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Dosimetric complex for long-term manned space flights
0735-245X/92 SS.00 + .OO Pcrgamon Rcu plc
Nucl. Tracks Radiat. Meas., Vol. 20, NO. I, pp. 7-11, 1992
Int. .I. Radial. Appl. Instun., Part D Printed in Great Britain
DOSIMETRIC
COMPLEX FOR LONG-TERM SPACE FLIGHTS
Yu. AKATOV,* E. E. KOVALEV,*V. A. SAKOVICH;
S. Dma,t
MAN-NED
I. Faxd@ and V. D. NGUYEN~
*Research Centrc for Spacecraft Radiation Safety of the Ministry of Health of the U.S.S.R., Moscow 123182, U.S.S.R.; tCe.ntral institute for Physics Research, Budapest, Hungary and SCentre d’Etudes Nuclkaircs de Fontenay-aux-Roses, France (Received 27 February 1991; in reviredform
24 June 1991)
Abstract-A dosimetric complex is proposed for use in long-term manned space tight on the orbital station and for the planned Mars expedition. This complex includes thermoluminescence dosimetcrs with on-board readout, a channel for continuous monitoring of dose rate and a channel for continuous measurement of absorbed and equivalent doses, as well as measurement of the LET spectra. The last four parameters are measured by a tissue-equivalent proportional counter.
INTRODUCTION
it should only be noted here that a particular type of radiation situation defines the short-term radiation safety (RS) measures. The second concept proposed by the ICRP (1984) requires that “any information necessary for personal irradiation to be estimated should be presented in the same tits as those used to express the basic limits”. This concept is of particular importance for space flights because the RS criterion has been taken to be the radiation risk oRrad (GOST 25645.215-85, 1986) which is a probabilistic quantity and, therefore, cannot be measured, but has to be calculated. The division of technologies, technical devices, and constructions into hazardous and safe types, without any quantitative estimates of the probabilities for averages of incurred damages of several different degrees, is gradually recognized to be no longer justified. The necessity of using such criteria is predetermined by the fact that solar proton Aares make it impossible to provide absolute safety of spacecraft crews. For this reason, not only a standard radiation level, but also the reliability of not exceeding such a level, was adopted in 1976 in the U.S.S.R. as space flight RS criteria (RSDC-75, 1976). The criteria were replaced later by the standard radiation risk level oRrad.st (GOST 25645.215-85, 1986) which makes it possible to allow adequately for both deterministic and probabilistic exposures to radiations. In this case, the radiation risk is meant (GOST 25645.214-85, 1984) to be an increase in the probability for cosmonauts to die during a flight because of somatic radiobiological effects caused by radiation. As noted by the ICRP (1984), other values, such as the mean equivalent dose in organs, the equivalent dose index, etc., cannot be measured directly. Therefore, the idea has been proposed by the ICRP (1984) of interpreting measurements on the basis of the models which describe quantitative relationships
RrJ>unoN monitoring has always been regarded as an important component of radiation protection and is a matter of permanent concern for such authoritative agencies as ICRP and ICRU. The manual “General Principles of Monitoring Radiation Protection of Workers” (ICRP, 1984) prepared at ICRP Committee 4 was based on extensive experience and, therefore, proved to be extremely helpful when preparing the standard “Requirements of Personal and On-Board Dosimetry” (GOST 25645.202-83, 1984) which is an integral part of the set of standards “Radiation Safety of Spacecraft Crews During Space Flights” (Kovalev and Sakovich, 1989) approved in 1983-1990 in the U.S.S.R. The concept of consistency of the task, scope, and accuracy of monitoring measures of the radiation environment expected during space flights was accepted to be the most general concept which could be traced back to the fundamental general concept recommended by the ICRP for guidance; namely, that radiation projection should be optimized by analyzing the balance between the radiation protection cost and the benefits from using the sources of ionizing radiations. In the case of space flights, the task of optimal usage of the mass distributed for various means of radiation protection and for safety as a whole becomes even more urgent. In view of the above, and by analogy with situations of different safety levels proposed by Beregovoy er al. (1989), the requirements of radiation monitoring were classified in GOST 25645.202-83 (1984) by introducing the “safe”, “standard”, “nonstandard”, “dangerous”, and “emergency” types of radiation situations. The quantitative discrimination among radiation situations will be discussed below; 7
8
YU. AKATOV
between measured and calculated values. Accordingly, the set of standards (Kovalev and Sakovich, 1989) includes a number of models which realize this concept, namely:
