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Real time quality factor and dose equivalent meter “CIRCE” and its use on-board the Soviet orbital station “MIR”
Acta Astronautica Vol. 23, pp. 217 226, 1991 Printed in Great Britain
0094-5765/91 $3.00 + 0.00 Pergamon Press plc
REAL TIME Q U A L I T Y F A C T O R AND DOSE E Q U I V A L E N T M E T E R " C I R C E " AND ITS USE O N - B O A R D T H E SOVIET O R B I T A L S T A T I O N " M I R "
V.D. Nguyen, P. Bouisset, N. Parmentier Commissariat a l'Energie Atomique, Fontenay aux Roses, France. I.A. Akatov, V.M. Petrov, S.B. Kozlova, E.E. Kovalev, A. Katovskaia Institute of Medical and Biological Problems, Moscow, USSR. M. Siegrist, J.F. Zwilling, B. Comet, J. Thoulouse CNES, Toulouse, France. J.L. Chr~tien, S.K. Krikalev, Cosmonauts.
Abstract During the French-Soviet space mission "Aragatz", the experiment CIRCE (Compteur Integrateur de R a y o n n e m e n t Complexe dans rEspace) recorded the dose rate and quality factor values inside the MIR station. This paper presents results obtained with a new active dose equivalent meter based on microdosimetric techniques and using a low pressure tissue equivalent proportional counter. In terms of lineal energy C I R C E device works in the 0.2 - 1200 keV.lim -1 range in tissue. Preliminary studies were performed in photon, neutron and heavy ion beams, and in the real stratosphere cosmic radiation field. Long term measurements o n - b o a r d MIR station from December 1988 to April 1989 gave an average quality factor value equal to 1.9:1: 0.3. T h r o u g h the South Atlantic A n o m a l y (SAA), the quality factor was equal to 1.4. The temporal orbital variations of the dose rates and quality factors have been established in space dosimetry for the first time. INTRODUCTION The complex radiation e n v i r o n m e n t in spaceflights raises a n u m b e r of problems in dosimetry, mainly due to a great n u m b e r of radiation sources including galactic cosmic rays, radiation originating from the sun, charged particles trapped by the earth's magnetic field, radiation induced in o n - b o a r d materials by high energy charged particles such as protons, alpha particles and heavier nuclei. The fluences and energy spectra of these radiations are dependent on altitude, orbit inclination, solar activities, spacecraft positions and orientations in orbit and types of shielding materials used in the spacecraft. During the last t w e n t y - f i v e years of the United-States and Soviet m a n n e d space programs (1), a large part of dosimetry measurements has been made using passive detectors and has yielded integrated dose and L E T spectra. Active detectors have been used only on several occasions to obtain information on the temporal variations of the radiation field inside a spacecraft (2, 3). Active detectors gave significant data on solar flares and photon bursts from electrons precipitating from the trapped belt. In addition to this situation, the extended duration of the stay and the increasing n u m b e r of people in space involve an adequate description of the external radiation field in the near earth orbits. In the above reports (1, 2, 3) the authors clearly show the importance and the need of real-time dosimetry for the current shuttle flights and particularly for the space station. Very little experimental data on the neutron contribution to the total dose is available for low altitude and low inclination orbits, and there are no experimental data for polar high altitude orbits. As a step in that direction, this paper presents the main results obtained by using CIRCE e q u i p m e n t for measuring dose equivalents and quality factors inside the Soviet space station MIR. The average altitude is 350 km, and the inclination 51".5. The rotating period is equal to 90 minutes. Long term measurements were p e r f o r m e d during six months in this orbit in order to record the temporal variations of the radiation field. INSTRUMENT DESCRIPTION
Mode of analysis The principles of the method and the main features and performance of the experimental device have been described elsewhere (4, s). A short s u m m a r y of the mode of analysis is given below. The method is based on the substitution of the linear energy transfer L i by the lineal energy Yi deduced from each pulse height h i generated by the detector described in the next paragraph. The pulse height h. is proportional to the energy locally imparted to the volume considered and therefore to the lineal energy1 Yi" Consequently, each pulse height h, is related to a linear energy transfer L, and to a specific quality factor Q(Li) value. The absorbed dosd D, the dose equivalent H and the dose haean quality factor Q are given by the following relations : D = (C/m)• hi H = ( C / m ) ~- h i q (Li) Q = H/D where C is the calibration factor for energy deposited per unit of pulse height and m is the mass of the tissue-equivalent gas inside the sensitive volume of the detector. 23-P
