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Dosimetry of aircrew exposure to radiation during solar maximum

Deliverables

In-flight determinations of route doses and dose rates for a wide range of flight profiles (altitude and latitude) during the period of maximum solar activity, in particular a comprehensive set of repeated measurements for Concorde. For measurements in aircraft, the NRPB uses a passive survey instrument with etched track detectors to measure the neutron component of the field plus neutron-like interactions of protons (via secondaries from strong force interactions) and TLDs to measure the non-neutron component. The survey instrument consists of a glass reinforced polyester (GRP) box of dimensions 255x250x125mm containing a central block of 36 etched track detectors arranged in 6 mutually orthogonal stacks of 6 dosemeters in order to have, in aggregate, a response independent of the direction characteristics of the radiation field, 30 thermoluminescence dosemeters(TLDs) and 2 electronic personal dosemeters (EPDs) to record the time profile of the radiation field.
The DIAS group employs solid state nuclear track detectors to investigate cosmic and solar energetic particles at aircraft altitudes. These particles include protons, neutrons and heavy ions such as helium, carbon etc. which are present in the primary radiation that strikes the upper atmosphere and are also present in the nuclear interaction products that result from collision with the molecules in the Earth’s atmosphere. The high speed particles leave a trail of damage in the detectors, which when duly processed, can be magnified, optically measured, and related to the charge and velocity of the particle. The relationship between the energy deposited in the detectors (the Linear Energy Transfer, LET) and the measured signal is determined by calibrating the detectors with protons and heavy ions of known energy at various accelerators. Once the LET values are known, the radiation dose can be determined. The DIAS detectors were calibrated over the range 5KeV/mm to 600KeV/mm which covered the LET region of interest for dose measurements at altitude in the Earth’s atmosphere. Repeated calibrations over a number of years showed excellent consistency and confirmed the reliability of this fundamental approach to high accuracy data for dosimetric purposes in aircraft and space.
Results from with two different types of instruments are reported. Both are based on tissue equivalent proportional counters. In one counter is evaluated according to the variance method, in which electric current measurements and determination of its variance are the essential quantities. In the second one the detector consists of many small proportional counters, which are evaluated traditionally. The variance method gives less detailed information than the other one based on traditional pulse height analysis and the response characteristics of the former instrument is therefore important to review. Such results are discussed in the report as well as in the reports to come. The results from both detectors agree with each other as well as with those reported by others within the accepted uncertainty during the whole time period. Results from measurements during Forbush events have been reported. Of particular interest are the results from measurements with several groups onboard the same flight, as systematic uncertainties then become visible. Such results are important for the conclusions concerning the radiation dose received onboard aircraft.
Development of stand alone detector systems consisting of TEPC detectors and charged particle detectors (CACS) for the dfosimetry of the radiation fields at aviation altitudes. The detectors are installed in suitcases of cabin baggage size. The flight cases are to be permanently installed on aircraft, or will be used as a mobile system, or will be equipped with a different detector (for example a neutron detector). The results will provide new insight into the particle composition of cosmic radiation in aircrafts and the correlation to the ambient dose equivalent. Owing to the strict legislation in Germany, the results are the basis of the radiation protection regulations for aircrew members.
Simulation results of the radiation exposure caused by the galactic cosmic radiation have been compared successfully with onboard instruments. The calculation was provided by the high-energy particle transport code FLUKA, the program package EPCARD version 3.2 and CARI-6. Experimental results of several active and passive dosimeters compared well for a wide range of aircraft positions and solar conditions within 30% uncertainty on average, for a confidence level of two standard deviations. The calculation results for solar energetic particles (SEPs) show that the radiation fields in the atmosphere depend significantly on the form of SEP primary spectra. Two important conclusions can be drawn: -The uncertainties of the primary spectra affect fluence rates much more than spectral shapes of secondary particles generated inside the atmosphere. This means that experimental devices which have been established in experiments studying the neutron component of the radiation induced by galactic cosmic rays may be used without serious problems of calibration in situations of higher radiation levels during a sporadic SEP event. -This situation can be different if particles of the electromagnetic cascade contribute significantly to the instrumental readout. The fluence rate of this component depends sensitively on the hardness of the SEP-spectrum and differs significantly for SEP-induced radiation and radiation induced by galactic cosmic rays. The numerical simulation of microdosimetric spectra measured by a tissue equivalent proportional counter (TEPC) is successfully demonstrated using the simulation code FLUKA. Comparison of measurements and simulation results in standard radiation conditions show good agreement. For ambient dose equivalent measurement at aircraft location close to the fuel tank the experimental data show a 25% dose reduction for high LET compared to simulation results in free air. Calculation results of numerical program codes will be used for dose assessment in future aircrew dosimetry. The evaluated results of the program codes will provide methods and tools for practical dosimetry. The calculation of Solar Energetic Events needs still further research and evaluation by experimental observations due to their complex physics. The results will provide a reliable basis to inform the public and aircraft crew about the radiation effect in air travelling.