(9 a model of the human body for calculating dose (COST 25645.203-83, 1984); (ii) methods for including spatial distribution of equivalent dose in the human body (GOST 25645.219-90, 1991) intended for finding the value of adequate dose (GOST 2X45.201-83, 1984) (iii) dependence of quality factor on linear energy (GOST 25645.218-90, 1991) and the relevant method for calculating microdosimetric characteristics (RD 50-25645.217-90, 1991); (iv) a model of generalized radiobiological effect (GOST 25645.214-85, 1984) allowing for temporal conditions of exposure to radiation; (v) a method for calculating radiation risk (RD 50-25645.205-83, 1984) intended for including the probabilistic nature of exposures to radiations and of the generalized radiobiological effect. The standard GOST 25645.215-85 (1986) prescribes not only the standard radiation risk level, but also the values of the limiting dose Gst corresponding to the standard radiation risk level when the probabilistic exposures to radiation are absent. For the purposes of radiation monitoring, therefore, the standard GOST 25645.202-83 (1984) includes the concept of the hourly adequate dose oG and its control, meaning oGst = Gst/T, where T is flight duration in h. The above makes it possible to explain the principle of delimitation between the types of radiation situations. Given the notation g = oG/oGst and r = oRrad/oRrad.st, the delimitation is defined by {gr} values of 0.1. 1.0, 10, and 100. Apart from the general concepts used in ICRP (1984) and in GOST 25645.202-83 (1984), some features of in-flight radiation monitoring should be noted which arise from the features of radiation environments and from the particulars of designing radiation safety systems. The orbits of the presentday near-Earth flights are selected to be radiationsafe. The doses received by cosmonauts (0.2-0.6 mSv day-‘) are much below tolerance doses, so the radiation risk is much smaller than its standard value. The real radtatton hazard, which would involve a complicated radiation monitoring system, arises during high-orbiting flights. flights at large inclination angles. Interplanetary flights. and flights in which nuclear reactors are used. In such flights, diverse radiation types prove to be dosimetrically important, while the temporal conditions of exposure to radiations are non-uniform and even random. In designing a spacecraft RS system, radiation risk is calculated using the standardized cosmic ray models (Kovalev and Sakovich, 1989) including the stochastic model of solar proton events (COST
et al.
25645.13486, 1986), and bearing in mind the prescribed sufficiently strict space flight programs and schedules, but allowing also for a definite probability of deviations from them. Therefore, the standard GOST 25645.202-83 (1984) introduces the concept of a design model for the radiation environment. During the Bight preparations and the flight proper, the RS Service is in operation. It controls the radiation environment not only with on-board instruments but also with instruments flown on other artificial satellites. The RS Service uses the results of control in combination with ground-based solar activity observation data to predict radiation situations and, if necessary, to suggest urgent measures of reducing the exposure of crews to radiation, up to the flight termination. Thus, the on-board radiation monitoring system of a manned spacecraft is an information subsystem of the RS system. The subsystem must yield information which would permit appropriate models to be used on Earth or on board a spacecraft to calculate the parameters defining a particular radiation situation, to find out if an adopted design model agrees with real radiation environments, and indicate the reasons for deviations from the model which could cause the situation to deteriorate. In the last case, not only controlling environments for the crew inside a spacecraft, but also observing the state of probable hazard sources, as well as controlling emergency radiation situations, are intended. The current means of radiation monitoring comprise essentially the elements of future onboard systems and make it possible to test the methods for recording radiations and processing data, as well as the technological design features which should meet rigorous service requirements. Since the time of pioneer space flights, personal thermoluminescent dosimeters (TLD) (Akatov et al., 1971) have been used in the U.S.S.R. Having been treated on the ground, the TLDs yield the value of absorbed dose, which is then scaled to tissue dose. The quality factor defining the equivalent dose is calculated using the composition known for orbital flights (Akopova et al., 1990) and inferred also from the LET-spectrometer data (Kovalev ef al., 1979; Nguyen et al., 1989). An attenuation of the dose in the human body is indicated by the in-flight experimental data obtained with shielded dosimeters and with phantoms (Akatov el al., 1969, 1984). Short-term on-board radiation monitoring is carried out with ionization chamber-based radiometers whose readings are transmitted to Earth by telemetry (Grigoriev et al., 1976). The space crews carry out radiation monitoring at different points of the spacecraft and find any deterioration of the radiation environment using portable direct-reading dosimeters connected to tissue-equivalent ionization chambers (Markelov and Chernykh, 1982). The onboard and personal radiation monitoring operations carried out by spacecraft crews are combined in a
DOSIMETRIC
COMPLEX
FOR SPACE FLIGHTS
Fwsonal sensors OCIOO
II
Detectors
interface I/
FIG. 1. Block diagram of the Fedor complex.