217
218
V . D . NGt;YE'4 et a/
Detector Low pressure tissue-equivalent proportional counters are usually used to measure absorbed doses based on the cavity chamber principles. Due to the low gas pressure and single-event registration with pulseheight analysis, such detectors are also able to determine dose equivalents by a method which is close to the definition of this quantity and differs from all other methods used in routine radiation protection instruments. Therefore, in order to measure the quality factors and dose equivalents a cylindrical tissue-equivalent gas proportional counter was used. The counter was tailored with special technology for the suspended central electrode, the 20 )am diameter wire was subjected to 10 g-acceleration tests along the 3 axes. The sensitive volume is 5 cm in diameter, 5 cm in height with 4 mm thick walls made of A-150 plastic (Figure 1). The outer protection shell is composed of two separate parts ; the following considerations were taken into account in the outer shell design. Since long-term measurements need absolute gas tightness, laser welding techniques were employed for the 316 L type stainless steel used ; the first part is made of 1.5 mm thick stainless steel. For an isotropic heavy charged particles radiation field, the dose at a specific organ depends on the thickness distribution function (6). The second part is therefore made of 6 mm thick polyethylene with conic holes (see details figure I) in order to spread out the thickness distribution function with respect to heavy charged particles. The overall thickness around the active gas volume of the counter is 2.2 g.cm -2. The counter was filled with propane based tissue equivalent gas to obtain a simulated size of 3.57 pro.
Pulse height processing and data processing unit The main features of the CIRCE device are illustrated in Figure 2. The range of four decades in the voltage signal requires two separate identical pulse processors working respectively with l and 64 gain amplifiers located in the pulse separator. The heart of the data processing unit consists of a MC 6809 microprocessor interfaced with a 8 k ROM where the external extension software is located, and with a 8 k RAM where the numerical data are stored. The overall dead time is evaluated at about 30 ,us. In term of linear energy transfer the device operates in the 0.2 - 1200 keV.pm -1 range. VALIDITY TESTS BEFORE SPACE F L I G H T Initially, the CIRCE device was set up for microdosimetric measurements in neutron and gamma mixed radiation field on earth {4, s) In order to investigate the behaviour of the dose equivalent meter CIRCE in a complex galactic cosmic radiation field, two experiments were carried out : energy deposition spectrum measurements in a 320 MeV.amu -1 12C beam at the DUBNA Synehrocyclotron (USSR), and dose equivalent measurements aboard an aircraft.
Energy deposition spectrum measurements (7) Energy deposition spectra were measured along the beam axis at 5 m from the scatterer. Lineal energy spectra at different positions relative to the Bragg peak are obtained by accumulating pulse height_ distributions from the counter, and by immediate data processing by the CIRCE device. At 2.2 g.cm the lineal energy mean values on frequenc~ distribution is equal to ~'F = 6.75 keV.pm -1 and on dose distribution is equal to : ~D = 21.2 keV.pm- .
Dose equivalent measurements in the stratosphere (7) Dose equivalents and quality factors were measured aboard the laboratory supersonic aircraft TU-144 on two different routes at an altitude between 17 and 18 kin. The first route was along the longitude 37°5 E between latitude 46* N and latitude 70 ° N, the second route along latitude 56 ° N from longitude 27* E to longitude 47* E. The dose equivalent rates vary from 7.7 to 12.2 pSv.h -1, and the average quality factor is equal to 2.2.
OPERATIONAL MODES In order to investigate the temporal and spatial variations of the dose equivalents and the quality factors on board the MIR space station, continuous one-hour's measurements were performed. This is called the normal measurement mode. Due to the topological structure of the South Atlantic Anomaly (SAA) with its very steep radial gradient of radiation, a second operational mode, called fast measurement mode, can be started on request at a predetermined instant. This mode consists of 30 seconds' measurements during half an hour, and then the system automatically switches back to the normal mode. The fast measurement mode was used to obtain a precise description of the SAA on 9 orbits indicated by an arrow in Figure 3. The starting times were calculated to start the fast measurement mode about 10 minutes before the MIR station entering the SAA.
