Individual Dosimetry of Aircraft Crew Methods and Results in Czech Republic. F. Spurný

Transcription

1 Individual Dosimetry of Aircraft Crew Methods and Results in Czech Republic F. Spurný Nuclear Physics Institute-Department of Radiation Dosimetry, Czech Academy of Sciences, Na Truhlářce 39/64, CZ-18086, Praha 8, Czech Republic Abstract. International Commission on Radiological Protection (ICRP) recommended in its Publication 60 that the radiation exposure due to the cosmic component at high altitudes be taken into account when appropriate as part of occupational exposure to the radiation. The recommendation mentioned was incorporated to the Czech regulation at The contribution describes first the elaboration of optimal procedure for the aircrew routine individual dosimetry. The procedure was proposed to the Czech authorities and received their accreditation. Routine individual dosimetry has started since Its results are presented, discussed and analyzed. The experimental studies started in Czech Republic already in The first campaign was realised in years, to assist in the development of routine individual dosimetry method mentioned. The second, during 1999, permitted to verify the validity of it, as well as the third one, realized during 2003 year. Since 2000 year, a Si-semiconductor spectrometer has been also used for onboard aircraft measurements, about 700 commercial Czech Airlines flights were already monitored. The dosemeters were used to estimate low and high linear energy transfer component of the radiation field on board. The results obtained were transformed to the ambient dose equivalent and compared with the values calculated by means of codes CARI and EPCARD. A reasonable agreement of both sets of data could be stated. Basic review of results obtained is presented and discussed from the point of view of individual dosimetry of aircrew. Particular attention is devoted to the results obtained during some extreme situations: an intense solar flare and some Forbush decreases. 1. Introduction The level of exposure to the cosmic radiation increases with the altitude. At the air transport altitudes, it can reach up to 10 µsv/h. In 1990, the ICRP recommended that the radiation exposure due to cosmic rays at high altitudes be taken into account where appropriate, as a part of occupational exposure to radiation [1]. Aircrew has become another group of workers for whom exposure to ionizing radiation is an occupational hazard [2-4]. Besides, there are significant differences in exposure conditions of aircraft crew and usual occupational exposures [4]: - 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 more than one half of females in aircrews, and they represent only a few percent for other occupationally exposed persons. Both these two factors increase the importance of correct estimation of aircrew exposure. The contribution describes how this task is assured in the Czech Republic and its results. First, the procedure used to perform routine individual dosimetry, accredited by the Czech authorities is described. It has started since 1998; its results are presented, discussed and analyzed. Afterwards, several campaigns of measurements started since 1991 are characterized, theirs results presented and discussed. Particular attention is devoted to the results obtained during some extreme situations: an intense solar events and some Forbush decreases. 2. Routine individual dosimetry of Czech aircraft crew 2.1. Method of aircrew exposure estimation In agreement with generally adopted philosophy [2-4] the calculation was chosen as the method of aircrew exposure estimation. Up to now the code CARI developed in the USA [5] was used. First, we searched for an optimal procedure to perform the exposure estimation. The real data taken from the 1997 year schedule were taken into account for that [6]. The main principle of this optimization consisted in the choice of the flight altitude for each type of aircraft, agreed with the company concerned; and the optimum and constant times for taking-off and landing. All other parameters necessary for the calculation, i.e. the time of flight and the actual solar activity level, are used in their 1

