedure of aircrew exposure estimation has been elaborated. The procedure was proposed to the Czech authorities to be used for all Czech companies concerned. It has been finally accepted, and the results concerning the exposure of aircrew during 1998 and 1999 are presented, analysed, and discussed. New experimental studies also followed. Mostly, active dosimeters were used to estimate low and high linear energy transfer component of the radiation field on board. The measurements on-board have been performed since 1999 in about 10 commercial flights. The results obtained were transformed to the effective dose and compared with the values of the same quantity calculated by means of the CARI. A reasonable agreement of both sets of data could be stated. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Radiation protection; Occupational exposure; High-energy radiation
*
Tel.: +420-2-8384-1772; fax: +420-2-8384-2788. E-mail address: [email protected] (F. Spurny´).
0531-5131/02 D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 3 1 - 5 1 3 1 ( 0 1 ) 0 0 5 2 1 - 0
F. Spurny´ / International Congress Series 1225 (2002) 121–129
122
1. Introduction The cosmic radiation is one of the contributors to the natural radiation environment. The level of exposure to it increases with altitude. At the air transport altitudes, it can reach 10 mSv/h. In 1990, the International Commission on Radiological Protection (ICRP) recommended that the radiation exposure due to cosmic rays at high altitudes be taken into account where appropriate, as part of occupational exposure to radiation [1]. Aircrew has become another group of workers for whom exposure to ionising radiation is an occupational hazard [2 –6]. There are significant differences in exposure conditions of aircraft crew and occupational exposures, generally, [6]:
the fraction of dose equivalent deposited at high linear energy transfer (LET) is much greater for aircrew, about 50%, compared to only a few percent for others; and, there are a little more than one half of females in aircrew, and they represent only a few percent of other occupationally exposed persons. Both these two factors increase the importance of correct estimation of aircrew exposure.
2. Cosmic radiation at subsonic aircraft altitude—aircrew exposure Cosmic radiation on the aircraft board is composed mostly of secondary particles created during the transport of cosmic rays in the Earth’s atmosphere [7]. These particles originate more than 90% from the galactic cosmic radiation. The exposure level is predictable, with the exception of very rare giant solar flares. If such event would occur, it had to be treated independently. Necessary approaches are not discussed in this contribution. At the altitudes of subsonic air transport (9– 12 km), the radiation field can be divided into the component with low linear energy transfer (LET) (mostly the electrons and the high energy protons) and the component with high LET (mostly neutrons with energies up to about 100 MeV). These components contribute to the dose equivalent, close to magnetic poles, roughly by one half. The importance of high LET component relatively decreases when going to equatorial regions. The exposure level varies only a little within an aircraft and depends on several parameters. One can estimate the exposure experimentally or by calculation. The values presented in this chapter were obtained from the code CARI, developed at the Civil Aeromedical Research Institute of the US Federal Aviation Administration in Oklahoma [3] and widely used for this purpose [2,4 –7]. The exposure level was expressed in the ambient dose equivalent H*(10). The variations of the dose equivalent rate with the geographic latitude and longitude at the flight altitude of 41 000 ft and the solar activity close to the minimum during the period 1958 –1998 are shown in Fig. 1. The combined influence of the solar activity and geomagnetic parameters is demonstrated in Fig. 2, where the relative values of H*(10) on some flight routes are presented for the solar activity variations during the periods 1958 – 1998 (heliocentric potential (HCP) between 400 and 1800 MV). One can see there that the
F. Spurny´ / International Congress Series 1225 (2002) 121–129
123
Fig. 1. Geographic variations of the dose equivalent rate at the altitude 41 000 ft, at HCP 420 MV.