8th IAA Man in Space Symposium
219
R E S U L T S AND DISCUSSION The results concern the m e a s u r e m e n t s made on December 1988, during the French and Soviet joint experiments, and those made on March and April 1989. Normal m e a s u r e m e n t results
The first set of measures was obtained in a quiet solar activity situation. Figure 4 - a shows the distributions of the dose equivalent rates H and absorbed dose rates D related to 319 o n e - h o u r ' s measures. Their mean values are respectively equal to 25.7 pSv.h -1 and 13.4 laGy.h -1. Figure 4 - b shows the distributions of the quality factors Q and of quality factors QHL due to L E T greater than 3.5 keV.pm -1. Their mean values are respectively equal to 1.9 and 7.7. The narrowness of the D and Q distributions related to the broadness of the H and QHL distributions indicate that a large part of the absorbed dose is due to low L E T events. These events are induced not only by g a m m a or electrons, but mainly by protons of energies greater than 14 MeV or by heavy ions of energies greater than 80 MeV.amu -1. Conversely in respect to the dose equivalent rate H, the contribution of the dose equivalent with LET greater than 3.5 keV.pm -1 represents 58 % of the total dose equivalent. This fact indicates the presence of very large L E T events. The second set of measures was performed during the period after the large solar flare at the beginning of March 1989. Figures 4-c and 4 - d show the same quantities H, D, Q and the QHL related to 732 measures. The distributions of H and D are almost identical to the first set of measures (Fig. 4-e). Their mean values are respectively equal to 33.3 pSv.h -1 and 18.8 }aGy.h -1. The distributions of Q and Q H L are sligthly different in the lower part of each distribution (Fig. 4-f). Their m e a n values are respectively equal to 1.8 and 7.4 (Table 1). During the period posterior to the solar flare which occurred on March 1989, these results, i.e. the increasing H and D and the decreasing Q, clearly show the increase of the absolute particle flux of low L E T in respect to the period before the solar flare. Table 2 shows the comparison of the values measured during different missions (8, 9, 10). The large discrepancies observed are not only due to orbital parameters but also to the nature and thickness of the vessel shielding. Systematically, 2 or 3 times a day the dose equivalent rates exceed 40 ~Sv.h -1. These values are related to the crossing through the SAA by the orbital station MIR about 2 or 3 times a day. A more precise description of the SAA area is given by the fast m e a s u r e m e n t mode. Fast m e a s u r e m e n t results
Figures 5 and 6 show 9 sets of 60 measures in December 1988 when the MIR station was crossing through the SAA related to 4 North South routes and 5 South North routes. Beginning and ending values of each route are in the same order as the above mean values. A comparison of figures 5 and 6 shows that systematically dose equivalent rates on South North routes are higher than those on North South routes by about 20 %. An orbit analysis indicates that South North routes were on the sun side, and North South routes were in the earth's shadow side. This established fact partly explains the above systematic difference. A very steep gradient is observed in the central area of the SAA (Fig. 7). Dose equivalent rates can reach values above 1 mSv.h -1. Conversely, the quality factor Q is equal to 1.4 and it is significantly lower than the mean value 2.1 outside of the SAA. Table 3 shows the comparison of absorbed doses measured by other authors (s, 11) and by the present work. It clearly indicates the large dose values in the SAA. Even if crossing durations vary from 8 to 10 minutes only, more than 30 % of total dose per day are due to the SAA radiation. ESTIMATION OF THE INTEGRATED EXPERIMENTS IN DECEMBER 1988
DOSE
DURING
THE
FRENCH
SOVIET
JOINT
The duration of the mission was 26 days. On this basis, the estimated dose equivalent inside the MIR station was equal to 16.0 mSv and the absorbed dose was equal to 8.4 mGy. T L D detectors exposed in the same conditions gave values in the 6 - 8 m G y range. The slight underestimation was due to the weak response of the detectors to heavy ions. The neutron c o m p o n e n t was measured by bubble detector (12). The neutron dose equivalent was equal to 0.6 mSv, representing about 4 % of the total dose. Table 4 s u m m a r i e s useful average dosimetric data related to one orbit aboard the MIR station, and Table 5 gives measured values during the m a n n e d spaceflight A R A G A T Z .