Hungarian-made “Pille” instrument (Fe& et al., 1981). The results of testing the TLD sensitivity with high-temperature luminosity peaks (Schmidt and Fellinger, 1987; Schmidt et al., 1988) have shown that the TLDs can be used in future to measure also the equivalent doses. Promising means of radiation monitoring include also the French-made “Circe” instrument equipped with a proportional counter. The instrument makes it possible to determine the absorbed and equivalent dose rates and, thereby, to find the quality factor values on different trajectory segments (Nguyen er al., 1989). The instrument was flown on board the Mir orbital station in 1988-1990 and operated within the French-Soviet Aragatsexperiment. The experience gained in designing and operating with the dosimetric Pille, Circe and other instruments has been used at the Central Institute of Physical Research of the Hungarian Academy of Sciences where, in cooperation with experts from the U.S.S.R. and France, the “Fedor” dosimetric instrumental complex is being designed. The complex includes TLDs and a proportional counter. In conformity with the requirements of GOST 25645.202-83 (1984) the Fedor complex will make it possible to measure personal doses received by cosmonauts, the dose distribution inside a spacecraft within a given period, the quality factors at prescribed points inside spacecraft, and the absorbed and equivalent dose rates. In addition, radiation risk and a particular radiation situation type can be estimated. Obviously, the Fedor instruments have to satisfy strict service requirements, to operate faultlessly within at least 1.5-2 yr, and to yield information in the forms amenable to being transmitted to Earth and to enabling definite decisions by the crew and on Earth.
Figure 1 is a block-diagram of the Fedor complex which includes (i) the unit for personal dosimetry; (ii) the unit for continuous monitoring of dose rate; (iii) the unit for continuous measuring of absorbed and equivalent dose rates, particle flux, and LET spectrum; (iv) the unit for processing, analyzing, storing and displaying information. The Pille instrument, used successfully in dosimetric studies on board Salyut and Mir stations and one of the Space Shuttles (Akatov et al., 1984; Feher et al., in press), is a prototype of the personal dosimetry unit. A CaSO 4 : Dy TLDcontaining vessel is an autonomous sensor in the unit. The detector data are read out automatically from a metering desk with the vessel inserted. The measurement data are fed to the processing and storing unit after being corrected for sensitivity of a particular sensor. The measured dose value is shown on the metering desk display. If a dose exceeds the prescribed value, which necessitates an additional heating to anneal the TLD, the operation may be performed by inserting the vessel into an additional socket in the desk. Any excess of the prescribed dose value is shown by a special indicator on the desk. The measured doses range from 0.01 to 1000 mGy, so the autonomous sensors may be carried permanently by cosmonauts to measure personal dose at different points of the human body surface during EVA, and measure the dose field when placed at different points of the spacecraft. The sensors may also be used in radiation monitoring of sensitive systems (for example, photographic films), in dosimetric accompaniment of some experiments, etc. The exact number of sensors in the complex is defined by the particular problems to be solved. The unit for continuously monitoring the absorbed dose makes use of a sensor similar to a personal dosimeter inserted permanently into the information
YU. AKATOV read-out unit and connected via a flexible cable of 20m length to the unit of processing, analyzing and storing data. The sensor data are read out automatically every hour or every day, depending on the particular radiation environment. The interval durations are determined automatically by comparing the dose rate measurement data from a respective unit with the value set on the desk, or manually. The measurement results are shown on the desk display and are stored in memory for transmission to Earth. If the above-mentioned operational conditions are changed, a signal is produced to indicate noticeable changes in the radiation environment. The location of the unit for constant monitoring of the absorbed dose may be changed by the crew on command from Earth, in conformity with the autonomous sensor readings, or to comply with particular experimental tasks, including radiation safety measures. The Fedor complex may be equipped with two such blocks. The Circe instrument is a prototype of the unit for continuously measuring the absorbed and equivalent dose rates which has been designed for finding further detailed characteristics of exposure to radiation. In