2 real values for the flight concerned. Solar activity level is taken as the average value during a month concerned. Aircrew routine individual dosimetry is performed in the following way: 1. Designed representative of the air company (3 companies are followed, A, B, and C) prepares the data sets characterizing the air traffic in the period followed. These sets are the list of route flights realized, and the list describing the participation of each aircrew concerned in them. 2. These data sets are used to create an input set for the CARI code. In this step, the correction of occasional errors is performed and the missing data (new airports, aircraft types, solar activity characteristics, etc.) are added. 3. The calculation of individual effective doses for each flight is performed. 4. The effective doses of each aircrew member identified by company code are calculated. In any of these steps the automatic check procedures exist to identify a problem arising, such checking is possible also externally. Generally, the choice mentioned should lead to a slight overestimation of the average exposure level. The analysis performed for more than 3000 flights realised during 1998 year showed that this overestimation was about 10% Results of routine individual dosimetry All procedure was developed during 1998 year, using the CARI 5E, i.e. the version calculating the effective dose, the quantity in which the radiation protection limits are expressed [1]. As the example of results, the average effective dose rates calculated for one of the concerned air companies are presented in the Figure 1. 0,4 Company A - E-rate distribution ,60 Distribution of E per year, Relative contribution 0,3 0,2 0,1 Relative contribution 0,45 0,30 0,15 company A company B company C 0, E-rate, µsv/h ,00 0,0-0,2 0,2-0,4 0,4-0,6 0,6-0,8 0,8-1,0 1,0-1,2 1,2-1,4 1,4-1,6 Annual E, msv FIG. 1. E- rates distribution for a company at FIG. 2. Annual E distributions, One can see there that these values vary from values below 0.2 up to 5.5 µsv/h. The lowest values correspond to the aircrew of propeller type aircraft. The variation for all other jet-type aircraft is still important, real flight program for each of aircrew members should be considered to estimate correctly its exposure. The relative distributions of annual effective dose values for all three concerned companies and the all period are presented in the FIG. 2, average values for the period are presented in the Table I. One can see there that: - The lowest average values are systematically found for company A, as well as their uncertainties characterizing the variability of annual doses for an aircraft crew member. It is related to the difference of this company when compared to other two. - The company A use frequently also propeller-type aircraft which fly lower and the data submitted concern also many members with annual exposure below 1 msv. Besides, this company flies also above North Atlantic, the distribution of effective dose exhibits two maxima, the higher corresponding to these routes. 1,6-1,8 1,8-2,0 2,0-2,2 2,2-2,4 2,4-2,6 2

3 - The distribution of values for the aircrew of two other companies, flying mostly short haul flights in Europe and to its tourist regions, is rather narrow, about 60% of them received the effective dose between 1.6 and 2.2 msv. - The average annual effective dose value is for all three companies around or a little lower than 2 msv, higher than for many other occupationally exposed groups of persons [7]. Table I. Mean annual E and theirs standard deviations for all companies followed. Year E (1σ), msv for Company A Company B Company C (0.52) 1.20(0.74) 1.80(0.26) (0.58) 1.71(0.41) 2.09(0.15) (0.57) 1.91(0.24) 2.13(0.19) (0.41) 1.98(0.37) 2.03(0.25) Since 2000, the version CARI 6 is used for calculation, the tests with the code EPCARD [8] were also started. The results obtained differ generally less than 10%. We estimate that the accuracy of results obtained is better than ± 20%, i.e. with full agreement with the general requirement [9]. 3. Experimental studies of aircraft crew exposure and theirs results 3.1. General overview First series of experimental studies of the aircrew exposure we realised in years, mostly onboard A and TU 154 aircraft of the Czech Airlines (CSA) company. Flight routes extended from the equator (Singapore) up to North Atlantic routes up to 65 o N, 20 return flights were monitored. The results obtained were already presented and analysed [10], a good agreement between different methods used was found. Another series of onboard aircraft experimental measurements was realized during 1999 year with the goal to verify the procedure used already at that time for routine individual dosimetry of aircraft crew of Czech air companies concerned. The basic results and conclusions following from this series are presented, analysed and discussed in next chapter 3.2. At 2000, we have acquired a Si-semiconductor energy deposition spectrometer, permitting to estimate onboard aircraft exposure level on the base of analysis of energy deposition spectra and the calibration in high-energy reference fields and/or direct comparison with the results of measurements with a reference instrument - tissue equivalent proportional counter (TEPC). Further important characteristic of this instrument is the possibility to perform autonomic onboard measurements during relatively long period, about two months. Six series of such long monitoring runs were realized onboard of an A aircraft of CSA during years. These runs and the results of measurements during them are more in detail described in the chapter 3.3. Finally, last series of measurements onboard of CSA aircraft was realized during 2003, again with the goal to verify the procedure used for routine individual dosimetry. These results are presented in the chapter Results of 1999 measurements The measurements were realised on: - intraeuropean flights to Northern Europe and to holiday s region of Spain and/or Greece; - north Atlantic routes (to New York and Toronto); and - a flight to south-east Asia (Bangkok). These route types were chosen to characterise the most possible exposure types for air crew members, measurements were performed between March and November 1999; on 11 round trip flights. For each flight, all navigation data have been received and the exposure for a flight has been also calculated by means of CARI 6 code and compared with the measured data. 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 3