220
V . D . NGUYEN et aL
CONCLUSION The temporal and spatial variations of space radiation pointed out by the present work in terms of quality factor and dose equivalent show the need of an active radiation measurements in manned space flights. With respect to the particular topological structure of the SAA, to its location and its extension which are randomly modulated by the solar activity, to its large contribution to the total dose inside the space station, an active detector as CIRCE device is an adequate instrument for long term dosimetric investigation. Such an instrument is useful in a wide variety of manned activities in near earth orbits with high inclinations, for instance 57* and more, where serious radiation problems must be taken into account in case of solar flares. The complex shielding of manned spacecraft causes large uncertainties in predictions of the flux, charge and energy spectra of charged particles, and consequently in the dose equi-valent rate evaluation. At the same time, the irradiation of tissue with cosmic ray nuclei, trapped energetic protons, recoils and spallation products, implies a knowledge of LET spectra to assess radiobiological consequences. The present work shows that, as an alterna-tive, the CIRCE device can act as a practical tool for systematic real-time dosimetry in shuttle flights and particularly in space stations. It can also give the answer to spatial and temporal variations of dosimetric quantities due to heavy charged particles and secondary neutron component, for which very little data exist at the present time.
REFERENCES 1. 2. 3. 4. 5. 6. 7.
8. 9. 10. 11. 12.
Benton, E.V., S u m m a r y o f radiation dosimetry results on U.S. and Soviet manned spacecraft COSPAR (1986). Kovalev, E.E., Benton, E.V., Marenny, A.M. Measurement o / L E T spectra aboard cosmos 936 biological satellite, Rad. Prot. Dos. 1, 3, 169 (1981). Parnell, T.A., Watts J.W., Fishman G.J., Benton, E.V., Frank, A.L., Gregory, J.C. The measured radiation environment within spacelabs 1 and 2 comparison with predictions COSPAR (1986). Nguyen, V.D., Luccioni C., Parmentier N. , Average quality factor and dose equivalent meter based on microdosimetry techniques, Rad. Prot. Dos., 10, 1,277 (1985). Nguyen, A dose equivalent meter based on the tissue-equivalent proportional counter, and problems encountered in its use, Rad. Prot. Dos. 9, 3,223 (1984). Bouisset, P., Nguyen V.D., Comet B., Bourrieau, J., Le Grand, J., Roux, Y., Parmentier N., Calcul de la dose induite aux organes critiques par les particules charg~es en orbites basses circumterrestres (to be published). Nguyen, V,D., Bouisset, P., Akatov, Y.A., Petrov, V.M., Siegrist, M., Zwilling, J.F., . Preliminary studies on the use o f C I R C E for manned space mission dosimetry, Workshop on implementation of dose equivalent meters based on microdosimetrie techniques in radiation protection, Schloss Elmau (RFA) (1988). Janni, J., S K Y L A B 2 radiation dosimetry systems and flight results, Report AFWL-TR-73-222, 1976. Akatov, Yu. A., et al., Thermoluminescent dose measurements on board Salyout type orbital stations, Adv. Space Res., 4(10) : 77 (1984). Kovalev, E.E., Benton, E.V., Marenny, A.M., Measurement o f L E T spectra aboard C O S M O S 936 biological satellite, Rad. Prot. Dos., 1, (3) : 169 (1981). Janni, J., Spacecraft cabin radiation distributions for the 4th and 6th Gemini flights, Aerosp. Med. 40 (12) : 1527 (1969). Ing, H., The status o f the bubble damage polymer detector, 13th International Conference on Solid State Nuclear Track detectors, Rome (1985).
Period .
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Dec. 1988 .
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Dose equivalent H (pSv.h "~)
25.7
33.3
Absorbed dose D (pGy.h -~)
13.4
18.8
7.7
7.4
1.9
1.8
High LET quality factor Total Quality factor ......
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¥-~'b3~'l"'--ff~Zn'valZZ~m~gre-d-I;~'~fli~if~Z
on board Soviet orbital station "MIR"
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8th IAA Man in Space Symposium
Mission .
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Orbit parameters Dose equivalent Absorbed dose (mSv/day) (mGy/day)
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Skylab 2 (1976) .
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Neutron dose (mSv/day) .
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R~f. .
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0.570
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0.216 .
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350 km
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0.071 .
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0.322
0.799
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350 km 51°.5
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380 km 28°.5
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Atlantis (1985) .
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Discovery (1985) .
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0.023 .
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(10) .
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present work
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0.451
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present work
51°'5 .
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Table 2 - Comparison of measured values averaged over the orbits of different spacecrafts
Mission .
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Orbit parameters Dose equiva|ent Absorbed ~ose Quality factor Ref (mSv.h -~) (mGy.h -~) .
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Table
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Table
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1.050 .
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(11) .
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0.755
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Dose equivalent (mSv/orbit) .
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1.4 .
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Absorbed dose (mGy/orbit) .