Different systems have been qualified and calibrated using high energy beams (neutrons, mixed fields…) representative of those encountered at flight altitude. An in-flight intercomparison of active and passive dosemeters from different laboratories has been realised during a common flight. More than 10 active systems (5 TEPCs, 2 silicon detectors, 1 GM, personal dosemeters) and different passive detectors (bubble, TLDs, track…) were used.
The radiation field in the atmosphere is complex and dose values depend on many things like geographic position flight altitude and solar activities. The reasons for this complexity are reviewed to make understanding of dose results and their use easier. The concept radiation dose is itself quite complex and two basic quantities are shortly described. The measurement result during the contract has been obtained from measurements with many different types of detectors for which the response to different radiation qualities have been investigated. Some basic differences between them is described. The report also discusses solar particle events and their influence on the aircrew dose. The report also includes some information concerning the legal requirements of aircrew dose determinations. The report has been written with the intention to be understood by those interested among aircrew, airline officers and officers in regulatory authorities.
Dose rate measurement results are the reference basis for the dose assessment of aircrew in flight altitudes. Furthermore, the measurement results provide comparison and the evaluation of cosmic radiation at flight altitudes calculated by numerical program codes.
Results of calibration and characterisation measurements of TEPC instruments in various mixed and complex radiation fields will provide the knowledge and database for any reference measurement of radiation exposure at flight altitudes. Well defined TEPC instruments can be used to evaluate any further software code for dose assessment. Well characterised TEPC instruments are necessary to guarantee the validity of dose assessment for aircrew.
Dose assessment by the DIAS detectors is achieved by the measurement of linear energy transfer (LET) spectra based on calibration of the detectors with protons, and heavy nuclei such as carbon, neon, silicon, iron and krypton, manly at the GSI (Darmstadt) and HIMAC (Japan) accelerators. It has also been demonstrated that protons and neutrons of similar energy produce similar LET spectra for the measurement approach used. Comparison of results obtained by DIAS with those of a standard TEPC at the CERN reference field show very good and consistent agreement, as do comparison with predictions of computer codes.
The main result of the many measurements carried out at aviation altitudes is the provision of an extensive data base on radiation dose rates for high linear energy transfer as a function of altitude, latitude and stage of solar cycle 23. The European Union Council Directive 96/29/EURATOM lays out the requirements for each member state with regard to monitoring of radiation doses received by aircrew. The results of the present work will contribute to these requirements both in the provision of dose equivalent rates and the development of passive instrumentation for dosimetry at aviation altitudes.
Different passive detectors can be easily used for the assessment of the dose due to the galactic cosmic ray. By contrast, the assessment of the cumulative exposure to solar cosmic rays is relatively much more difficult. The possibility of exposure to solar flare radiation giving rise to excess annual dose is so remote that should such a rare occurs it should be treated as unplanned exposure, the dose calculated retrospectively. To this end, new methods of retrospective dosimetry of solar flares have been investigated based on detectors, which can be easily found on-board as consumer products. Indeed on aircrafts, it is possible to find consumer products, which could provide two different types of neutron detectors and/or dosemeters such as polycarbonate and CR-39 plastics.