4 electronic dosemeters. To characterise the contribution of high LET (neutron) component we used bubble damage neutron detectors (BDND s) 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, available as well as BDND from the Bubble Technology Industries, Chalk River, was used to determine the dose equivalent on the base of an estimation of neutron spectra. It is composed of 6 sets of detectors with 6 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 60 Co photons, 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 reference radiation. The results obtained with all methods characterising low and/or high LET component on the board have been combined together and corrected taking into account the response in CERN high energy reference field (concrete shielding) [11]. First, we have analysed, to what extent our experimental results reflect some of general tendencies. The results of such confrontation are presented in Table II. Table II. Relative contributions of low and high LET components to the total dose equivalent. Destination from Prague Ratio of values high/low Contribution of high LET LET component component to the total Northern America ) ) Northern Europe Southern Europe Abu Dhabi Abu Dhabi - Bangkok ) Uncertainty of values is estimated to about ± 15 % (1σ) One can see in the Table II that the tendencies expected be well confirmed by our experimental results, i.e. the relative importance of both component of radiation field at northern parts is close to be the same, the importance of high LET component clearly diminishes when going to the south. Experimentally determined values of H*(10) were compared with the values of the effective dose E calculated by means of the CARI code. 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. We observed that both sets of values agree well. The ratio of experimental values of H*(10) and calculated E-values was in average equal to - (9±6) %. The values of E should be in the fields on board higher than the values of H*(10), about 20 % in the case of northern routes, a little less for the routes close to the equator [12]. It seemed that the code CARI 6 underestimated a little the actual exposure level in the value of effective dose Results of long-term monitoring with a Si-semiconductor energy deposition spectrometer The Mobile Dosimetry Unit (MDU) [13-15] monitors simultaneously the dose and number of energy deposition events in a Si-detector. It is designed as handy equipment. The amplitude of the pulses is proportional to the energy loss in the detector. Final adjustment of the energy scale is made through the 60 kev photons of 241 Am. The amplitudes are digitised and organised in a 256-channel spectrum. The dose D [Gy] is calculated from the spectrum as: D = K Σ(E i A i )/MD, (1) where MD is the mass of the detector in [kg]; E i is the energy loss in the channel i; A i is the number of events in it; and K is a coefficient. The operational time of the instrument was with 14 Ah-batteries about 1400 hours. MDU units were exposed in photon and neutron reference radiation fields. The measured and calculated spectra of deposited energy for 200 kev, 137 Cs and 60 Co photons [14], and for natural radiation background showed that the impulsions with energy deposited above 1 MeV are practically absent. Different was the situation for fast and high-energy neutrons (CERN high-energy reference field). The energy deposited was going up to the maximum energy registrable, 21 MeV. We decided 4

5 therefore to take as a measure of neutron response of a MDU unit the energy deposited in Si with the impulsion s height above 1 MeV. To interpret the data in H*(10), CERN reference values for neutron component were considered as well as the results of direct comparison of data measured by MDU and a TEPC during several common flights [16]. Since the March 2001, the spectrometer has been used during six long-term exposures, with more than 600 flights in total. All necessary flight parameters were acquired from colleagues of Czech Airlines, which permitted to calculate the effective dose E on board by means of CARI 6 and EPCARD 3.2. codes and compare them with the apparent H*(10) values obtained as mentioned above. More detailed characteristics of long-term measurements are given in the Table III, the record of detectors data for the first of them is presented in the FIG. 3. Table III. Characteristics of long-term onboard aircraft monitoring realised in with LIULIN semiconductor spectrometer. Run No. Period Returned flights monitored 1 Flight hours /03-07/05/01 PRG-NY(25), PRG-TOR(13), PRG-DUB(3) /05-24/07/01 PRG-NY(41), PRG-TOR(12) /08-16/10/01 PRG-NY(26), PRG-TOR(13), PRG-DUB(2) /10-10/12/01 PRG-NY(20), PRG-TOR(7), PRG-AMS(1) /05-28/06/02 PRG-NY(22), PRG-TOR(13), PRG-DUB(8), PRG- LHR(5), PRG-MAD(5) /10-05/12/02 PRG-NY (23), PRG-TOR (9), PRG-DUB (5), CMB- DUB(5) PRG Prague, NY New York, TOR Toronto, DUB Dubai, AMS Amsterdam, LHR London, MAD Madrid, CMB-Colombo; in brackets number of returned flights 2 from taking off to landing FIG. 3. Record of measurements for the period 22/03-07/05/01. The comparison of exposure values calculated from navigation data and the results obtained from MDU readings by means of procedure mentioned above are presented in the Table IV. One can see that the data sets agree well, only the values of effective dose are, when EPCARD code is used, a little, in average by 12 %, higher. Such difference is from the point of view of radiation protection largely acceptable [9]. The data from long-term measurements were also treated in the same way as for individual flights. The examples of flight records are presented in FIGs. 4 and 5. One can see there again that the values of total apparent H*(10) are in rather good agreement with the E-values calculated by means of CARI 6 code. Such tendency was observed in the most of 625 flights treated in the same way, the average relative uncertainties of the differences between calculated and treated measured data did not exceed 10 %. 5