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0.039
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Present work .
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2.1
0.052 .
.
1.9
0.015
0.081 .
.
Quality factor
0.020
0.031
Over orbits crossing SAA .
.
0.126
Over orbits out of SAA .
.
0.828
Over all orbits .
.
Comparison of maximum dose rates measured on the South Atlantic Anomaly aboard different spacecrafts
-
.
.
350 km 51°.5
Route configuration .
.
296 x 166 km 32°.5
MIR (Dec. 1988) .
.
435 km 50*
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.
1.6 .
.
Estimated average values related to one orbit aboard Soviet orbital station "MIR" (350 km, 51°.5, 90 minutes)
222
V . D . NGUYEN et al.
Duration : 26 days . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Detector
Dose equivalent (roSy)
Absorbed dose (mGy)
Quality factor
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CIRCE
16.0
TLD
8.4
1.9
6 to 8
Bubble detector
0.6 (neutron only)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Table 5 - Dosimetric data related to "ARAGATZ" mission aboard Soviet orbital station "MIR" (Dec. 1988)
316 L stainless steel ]
A-150 plastic
]
polyethylene g 94
.403
1
V, f i l l
1
-} i
I
L
20 pm golden tungsten
Details of polyethylene cap
central wire Figure 1- Schematic geometry of the tissue equivalent proportional counter
i
8th IAA Man in Space Symposium
223
PULSE h
PROCESSOR
PULSE SEPARATOR PULSE PROCESSOR T,E. PROPORTIOtLAL COUNTER
F i g u r e 2 - M a i n features o f the dose equivalent m e t e r C I R C E
, . . . .
16052
16093
16082*
F i g u r e 3 - P r e d e t e r m i n e d orbits for f a s t measurements through the S A A . The underlined numbers indicate the orbit number, the arrows indicate the directions of the space station. * on this o r b i t no m e a s u r e m e n t could be carried out .
224
V.D. NGUYEN et air
DECEMBER 1988 RESULTS
W
0
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10
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M A R C H - APRIL 1989 RESULTS
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,
I
,
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H : Dose e q u i v a l e n t pSv.h - I D : Absorbed dose pGy.h -1
Figure
I
•
10
8
4 '
10
12
Q : Total quality f a c t o r QHL : High LET quality f a c t o r
of
doses
and
Soviet
orbital
station
quality
MIR
factors
measured
on
board
8th IAA Man in Space Symposium
225
1.2"
0.9m
°
H
0,~-
..,lllillll.li.,,,i,,, .....
..
"'~'1[1111
• ---"...,]i!II ..... 2~ •
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320 •
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LONGITI.IDE
F i g u r e 5 - D o s e e q u i v a l e n t r a t e s a b o a r d t h e S o v i e t space s t a t i o n M I R on S A A N o r t h South r o u t e s . A b o v e ~0 }JSv.h -1 d i a g r a m s a r e d r a w n i n b l a c k .
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--50 °
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LONGI TLIOE
F i g u r e 6 - Dose e q u i v a l e n t rates aboard the S o v i e t space s t a t i o n M I R on S A A South N o r t h r o u t e s . A b o v e 40 p S v . h" l d i a g r a m s a r e d r a w n in b l a c k .
226
V . D . NGUYEN el a/.
1.2-
0,9
275 °
290 °
305 °
320 °
335 °
3S0 °
LONGITUDE
F i g u r e 7- D o s e e q u i v a l e n t r a t e s m e a s u r e d a b o a r d t h e S o v i e t s p a c e s t a t i o n MIR t h r o u g h SAA in t h r e e d i m e n s i o n r e p r e s e n t a t i o n .
0094-5765/91 $3.00 + 0.00 Pergamon Press plc
REAL TIME Q U A L I T Y F A C T O R AND DOSE E Q U I V A L E N T M E T E R " C I R C E " AND ITS USE O N - B O A R D T H E SOVIET O R B I T A L S T A T I O N " M I R "
V.D. Nguyen, P. Bouisset, N. Parmentier Commissariat a l'Energie Atomique, Fontenay aux Roses, France. I.A. Akatov, V.M. Petrov, S.B. Kozlova, E.E. Kovalev, A. Katovskaia Institute of Medical and Biological Problems, Moscow, USSR. M. Siegrist, J.F. Zwilling, B. Comet, J. Thoulouse CNES, Toulouse, France. J.L. Chr~tien, S.K. Krikalev, Cosmonauts.