the unit, the spectrum of the pulse amplitudes supplied by the tissue-equivalent proportional counter is so processed that either the LET spectrum is obtained or the equivalent dose rate is found using the dependence of quality factor on LET or on linear energy stored in the unit memory. The LET spectrum is determined within the 0.2-1200 keV pm-’ range, and the equivalent dose rate within 0.005-S mGy h-r range. The quality factor is determined in the l-20 s interval with a 20% error. The stability of sensitivity is controlled by constant automatic calibration at a rate of once per day, using a built-in alpha-source. Under the basic operational conditions, the absorbed and equivalent doses, the particle flux and the quality factors which characterize them are measured each 0.5 h. A 5 12 channel LET spectrum is measured daily. Under accelerated operational conditions which are set automatically by a prescribed dose rate, or manually, the dose is measured each 0.5 min within a 10 min interval. The LET spectrum is measured within the same 10 min interval. When the accelerated operational conditions are set manually, the main working conditions are restored automatically within 10 min. A commercial PC compatible with IBM XT, or one of the on-board computers installed normally on manned spacecraft, is planned to be used as the unit for processing, analyzing, storing and displaying data. All experimental data are transmitted to a recoverable medium. The most important data fraction reIating mainly to radiation-hazard situations will be transmitted to Earth by telemetry channels. The Fedor complex is designed to solve the basic problems relevant to radiation monitoring during long-term orbital manned flight or during a Martian mission.
ef al. REFERENCES
Akatov Yu. A., Arkhangelsky V. V., Alexsandrov A. P., Feher I., Deme S., Szabi, B., Vagvcilgyi J., Saab6 P. P., Csrike a., Ranky M. and Farkas B. (1984) Thermoluminescent dose measurements on board Salyut-type orbital station. Adv. Space Res. 4(10), 77-81. Akatov Yu. A., Arkhangelsky V. V., Kokorev S. I., Sysoev P. Yu., Fehtr I. and Deme S. (1984) Experimental results obtained in framework of Phantom project on board Salyut ‘I-Soyuz orbital complex. Abs. Reps. at XVIIth Mtg Working Group Space Biol. Med. Intercosmos Program, Bmo, Czechoslovakia, IO-16 June. Akatov Yu. A., Arkhangelsky V. V., Markelov V. V., Skvortsov S. S., Smirenny L. N., Turkin V. N. and Chemykh I. V. (1971) Dosimetry of cosmic radiation. In Physical and Radiobiological Studies on Board Artificial Earth Satellites, pp. 33-40, Moscow. Akatov Yu. A., Kovalev E. E., Petrov V. M., Skvortsov S. S., Skuredin V. P. and Smirenny L. N. (1969) Results of experimental study of dosimetry and shielding on board Cosmos-l 10. Kosmich. Issl. VII(Z), 317-319. Akopova A. B., Magradze N. V., Dudkin V. E., Kovalev -E. E., Potapov-Yu. V., Benton E. V., Frank A. L., Benton E. R.. Pamell T. A. and Watts J. W. Jr 11990) Linear energy transfer (LET) spectra of cosmic‘ radi: ation in low Earth orbit. Nucl. Tracks Radial. Meas. 17, 93-97. Eleregovoy G. T., Yaropolov V. I., Baranetsky I. I., Vysokanov V. A. and Shatrov Yu. T. (1989) Handbook on Space Flight Safety, pp. 268-269. Mashinostroenie, Moscow. Feher I., Deme S., Szabo B., Vagvdlgyi J., Szabi, P. P., CsGke A., Ranky M. and Akatov Yu. A. (1981) A new thermoluminescent dosimeter system for space research. Adc. Space Res. 1, 61-66. Fehtr I., Richmond R. G., Ride S. K. and Atwell W. (in press) Real-time thermoluminescent dosimeter measurements on board the Space Shuttle orbiter. GOST 25645.202-83 (I 984) Requirements for individual crew and onboard dosimetry. Gosstandart S.S.S.R., Moscow. GOST 25645.214-85 (1984) Model of a generalised radiobiological effect. Gosstandart U.S.S.R., Moscow. GOST 25645.203-83 (1984) A human body model for tissue dose calculations. Gosstandart U.S.S.R., Moscow. GOST 25645.201-83 (1984) Radiation safety of spacecraft crews during flight. Terms and definitions. Gosstandart U.S.S.R., Moscow. GOST 25645.134-86 (1986) Solar cosmic rays. A model of proton fluxes. Gosstandart U.S.S.R., Moscow. GOST 25645.215-85 (1986) Radiation protection standards for flight duration up to three years. Gosstandart U.S.S.R., Moscow. GOST 25645.21890 (1991) Dependence of quality factor on linear energy. Gosstandart U.S.S.R.. Moscow. GOST 25645.21990 (1991) Methods for including the spatial distribution of equivalent dose in the human body. Gosstandart U.S.S.R., Moscow. Grigoriev Yu. Cl., Kovalev E. E., Petrov V. M., Etlmov V. I., Markelov V. V., Akatov Yu. A. and Teltsov A. V. (1976) Radiation safety of crews of Soyuz spacecraft. In Space Flights of Soyuz Spacecrafi, pp. 105-126, Nauka, Moscow. ICRP (1984) Publication 35: General Principles of Monitoring Radiation Protection of Workers. Pergamon Press, Oxford.