6 Table IV. CSA aircraft (A ) long-term exposures 2001 comparison of calculated and MDU results treated using CERN calibration. Component Run number Method CARI6 E, msv total EPCARD3.2 neutrons H * (10) non neutrons msv total EPCARD3.2 neutrons E-ISO non neutrons msv total Liulin-MDU neutrons H app non neutrons msv total σ(rel) 10% for others, 20% for neutrons and total FIG. 4. Flight profiles New York Prague. FIG. 5. Flight profiles Bahrajn Prague. During the 1 st monitoring period, an intense solar flare, Ground Level Event (GLE60) occurred. We really observed at the 15 th April a maximum in full record (see FIG. 3). A good time and formcorrelation s of our measurements with independent data available from Oulu (Finland) cosmic ray monitor station and/or from the GOESS system satellites was also confirmed [17]. Forbush decreases occurred during 2001 year also, at 12/04 and 06/11. Our MDU was flying onboard aircraft, at both dates from Prague to New York. Another, very deep, Forbush decrease occurred the 29/10/03, the author flew during it from Sofia to Prague [18]. We treated measured data and compared them with the calculation by means of EPCARD3.2 code [8] (Table V). One can see there that both solar flare and Forbush decrease have an influence on exposure level both quantitatively and qualitatively. Total exposure was for GLE 60 about 44 % higher, for Forbush decrease up to 26 % lower. The influence is more important for neutrons. The changes would indicate that the neutron spectrum is softer during a solar flare, harder during the Forbush decrease. Table V. The influence of extremes on the exposure level measured on aircraft board. Flight Ratio MDU /EPCARD, H*(10) non neutrons neutrons total PRG-JFK, 14/04/01 - quiet JFK-PRG, 16/04/01 - quiet PRG-JFK, 15/04/01 - with GLE PRG-JFK, 12/04/01 - Forbush PRG-JFK, 06/11/01 - Forbush SOF PRG, 29/10/03 Forbush

7 3.4. Results of 2003 measurements; comparison with routine procedure The measurements were realised on: - 2 intraeuropean flights (Helsinki, Madrid); - 1 North-Atlantic route (Montreal-Toronto); and - 4 routes to and/or over the equator (Dubai, Colombo, Singapore, Brisbane). The same equipments as in 1999 were used together with one or two units of MDU. As in previous tests, all navigation data were available to calculate the exposure level. The results of measurements and calculation by means of both EPCARD 3.2 and CARI 6 codes are presented in the Table VI. Table VI. Measured and calculated total exposures in µsv during 2003 run. Route types Measured, H*(10) Calculated detectors set MDU H*(10)-EPCARD 3.2 E-CARI 6 Intraeuropean North Atlantic Dubai-Colombo Singapore-Brisbane One can see there that the agreement of all sets of results in the most of cases is reasonable. The best is for intraeuropean routes, the largest differences are observed in the case of returned flights to Dubai and Colombo. But, even in this case, the average value of H*(10) measured and calculated by means of EPCARD 3.2 code is equal to 74.5 (13.4) µsv, i.e. with 1 relative σ equal to 18%. It is still acceptable from the point of view of general requirements. When all flights are considered, the average value is equal to 247 (23) µsv, i.e. 1 relative σ is only about 9%. Nevertheless, it seems that the results obtained by means of MDU overestimate the exposure level, particularly for flights to equator regions, i.e. with high geomagnetic rigidity. We compared the results of measurements also with the total E calculated by means of routine procedure used for individual dosimetry, in which simplified flight parameters are considered. This approach has given the total E about 20 % higher, mainly due to the difference in flight altitude. Routine procedure supposes feet, actually it was in average about feet. 3. Conclusions 1. The development of the procedure for the routine individual dosimetry of the aircraft crew of Czech air companies and its introduction to the practice have permitted to assure regular check of the exposure level of all persons concerned since more than 5 years. The results show that the average annual effective dose is about or lower than 2 msv, with very few cases exceeding slightly 3 msv. 2. Several series of the comparison of measured and calculated values demonstrated a good agreement of both approaches when actual navigation data are considered. When simplified flight parameters used in the routine procedure are considered, the measured values are typically about % lower, i.e. routine procedure slightly overestimate the level of exposure, in agreement with the general principle adopted in radiation protection. 3. The semiconductor spectrometer MDU was introduced for onboard measurements, necessary procedure of data treatment was developed and tested, to profit of the simplicity of its use and reliability to perform also long-term onboard monitoring. Its spectrometric properties seem to allow obtaining both qualitative and quantitative characteristics of onboard radiation fields. These properties were also able to estimate quantitative and qualitative changes of the onboard field during a solar flare and some Forbush decreases. Additional effort is needed to improve its performance, particularly in equatorial regions. Common measurements with a reference tissueequivalent proportional counter should help to compare directly measured and interpreted data. All these approaches are in further progress in our laboratories. 7