Abstract During the French-Soviet space mission "Aragatz", the experiment CIRCE (Compteur Integrateur de R a y o n n e m e n t Complexe dans rEspace) recorded the dose rate and quality factor values inside the MIR station. This paper presents results obtained with a new active dose equivalent meter based on microdosimetric techniques and using a low pressure tissue equivalent proportional counter. In terms of lineal energy C I R C E device works in the 0.2 - 1200 keV.lim -1 range in tissue. Preliminary studies were performed in photon, neutron and heavy ion beams, and in the real stratosphere cosmic radiation field. Long term measurements o n - b o a r d MIR station from December 1988 to April 1989 gave an average quality factor value equal to 1.9:1: 0.3. T h r o u g h the South Atlantic A n o m a l y (SAA), the quality factor was equal to 1.4. The temporal orbital variations of the dose rates and quality factors have been established in space dosimetry for the first time. INTRODUCTION The complex radiation e n v i r o n m e n t in spaceflights raises a n u m b e r of problems in dosimetry, mainly due to a great n u m b e r of radiation sources including galactic cosmic rays, radiation originating from the sun, charged particles trapped by the earth's magnetic field, radiation induced in o n - b o a r d materials by high energy charged particles such as protons, alpha particles and heavier nuclei. The fluences and energy spectra of these radiations are dependent on altitude, orbit inclination, solar activities, spacecraft positions and orientations in orbit and types of shielding materials used in the spacecraft. During the last t w e n t y - f i v e years of the United-States and Soviet m a n n e d space programs (1), a large part of dosimetry measurements has been made using passive detectors and has yielded integrated dose and L E T spectra. Active detectors have been used only on several occasions to obtain information on the temporal variations of the radiation field inside a spacecraft (2, 3). Active detectors gave significant data on solar flares and photon bursts from electrons precipitating from the trapped belt. In addition to this situation, the extended duration of the stay and the increasing n u m b e r of people in space involve an adequate description of the external radiation field in the near earth orbits. In the above reports (1, 2, 3) the authors clearly show the importance and the need of real-time dosimetry for the current shuttle flights and particularly for the space station. Very little experimental data on the neutron contribution to the total dose is available for low altitude and low inclination orbits, and there are no experimental data for polar high altitude orbits. As a step in that direction, this paper presents the main results obtained by using CIRCE e q u i p m e n t for measuring dose equivalents and quality factors inside the Soviet space station MIR. The average altitude is 350 km, and the inclination 51".5. The rotating period is equal to 90 minutes. Long term measurements were p e r f o r m e d during six months in this orbit in order to record the temporal variations of the radiation field. INSTRUMENT DESCRIPTION
Mode of analysis The principles of the method and the main features and performance of the experimental device have been described elsewhere (4, s). A short s u m m a r y of the mode of analysis is given below. The method is based on the substitution of the linear energy transfer L i by the lineal energy Yi deduced from each pulse height h i generated by the detector described in the next paragraph. The pulse height h. is proportional to the energy locally imparted to the volume considered and therefore to the lineal energy1 Yi" Consequently, each pulse height h, is related to a linear energy transfer L, and to a specific quality factor Q(Li) value. The absorbed dosd D, the dose equivalent H and the dose haean quality factor Q are given by the following relations : D = (C/m)• hi H = ( C / m ) ~- h i q (Li) Q = H/D where C is the calibration factor for energy deposited per unit of pulse height and m is the mass of the tissue-equivalent gas inside the sensitive volume of the detector. 23-P
217
218
V . D . NGt;YE'4 et a/
Detector Low pressure tissue-equivalent proportional counters are usually used to measure absorbed doses based on the cavity chamber principles. Due to the low gas pressure and single-event registration with pulseheight analysis, such detectors are also able to determine dose equivalents by a method which is close to the definition of this quantity and differs from all other methods used in routine radiation protection instruments. Therefore, in order to measure the quality factors and dose equivalents a cylindrical tissue-equivalent gas proportional counter was used. The counter was tailored with special technology for the suspended central electrode, the 20 )am diameter wire was subjected to 10 g-acceleration tests along the 3 axes. The sensitive volume is 5 cm in diameter, 5 cm in height with 4 mm thick walls made of A-150 plastic (Figure 1). The outer protection shell is composed of two separate parts ; the following considerations were taken into account in the outer shell design. Since long-term measurements need absolute gas tightness, laser welding techniques were employed for the 316 L type stainless steel used ; the first part is made of 1.5 mm thick stainless steel. For an isotropic heavy charged particles radiation field, the dose at a specific organ depends on the thickness distribution function (6). The second part is therefore made of 6 mm thick polyethylene with conic holes (see details figure I) in order to spread out the thickness distribution function with respect to heavy charged particles. The overall thickness around the active gas volume of the counter is 2.2 g.cm -2. The counter was filled with propane based tissue equivalent gas to obtain a simulated size of 3.57 pro.