exposure levels are at solar minimum for the routes close to magnetic poles (New York, Montreal) up to twice higher than at a solar maximum. For the routes situated close to the equator, these variations are much lower, while for the route between Abu Dhabi and Bangkok, they do not exceed 10%. As far as the flight altitude influence is concerned, the exposure level increases more than twice between 9 and 12 km, and the increase is a little higher in the period of low solar activity. Integral values of the dose equivalent for a route depend also on the duration of a flight. It was estimated that the exposure rate levels are, at the HCP, about 500 MV, for short-haul flights (shorter than 3 h) typically around the 2– 3 mSv/h, while for long-haul flights, are roughly twice higher, mostly due to smaller relative contribution of ascent and descent times. Annual exposure for an aircrew depends also on the total annual flight time. Annual dose limits may differ in different countries, it is generally limited to less than 1000. It was estimated that 60% of total UK aircraft crew flying mostly short-haul routes would receive, annually in average, about 2 mSv, and 40% of them flying mostly long-haul routes, about 4 mSv [6]. Such values are higher than average values for most of the other occupationally exposed workers. For example, the average annual dose in the UK was 0.8 mSv in 1996 [6], the average values for Czech Republic in 1975 – 1989 were between 0.58 and 0.94 mSv for medical occupational exposures, and between 1.10 and 1.51 mSv for industrial application of ionising radiation, with descending tendency [8].
124
F. Spurny´ / International Congress Series 1225 (2002) 121–129
Fig. 2. The influence of solar activity during some flights.
3. Preliminary studies in the Czech Republic First experimental studies of the aircrew exposure were realised in Czech Republic in the years 1991– 1994 and were already published [9]. The results agreed reasonably well with the results obtained by other authors and compiled by the author in Ref. [2]. The ICRP 60 recommendations were already incorporated to the Czech regulation since 1997 [10]. Since the end of that year, a preliminary analysis of the exposure of Czech air company’s aircrew members has been started with the goal to develop an ‘‘optimum’’ procedure of aircrew exposure level estimation. The analysis was based on the CARI code already mentioned [3]. It was found that the calculation based on optimal flight parameters, which varies considering the aircraft type, gives results that are satisfying to the general requirements of individual dosimetry [11,12]. The main principle of this modelling, agreed by air companies, was the choice of an optimum flight altitude (closer to higher level for each aircraft to be on the safe side of estimation) and constant time for taking-off and landing.
4. Individual dosimetry of Czech air company’s aircrew The procedure was further developed, and the code CARI has been proposed in the version 5E, which calculates already the exposure level in the effective dose E, the quantity used for the limitation. It was finally proposed to the Czech authorities to be used for companies concerned. It has been accepted and used since 1998. Fig. 3 shows the aircrew
F. Spurny´ / International Congress Series 1225 (2002) 121–129
125
Fig. 3. Effective dose relative distributions of Czech air companies in 1999.
effective dose distributions for all air companies concerned for the year 1999. Fig. 4 shows, for one of them, the comparison of these distributions for the years 1998 and 1999. The following main characteristics can be deduced from these figures:
average effective dose value for all companies were, in 1999, a little lower by 2 mSv; for two companies flying only short-haul routes in Europe and to its holiday regions, there is one broad maximum, while the distribution for the third one, flying also long-haul flights to the North America and the Far Orient is characterised by two maxims; these maxims are still better seen in the comparison of two years (Fig. 4).
5. On-board measurements during 1999 [13] Several route types were chosen to characterise the most possible exposure types for aircrew members of air companies: intraeuropean flights to Northern Europe and to holiday regions of Spain and/or Greece; North Atlantic routes (to New York and Toronto); and a flight to Southeast Asia (Bangkok). Measurements have been performed between March and November 1999 on 11 round trip flights. For each flight, all necessary parameters registered by means of the board computer have been received. The exposure for individual flight has also been calculated by means of the CARI 5E code and were compared with the measured data.
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Fig. 4. Comparison of effective dose relative distributions of CSA air company in the years 1998 and 1999.