8 References Recommendations of the ICRP, Annals ICRP, vol. 21, ICRP Publication 60, No.1, (1991). 2. EURADOS Report , Exposure of aircrew to cosmic radiation, I.R. McAulay, et al. (eds.), ISBN , Luxembourg (1996). 3. Recommendation for the Implementation of the Title VIII of the European BSS Directive Concerning Significant Increase in Exposure due to Natural Radiation Sources, EC, DG XI, ISBN , EC (1997). 4. Bartlett, D.T., Radiation protection concepts and quantities for the occupational exposure to cosmic radiation, Radiat. Prot. Dosim. 86(4), p (1999). 5. Friedberg, W., Snyder, W., Faulkner, D.N., Radiation exposure of air carrier crew members, US FAA Report DOT/FAA/AM-92-2, (1992). 6. Spurný, F., et al., Aircrew individual dosimetry of air companies in the Czech Republic - the choice of method and its experimental verification, Nuclear Energy Safety, 10(48), p , (2002). 7. Prouza, Z., et al., Occupational radiation exposures in the Czech and Slovak Republic, Radiat. Prot. Dosim. 54(3/4), p , (1999). 8. Schraube, H., Mareš, V., Roessler, S., Heinrich, W., Experimental verification and calculation of aviation route dosis, Radiat. Prot. Dosim. 86, p ,5 (1999). 9. International Commission on Radiological Protection 75: General Principles for the Radiation Protection of Workers, Pergamon Press, Oxford Spurný F., Experimental Approach to the Exposure of Aircrew to Cosmic Radiation, Radiat. Prot. Dosim. 70, p , (1997). 11. Mitaroff, A. and Silari, M, The CERN-EU high-energy reference field (CERF) facility for dosimetry at commercial flight altitudes and in space, Radiat. Prot. Dosim. 102, p. 7-22, (2002). 12. Bartlett DT., Aspects of the Exposure of Aircraft Crew to Radiation, Radiat. Prot. Dosim. 81, p , (1999). 13. Spurný, F. and Dachev, Ts., Aircrew onboard Dosimetry with a Semiconductor Spectrometer, Radiat. Prot. Dosim. 100, p , (2002). 14. Spurný, F., Gschwindt, R., Makovička, L. and Dachev, Ts., Response of a Semiconductor Spectrometer to Photons, Nuclear Energy Safety 11(49), p , (2003). 15. Spurný, F., and Dachev, Ts., Long-term Monitoring of the onboard Aircraft Exposure Level with a Si-diode based Spectrometer, Invited paper, Adv. Space Research 32, p , (2003). 16. Lindborg, L., McAulay, I., Bartlett, T.D., Beck, P., Schraube, H., Schnuer, K., and Spurný, F., (eds.), Exposure of Aircraft Crew to Cosmic Radiation, EURADOS WG 5 Report, EC DG TREN, Luxemburg Spurný F., Dachev Ts., Kudela K., Increase of onboard aircraft exposure level during a solar flare, Nuclear Energy Safety 11(49), p , (2003). 18. Spurný, F., Kudela, K., and Dachev, Ts., Airplane radiation dose decrease during a strong Forbush decrease. Geophysical Res. Letters, submitted for publication 8