Pulse height processing and data processing unit The main features of the CIRCE device are illustrated in Figure 2. The range of four decades in the voltage signal requires two separate identical pulse processors working respectively with l and 64 gain amplifiers located in the pulse separator. The heart of the data processing unit consists of a MC 6809 microprocessor interfaced with a 8 k ROM where the external extension software is located, and with a 8 k RAM where the numerical data are stored. The overall dead time is evaluated at about 30 ,us. In term of linear energy transfer the device operates in the 0.2 - 1200 keV.pm -1 range. VALIDITY TESTS BEFORE SPACE F L I G H T Initially, the CIRCE device was set up for microdosimetric measurements in neutron and gamma mixed radiation field on earth {4, s) In order to investigate the behaviour of the dose equivalent meter CIRCE in a complex galactic cosmic radiation field, two experiments were carried out : energy deposition spectrum measurements in a 320 MeV.amu -1 12C beam at the DUBNA Synehrocyclotron (USSR), and dose equivalent measurements aboard an aircraft.
Energy deposition spectrum measurements (7) Energy deposition spectra were measured along the beam axis at 5 m from the scatterer. Lineal energy spectra at different positions relative to the Bragg peak are obtained by accumulating pulse height_ distributions from the counter, and by immediate data processing by the CIRCE device. At 2.2 g.cm the lineal energy mean values on frequenc~ distribution is equal to ~'F = 6.75 keV.pm -1 and on dose distribution is equal to : ~D = 21.2 keV.pm- .
Dose equivalent measurements in the stratosphere (7) Dose equivalents and quality factors were measured aboard the laboratory supersonic aircraft TU-144 on two different routes at an altitude between 17 and 18 kin. The first route was along the longitude 37°5 E between latitude 46* N and latitude 70 ° N, the second route along latitude 56 ° N from longitude 27* E to longitude 47* E. The dose equivalent rates vary from 7.7 to 12.2 pSv.h -1, and the average quality factor is equal to 2.2.
OPERATIONAL MODES In order to investigate the temporal and spatial variations of the dose equivalents and the quality factors on board the MIR space station, continuous one-hour's measurements were performed. This is called the normal measurement mode. Due to the topological structure of the South Atlantic Anomaly (SAA) with its very steep radial gradient of radiation, a second operational mode, called fast measurement mode, can be started on request at a predetermined instant. This mode consists of 30 seconds' measurements during half an hour, and then the system automatically switches back to the normal mode. The fast measurement mode was used to obtain a precise description of the SAA on 9 orbits indicated by an arrow in Figure 3. The starting times were calculated to start the fast measurement mode about 10 minutes before the MIR station entering the SAA.