DOSIMETRIC
COMPLEX
Kovalev E. E., Bryksin V. N., Vinogradov yu. A., Dudkin V. E., Koslova S. B., Marenny A. M., Markelov V. V., Ncfedov N. A., Potapov Yu. V., Redko V. I. and Khovanskaya A. I. (1979) Measurements of linear energy transfer of cosmic radiation on board Cosmos782. Kosmich. Issl. XVII(4), 634-636. Kovalev E. E. and Sakovicb V. A. (1989) State Standards for radiation of spaa flights. Nucl. Tracks Radial. Meas. H&49-52; J.-AtomicEnergy 68,381-384 (1990). Markelov V. V. and Chemvkh I. V. (1982) Radiation monitoring with direct-&ding dosimete& on board Salyut stations. In Space Biologyand Medicine, No. 5, pp. 81-83, Meditsina, Moscow. Nguyen V. D., Bouisset P., Parmentier N., Akatov Yu. A., Petrov V. M., Kozlova S. B., Kovalev E. E., Kotovskaia A. R., Siegrist M., Zwilling J. F., Corn& B.. Thoulouse J.. Chrttien J. L. and Krikalev S. K. (1989) Real-time’ quality factor and dose equivalent
FOR SPACE FLIGHTS
11
meter “CIRCE” and its use on board the Soviet orbital station “Mir”. Proc. Int. Acad. Astr. 8th &mu. IAA-T-129, Tashkent, U.S.S.R. RD 50-25645.205-83 (19841 A method for calculatinn radiation risk. Goss&&t U.S.S.R., Moscow. RD 50-25645.217-90 (1991) Microdosimetry characteristics of cosmic rays. Gosstandart U.S.S.R., Moscow. RSDC-75 (1976) Radiation Shielding Design Criteria for Space Flights, U.S.S.R. Ministry of Health, Moscow. Schmidt P. and Fell&r J. (1987) Experimental determination of the response of some TL materials to heavy charged particles. Technische Universitiit D&en, Preprints No. 05-04-87 and 05-05-87 (in German). Schmidt P., Fellinger J. and Hilbner K. (1988) Experimental determination of relative light conversion factor of TLD-100 for protons. Nucl. Instrum. Meth. A272, 875-879.
Nucl. Tracks Radiat. Meas., Vol. 20, NO. I, pp. 7-11, 1992
Int. .I. Radial. Appl. Instun., Part D Printed in Great Britain
DOSIMETRIC
COMPLEX FOR LONG-TERM SPACE FLIGHTS
Yu. AKATOV,* E. E. KOVALEV,*V. A. SAKOVICH;
S. Dma,t
MAN-NED
I. Faxd@ and V. D. NGUYEN~
*Research Centrc for Spacecraft Radiation Safety of the Ministry of Health of the U.S.S.R., Moscow 123182, U.S.S.R.; tCe.ntral institute for Physics Research, Budapest, Hungary and SCentre d’Etudes Nuclkaircs de Fontenay-aux-Roses, France (Received 27 February 1991; in reviredform
24 June 1991)
Abstract-A dosimetric complex is proposed for use in long-term manned space tight on the orbital station and for the planned Mars expedition. This complex includes thermoluminescence dosimetcrs with on-board readout, a channel for continuous monitoring of dose rate and a channel for continuous measurement of absorbed and equivalent doses, as well as measurement of the LET spectra. The last four parameters are measured by a tissue-equivalent proportional counter.