The measurements of the low LET component were performed with Reuter Stokes argon-filled high-pressure steel ionisation chamber RSS 112, NB 3201 scintillation counter, and two types of individual electronic dosimeters, both based on a Si-diode. To characterise the contribution of high LET (neutron) component, we used bubble damage neutron detectors (BDNDs) with a 100 keV neutron energy threshold and superheated drop detectors (SDD) with three energy thresholds (0.1, 1.0, and 6.0 MeV). Bubble damage spectrometer as well as BDND available from the Bubble Technology Industries, Chalk River, was used to determine the dose equivalent on the basis of an estimation of neutron spectra. It is composed of six sets of detectors with six ‘‘thresholds’’ in neutron energy: 0.01, 0.10, 0.60, 1.0, 2.5, and 10 MeV. Low LET radiation-measuring instruments were calibrated with 60Co photons, and high LET radiation measuring equipment with an AmBe neutron source. For both components, the response has been primarily expressed in terms of the ambient dose equivalent of the Table 1 Relative contributions of low and high LET components to the total dose equivalent Destination from Prague
Ratio of values high to low LET component
Contribution of high LET component to the total
Northern America Northern Europe Southern Europe Abu Dhabi Abu Dhabi – Bangkok
1.15a 1.09 0.86 0.514 0.439
0.535a 0.521 0.462 0.338 0.291
a
Uncertainty of values is estimated to about ± 15% (1r).
F. Spurny´ / International Congress Series 1225 (2002) 121–129
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Table 2 Comparison of experimental and calculated total exposure levels on board of aircraft Flight route
H*(10) measured, mSv
E, calculated by CARI 5E, mSv
Prague – Oslo – Prague Prague – Helsinki – Prague Prague – Madrid – Prague Prague Stockholm – Prague Prague – New York (Newark) New York (Newark) – Prague Prague – St. Petersburg – Prague Prague – Moscow – Prague Prague – Montreal – Toronto Toronto – Prague Prague – Prevesa – Prague Prague – Valencia – Prague Prague – Abu Dhabi – Bangkok Bangkok – Abu Dhabi – Prague
14.8 21.2 18.5 15.0 42.4 33.6 20.7 17.7 42.6 45.0 15.5 17.6 26.8 29.5
12.1 17.7 16.2 15.8 38.0 32.2 18.2 17.5 37.4 36.9 14.6 17.9 26.9 28.2
reference radiation. The results obtained with all methods characterising low and/or high LET component on-board have been combined and corrected, taking into account the response in CERN high-energy reference field (concrete shielding) [14]. First, we have analysed to what extent our experimental results would reflect some of general tendencies. The results of such confrontation are presented in Table 1. In Table 1, one can see that the tendencies expected are well confirmed by our experimental results, i.e., the relative importance of both component of radiation field at northern parts is close to being the same, the importance of high LET component clearly diminishes when going to the south. Experimentally determined values of H*(10) and the theoretical values of the effective dose E are compared in Table 2. The calculation has been performed for actual flight altitude profile and geomagnetic parameters of the route, taking into account the actual value of heliocentric potential (solar activity) in a month. One can see that both sets of values do not differ much. The experimental values of H*(10) are a little higher, an average of (9 ± 6)%, than the theoretical E-values. The values of E should be higher than the values of H*(10) in the fields on-board, about 20% in the case of ‘‘northern routes’’, and a little less for the routes close to the equator [15]. It seems that the code CARI 5E underestimates a little the actual exposure level.
6. Conclusions (1) It is believed that both the experimental and calculated values presented in this contribution represent a reliable [12] estimation (1r about ± 20%) of the aircrew exposure levels. (2) Of course, our knowledge of the exposure level of aircrew must be improved. As far as the calculation is concerned, a new version of the code (CARI 6) appeared at the
128
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beginning of year 2000. We have tested it already and found that the values obtained are at subsonic flight altitudes about 10% higher than those calculated with CARI 5E. In such case, the agreement of experimental and calculated data would still be better. Other codes were also developed and will be tested in the frame of new EURADOS working group activities and new EC research project [16,17]. (3) Moreover, more complex experimental measurements on-board complemented by the calibration in high-energy laboratory reference fields must continue. New measurements are in progress in several laboratories, particularly in the frame of EC projects supporting these studies [16]. Further measurements would be more concentrated to the equatorial regions, and it would also be very important to succeed with the measurements during an important solar flare.