It is necessary to determine the neutron and photon response characteristics of the NRPB passive survey instrument used for the measurement of cosmic radiation fields in aircraft and in low earth orbit. Both the energy and angle dependence of response need to be determined. This work is important for the assessment of the radiation exposure of aircraft crew. The NRPB passive survey instrument can be used for routine measurements to validate calculated route doses. Exposure to cosmic radiation is considered to be occupational exposure, and in Europe, requirements for dose assessment are given in the European Council Directive 96/29/EURATOM, which have been, or are being incorporated into the national legislation of EU Member States, and in the forthcoming revised Joint Aviation Regulations to be issued by the Joint Aviation Authority (which covers the activities of the civil airlines of 35 European states). The neutron and photon response characteristics of the NRPB passive survey instrument have been determined for neutron energies from 144keV to about 200MeV and for photons of energies 662keV and 4.4 and 6/7MeV. The angle dependence of response has been measured. The reproducibility of the PADC detector sensitivity has been determined by a series of repeated measurements in the CERN high energy calibration facility which simulates the neutron component of the cosmic radiation field in aircraft. An assessment of the uncertainties has been made.
Exposure to cosmic radiation is considered to be occupational exposure, and in Europe, requirements for dose assessment are given in the European Council Directive 96/29/EURATOM, which have been, or are being incorporated into the national legislation of EU Member States, and in the forthcoming revised Joint Aviation Regulations to be issued by the Joint Aviation Authority (which covers the activities of the civil airlines of 35 European states). The uncertainties of dose assessment need to be estimated. This result is concerned with: - The estimation of the accuracy of dose assessment depending on measurement method, and - The comparison of calculated and measured route doses.
In-flight dose rates and LET spectra for different airlines and flights at both supersonic and subsonic altitudes during the period of solar maximum activity were determined. Thousands hours of measurements aboard aircraft were performed, including measurements during a GLE on April 2001. The comparisons of dose equivalent measured and calculated by theoretical models were obtained. Improvements of models have been realised, especially those concerning GLEs.
The data obtained with the ANPA stack on hundreds of repeated return long-haul flights on the routes Milan-Los Angeles, Milan-Tokyo and Rome Rio-de Janeiro have been used to study the reproducibility of the route dose at different phases of the solar cycle. These data have proved to be highly valuable to study the dependence of the route dose on altitude, latitude and on the particular location within the aircraft.
Calibration and detector characterisation in different radiation fields. The TEPC/CACS system has been exposed to neutron beams having energies ranging from 0.5MeV up to 200MeV. Up to 20MeV PTB provides ISO reference fields. Above 20MeV, only facilities in Belgium, Sweden and Southafrica have suitable neutron beams installed. From these measurements the calibration for the so-called high-LET radiations as neutrons is obtained. During the project the calibration in high-energy photon reference fields at photon energies of 6-7MeV emerged as a very important issue. Therefore, the ISO R-F photon reference fields at PTB are used of the calibration of the so-called low-LET radiations like photons and electrons.
If a software code can be used for the dose assessment of aircrew dosimetry an evaluation procedure have to be passed. A software code has to show a minimum set of requirements using input data, well-defined models and providing the correct output data in the appropriate units.
The APAT group employs a stack formed by several different passive detectors (track detectors, bubble detectors, TLDs) and personal electronic dosimeters. By the proper choice of the detectors to be used, it is possible to obtain a stack respectively for the spectrometry of low and high energy neutrons and for the cosmic ray dosimetry. Several new etch-track detectors are used which are based on the registration of neutron-induced recoil tracks in polycarbonate and CR-39 detectors and/or neutron-induced fission tracks in heavy elements. The successful applications of these detectors for the measurement of sufficiently low cosmic ray neutron doses have been made possible by exploiting a new track detection principle, of which APAT claims the intellectual and proprietary rights. The above detectors have been calibrated with neutrons beams with energy up to about 200MeV. For energies above 200MeV, the responses of all these detectors have been studied with monoenergetic proton beams. The exposure facilities used for these studies have been made available from throughout Europe, South Africa, Russia and North America within the present and previous CEC contracts. Two of these novel detectors are based on neutron induced-fission in bismuth and gold, which are the only two detectors capable of the selective measurement of neutrons with energy larger than 50MeV.