INTRODUCTION
it should only be noted here that a particular type of radiation situation defines the short-term radiation safety (RS) measures. The second concept proposed by the ICRP (1984) requires that “any information necessary for personal irradiation to be estimated should be presented in the same tits as those used to express the basic limits”. This concept is of particular importance for space flights because the RS criterion has been taken to be the radiation risk oRrad (GOST 25645.215-85, 1986) which is a probabilistic quantity and, therefore, cannot be measured, but has to be calculated. The division of technologies, technical devices, and constructions into hazardous and safe types, without any quantitative estimates of the probabilities for averages of incurred damages of several different degrees, is gradually recognized to be no longer justified. The necessity of using such criteria is predetermined by the fact that solar proton Aares make it impossible to provide absolute safety of spacecraft crews. For this reason, not only a standard radiation level, but also the reliability of not exceeding such a level, was adopted in 1976 in the U.S.S.R. as space flight RS criteria (RSDC-75, 1976). The criteria were replaced later by the standard radiation risk level oRrad.st (GOST 25645.215-85, 1986) which makes it possible to allow adequately for both deterministic and probabilistic exposures to radiations. In this case, the radiation risk is meant (GOST 25645.214-85, 1984) to be an increase in the probability for cosmonauts to die during a flight because of somatic radiobiological effects caused by radiation. As noted by the ICRP (1984), other values, such as the mean equivalent dose in organs, the equivalent dose index, etc., cannot be measured directly. Therefore, the idea has been proposed by the ICRP (1984) of interpreting measurements on the basis of the models which describe quantitative relationships
RrJ>unoN monitoring has always been regarded as an important component of radiation protection and is a matter of permanent concern for such authoritative agencies as ICRP and ICRU. The manual “General Principles of Monitoring Radiation Protection of Workers” (ICRP, 1984) prepared at ICRP Committee 4 was based on extensive experience and, therefore, proved to be extremely helpful when preparing the standard “Requirements of Personal and On-Board Dosimetry” (GOST 25645.202-83, 1984) which is an integral part of the set of standards “Radiation Safety of Spacecraft Crews During Space Flights” (Kovalev and Sakovich, 1989) approved in 1983-1990 in the U.S.S.R. The concept of consistency of the task, scope, and accuracy of monitoring measures of the radiation environment expected during space flights was accepted to be the most general concept which could be traced back to the fundamental general concept recommended by the ICRP for guidance; namely, that radiation projection should be optimized by analyzing the balance between the radiation protection cost and the benefits from using the sources of ionizing radiations. In the case of space flights, the task of optimal usage of the mass distributed for various means of radiation protection and for safety as a whole becomes even more urgent. In view of the above, and by analogy with situations of different safety levels proposed by Beregovoy er al. (1989), the requirements of radiation monitoring were classified in GOST 25645.202-83 (1984) by introducing the “safe”, “standard”, “nonstandard”, “dangerous”, and “emergency” types of radiation situations. The quantitative discrimination among radiation situations will be discussed below; 7
8
YU. AKATOV
between measured and calculated values. Accordingly, the set of standards (Kovalev and Sakovich, 1989) includes a number of models which realize this concept, namely:
(9 a model of the human body for calculating dose (COST 25645.203-83, 1984); (ii) methods for including spatial distribution of equivalent dose in the human body (GOST 25645.219-90, 1991) intended for finding the value of adequate dose (GOST 2X45.201-83, 1984) (iii) dependence of quality factor on linear energy (GOST 25645.218-90, 1991) and the relevant method for calculating microdosimetric characteristics (RD 50-25645.217-90, 1991); (iv) a model of generalized radiobiological effect (GOST 25645.214-85, 1984) allowing for temporal conditions of exposure to radiation; (v) a method for calculating radiation risk (RD 50-25645.205-83, 1984) intended for including the probabilistic nature of exposures to radiations and of the generalized radiobiological effect. The standard GOST 25645.215-85 (1986) prescribes not only the standard radiation risk level, but also the values of the limiting dose Gst corresponding to the standard radiation risk level when the probabilistic exposures to radiation are absent. For the purposes of radiation monitoring, therefore, the standard GOST 25645.202-83 (1984) includes the concept of the hourly adequate dose oG and its control, meaning oGst = Gst/T, where T is flight duration in h. The above makes it possible to explain the principle of delimitation between the types of radiation situations. Given the notation g = oG/oGst and r = oRrad/oRrad.st, the delimitation is defined by {gr} values of 0.1. 1.0, 10, and 100. Apart from the general concepts used in ICRP (1984) and in GOST 25645.202-83 (1984), some features of in-flight radiation monitoring should be noted which arise from the features of radiation environments and from the particulars of designing radiation safety systems. The orbits of the presentday near-Earth flights are selected to be radiationsafe. The doses received by cosmonauts (0.2-0.6 mSv day-‘) are much below tolerance doses, so the radiation risk is much smaller than its standard value. The real radtatton hazard, which would involve a complicated radiation monitoring system, arises during high-orbiting flights. flights at large inclination angles. Interplanetary flights. and flights in which nuclear reactors are used. In such flights, diverse radiation types prove to be dosimetrically important, while the temporal conditions of exposure to radiations are non-uniform and even random. In designing a spacecraft RS system, radiation risk is calculated using the standardized cosmic ray models (Kovalev and Sakovich, 1989) including the stochastic model of solar proton events (COST
et al.