Acknowledgements These studies were supported through the grant of the Grant Agency of the Czech Republic No. 202/99/0151. We are also much obliged to CSA Air for the support and the interest in studies presented in this contribution. References [1] 1990 Recommendations of the International Commission on Radiological Protection, Ann. ICRP vol. 21 ICRP Publication 60, 1991, No. 1. [2] Exposure of aircrew to cosmic radiation, in: I.R. McAulay, et al (Eds.), EURADOS Report 1996-01, ISBN 92-827-7994-7, Luxembourg, 1996. [3] W. Friedberg, W. Snyder, D.N. Faulkner, Radiation exposure of air carrier crew members II, US FAA Report DOT/FAA/AM-92-2, 1992. [4] International Conference on Cosmic Radiation and Aircrew Exposure. European Communities, Radiat. Prot. Institute of Ireland, Assoc. European Airlines; July 1998; Proceedings published as a special volume of Radiat. Prot. Dosim. 86, 1999. [5] Recommendations for the Implementation of the Title VIII of the European Basic Safety Standards Directive Concerning Significant Increase in Exposure due to Natural Radiation Sources. EC, DG XI, ISBN 92-8275336-0, EC 1997. [6] D.T. Bartlett, Radiation protection concepts and quantities for the occupational exposure to cosmic radiation, Radiat. Prot. Dosim. 86 (4) (1999) 263 – 268. [7] K. O’Brien, W. Friedberg, H.H. Sauer, D.F. Smart, Atmospheric cosmic rays and solar energetic particles at aircraft altitudes, Environ. Int. 22 (Suppl. 1) (1996) S9 – S44. [8] Z. Prouza, F. Spurny´, V. Klener, I. Fojtikova´, P. Fojtik, J. Podskubkova´, Occupational radiation exposures in the Czech and Slovak Republic, Radiat. Prot. Dosim. 54 (1994) 333 – 336. [9] F. Spurny´, Experimental approach to the exposure of aircrew to cosmic radiation, Radiat. Prot. Dosim. 70 (1997) 409 – 412. [10] Directive of the State Office for Nuclear Safety concerning the Radiation Protection (in Czech). Sbı´rka za´kon. 184/1997, Sbı´rka za´kon R a´ stka 66, emitted the 19th August 1997. [11] F. Spurny´, A. Malusˇek, Exposure level of CSA a.s. aircrew. I. Contribution of different routes in 1997 (in ´ JF AV R 447/98; Prague, November 1998. Czech), Report ODZ U [12] International Commission on Radiological Protection 75: General Principles for the Radiation Protection of Workers, Pergamon, Oxford, 1997. [13] F. Spurny´, et al., Experimental determination of the air crew members of the CSA air company—Comparison with theoretical calculations (in Czech), Report DRD NPI AS CR 466/99, Prague, June 1999.
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[14] M. Ho¨fert, G.R. Stevenson, The CERN-CEC High Energy Reference Field Facility. Presented at the American Nuclear Society Meeting—8th Inter. Conf. on Radiation Shielding, Arlington, Texas. [15] D.T. Bartlett, Aspects of the exposure of aircraft crew to radiation, Radiat. Prot. Dosim. 81 (1999) 243 – 245. [16] EC Contract No.F15P-CT99-00239. Co-ordinator O’Sullivan D (DIAS Dublin), 2000 – 2003. [17] EURADOS WG 5: ‘‘Aircrew exposure’’, chairman Ulrich J. Schrewe, since 2001.