25645.13486, 1986), and bearing in mind the prescribed sufficiently strict space flight programs and schedules, but allowing also for a definite probability of deviations from them. Therefore, the standard GOST 25645.202-83 (1984) introduces the concept of a design model for the radiation environment. During the Bight preparations and the flight proper, the RS Service is in operation. It controls the radiation environment not only with on-board instruments but also with instruments flown on other artificial satellites. The RS Service uses the results of control in combination with ground-based solar activity observation data to predict radiation situations and, if necessary, to suggest urgent measures of reducing the exposure of crews to radiation, up to the flight termination. Thus, the on-board radiation monitoring system of a manned spacecraft is an information subsystem of the RS system. The subsystem must yield information which would permit appropriate models to be used on Earth or on board a spacecraft to calculate the parameters defining a particular radiation situation, to find out if an adopted design model agrees with real radiation environments, and indicate the reasons for deviations from the model which could cause the situation to deteriorate. In the last case, not only controlling environments for the crew inside a spacecraft, but also observing the state of probable hazard sources, as well as controlling emergency radiation situations, are intended. The current means of radiation monitoring comprise essentially the elements of future onboard systems and make it possible to test the methods for recording radiations and processing data, as well as the technological design features which should meet rigorous service requirements. Since the time of pioneer space flights, personal thermoluminescent dosimeters (TLD) (Akatov et al., 1971) have been used in the U.S.S.R. Having been treated on the ground, the TLDs yield the value of absorbed dose, which is then scaled to tissue dose. The quality factor defining the equivalent dose is calculated using the composition known for orbital flights (Akopova et al., 1990) and inferred also from the LET-spectrometer data (Kovalev ef al., 1979; Nguyen et al., 1989). An attenuation of the dose in the human body is indicated by the in-flight experimental data obtained with shielded dosimeters and with phantoms (Akatov el al., 1969, 1984). Short-term on-board radiation monitoring is carried out with ionization chamber-based radiometers whose readings are transmitted to Earth by telemetry (Grigoriev et al., 1976). The space crews carry out radiation monitoring at different points of the spacecraft and find any deterioration of the radiation environment using portable direct-reading dosimeters connected to tissue-equivalent ionization chambers (Markelov and Chernykh, 1982). The onboard and personal radiation monitoring operations carried out by spacecraft crews are combined in a
DOSIMETRIC
COMPLEX
FOR SPACE FLIGHTS
Fwsonal sensors OCIOO
II
Detectors
interface I/
FIG. 1. Block diagram of the Fedor complex.
Hungarian-made “Pille” instrument (Fe& et al., 1981). The results of testing the TLD sensitivity with high-temperature luminosity peaks (Schmidt and Fellinger, 1987; Schmidt et al., 1988) have shown that the TLDs can be used in future to measure also the equivalent doses. Promising means of radiation monitoring include also the French-made “Circe” instrument equipped with a proportional counter. The instrument makes it possible to determine the absorbed and equivalent dose rates and, thereby, to find the quality factor values on different trajectory segments (Nguyen er al., 1989). The instrument was flown on board the Mir orbital station in 1988-1990 and operated within the French-Soviet Aragatsexperiment. The experience gained in designing and operating with the dosimetric Pille, Circe and other instruments has been used at the Central Institute of Physical Research of the Hungarian Academy of Sciences where, in cooperation with experts from the U.S.S.R. and France, the “Fedor” dosimetric instrumental complex is being designed. The complex includes TLDs and a proportional counter. In conformity with the requirements of GOST 25645.202-83 (1984) the Fedor complex will make it possible to measure personal doses received by cosmonauts, the dose distribution inside a spacecraft within a given period, the quality factors at prescribed points inside spacecraft, and the absorbed and equivalent dose rates. In addition, radiation risk and a particular radiation situation type can be estimated. Obviously, the Fedor instruments have to satisfy strict service requirements, to operate faultlessly within at least 1.5-2 yr, and to yield information in the forms amenable to being transmitted to Earth and to enabling definite decisions by the crew and on Earth.
Figure 1 is a block-diagram of the Fedor complex which includes (i) the unit for personal dosimetry; (ii) the unit for continuous monitoring of dose rate; (iii) the unit for continuous measuring of absorbed and equivalent dose rates, particle flux, and LET spectrum; (iv) the unit for processing, analyzing, storing and displaying information. The Pille instrument, used successfully in dosimetric studies on board Salyut and Mir stations and one of the Space Shuttles (Akatov et al., 1984; Feher et al., in press), is a prototype of the personal dosimetry unit. A CaSO 4 : Dy TLDcontaining vessel is an autonomous sensor in the unit. The detector data are read out automatically from a metering desk with the vessel inserted. The measurement data are fed to the processing and storing unit after being corrected for sensitivity of a particular sensor. The measured dose value is shown on the metering desk display. If a dose exceeds the prescribed value, which necessitates an additional heating to anneal the TLD, the operation may be performed by inserting the vessel into an additional socket in the desk. Any excess of the prescribed dose value is shown by a special indicator on the desk. The measured doses range from 0.01 to 1000 mGy, so the autonomous sensors may be carried permanently by cosmonauts to measure personal dose at different points of the human body surface during EVA, and measure the dose field when placed at different points of the spacecraft. The sensors may also be used in radiation monitoring of sensitive systems (for example, photographic films), in dosimetric accompaniment of some experiments, etc. The exact number of sensors in the complex is defined by the particular problems to be solved. The unit for continuously monitoring the absorbed dose makes use of a sensor similar to a personal dosimeter inserted permanently into the information
YU. AKATOV read-out unit and connected via a flexible cable of 20m length to the unit of processing, analyzing and storing data. The sensor data are read out automatically every hour or every day, depending on the particular radiation environment. The interval durations are determined automatically by comparing the dose rate measurement data from a respective unit with the value set on the desk, or manually. The measurement results are shown on the desk display and are stored in memory for transmission to Earth. If the above-mentioned operational conditions are changed, a signal is produced to indicate noticeable changes in the radiation environment. The location of the unit for constant monitoring of the absorbed dose may be changed by the crew on command from Earth, in conformity with the autonomous sensor readings, or to comply with particular experimental tasks, including radiation safety measures. The Fedor complex may be equipped with two such blocks. The Circe instrument is a prototype of the unit for continuously measuring the absorbed and equivalent dose rates which has been designed for finding further detailed characteristics of exposure to radiation. In the unit, the spectrum of the pulse amplitudes supplied by the tissue-equivalent proportional counter is so processed that either the LET spectrum is obtained or the equivalent dose rate is found using the dependence of quality factor on LET or on linear energy stored in the unit memory. The LET spectrum is determined within the 0.2-1200 keV pm-’ range, and the equivalent dose rate within 0.005-S mGy h-r range. The quality factor is determined in the l-20 s interval with a 20% error. The stability of sensitivity is controlled by constant automatic calibration at a rate of once per day, using a built-in alpha-source. Under the basic operational conditions, the absorbed and equivalent doses, the particle flux and the quality factors which characterize them are measured each 0.5 h. A 5 12 channel LET spectrum is measured daily. Under accelerated operational conditions which are set automatically by a prescribed dose rate, or manually, the dose is measured each 0.5 min within a 10 min interval. The LET spectrum is measured within the same 10 min interval. When the accelerated operational conditions are set manually, the main working conditions are restored automatically within 10 min. A commercial PC compatible with IBM XT, or one of the on-board computers installed normally on manned spacecraft, is planned to be used as the unit for processing, analyzing, storing and displaying data. All experimental data are transmitted to a recoverable medium. The most important data fraction reIating mainly to radiation-hazard situations will be transmitted to Earth by telemetry channels. The Fedor complex is designed to solve the basic problems relevant to radiation monitoring during long-term orbital manned flight or during a Martian mission.
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