8th IAA Man in Space Symposium
219
R E S U L T S AND DISCUSSION The results concern the m e a s u r e m e n t s made on December 1988, during the French and Soviet joint experiments, and those made on March and April 1989. Normal m e a s u r e m e n t results
The first set of measures was obtained in a quiet solar activity situation. Figure 4 - a shows the distributions of the dose equivalent rates H and absorbed dose rates D related to 319 o n e - h o u r ' s measures. Their mean values are respectively equal to 25.7 pSv.h -1 and 13.4 laGy.h -1. Figure 4 - b shows the distributions of the quality factors Q and of quality factors QHL due to L E T greater than 3.5 keV.pm -1. Their mean values are respectively equal to 1.9 and 7.7. The narrowness of the D and Q distributions related to the broadness of the H and QHL distributions indicate that a large part of the absorbed dose is due to low L E T events. These events are induced not only by g a m m a or electrons, but mainly by protons of energies greater than 14 MeV or by heavy ions of energies greater than 80 MeV.amu -1. Conversely in respect to the dose equivalent rate H, the contribution of the dose equivalent with LET greater than 3.5 keV.pm -1 represents 58 % of the total dose equivalent. This fact indicates the presence of very large L E T events. The second set of measures was performed during the period after the large solar flare at the beginning of March 1989. Figures 4-c and 4 - d show the same quantities H, D, Q and the QHL related to 732 measures. The distributions of H and D are almost identical to the first set of measures (Fig. 4-e). Their mean values are respectively equal to 33.3 pSv.h -1 and 18.8 }aGy.h -1. The distributions of Q and Q H L are sligthly different in the lower part of each distribution (Fig. 4-f). Their m e a n values are respectively equal to 1.8 and 7.4 (Table 1). During the period posterior to the solar flare which occurred on March 1989, these results, i.e. the increasing H and D and the decreasing Q, clearly show the increase of the absolute particle flux of low L E T in respect to the period before the solar flare. Table 2 shows the comparison of the values measured during different missions (8, 9, 10). The large discrepancies observed are not only due to orbital parameters but also to the nature and thickness of the vessel shielding. Systematically, 2 or 3 times a day the dose equivalent rates exceed 40 ~Sv.h -1. These values are related to the crossing through the SAA by the orbital station MIR about 2 or 3 times a day. A more precise description of the SAA area is given by the fast m e a s u r e m e n t mode. Fast m e a s u r e m e n t results
Figures 5 and 6 show 9 sets of 60 measures in December 1988 when the MIR station was crossing through the SAA related to 4 North South routes and 5 South North routes. Beginning and ending values of each route are in the same order as the above mean values. A comparison of figures 5 and 6 shows that systematically dose equivalent rates on South North routes are higher than those on North South routes by about 20 %. An orbit analysis indicates that South North routes were on the sun side, and North South routes were in the earth's shadow side. This established fact partly explains the above systematic difference. A very steep gradient is observed in the central area of the SAA (Fig. 7). Dose equivalent rates can reach values above 1 mSv.h -1. Conversely, the quality factor Q is equal to 1.4 and it is significantly lower than the mean value 2.1 outside of the SAA. Table 3 shows the comparison of absorbed doses measured by other authors (s, 11) and by the present work. It clearly indicates the large dose values in the SAA. Even if crossing durations vary from 8 to 10 minutes only, more than 30 % of total dose per day are due to the SAA radiation. ESTIMATION OF THE INTEGRATED EXPERIMENTS IN DECEMBER 1988
DOSE
DURING
THE
FRENCH
SOVIET
JOINT
The duration of the mission was 26 days. On this basis, the estimated dose equivalent inside the MIR station was equal to 16.0 mSv and the absorbed dose was equal to 8.4 mGy. T L D detectors exposed in the same conditions gave values in the 6 - 8 m G y range. The slight underestimation was due to the weak response of the detectors to heavy ions. The neutron c o m p o n e n t was measured by bubble detector (12). The neutron dose equivalent was equal to 0.6 mSv, representing about 4 % of the total dose. Table 4 s u m m a r i e s useful average dosimetric data related to one orbit aboard the MIR station, and Table 5 gives measured values during the m a n n e d spaceflight A R A G A T Z .
220
V . D . NGUYEN et aL
CONCLUSION The temporal and spatial variations of space radiation pointed out by the present work in terms of quality factor and dose equivalent show the need of an active radiation measurements in manned space flights. With respect to the particular topological structure of the SAA, to its location and its extension which are randomly modulated by the solar activity, to its large contribution to the total dose inside the space station, an active detector as CIRCE device is an adequate instrument for long term dosimetric investigation. Such an instrument is useful in a wide variety of manned activities in near earth orbits with high inclinations, for instance 57* and more, where serious radiation problems must be taken into account in case of solar flares. The complex shielding of manned spacecraft causes large uncertainties in predictions of the flux, charge and energy spectra of charged particles, and consequently in the dose equi-valent rate evaluation. At the same time, the irradiation of tissue with cosmic ray nuclei, trapped energetic protons, recoils and spallation products, implies a knowledge of LET spectra to assess radiobiological consequences. The present work shows that, as an alterna-tive, the CIRCE device can act as a practical tool for systematic real-time dosimetry in shuttle flights and particularly in space stations. It can also give the answer to spatial and temporal variations of dosimetric quantities due to heavy charged particles and secondary neutron component, for which very little data exist at the present time.
REFERENCES 1. 2. 3. 4. 5. 6. 7.
8. 9. 10. 11. 12.
Benton, E.V., S u m m a r y o f radiation dosimetry results on U.S. and Soviet manned spacecraft COSPAR (1986). Kovalev, E.E., Benton, E.V., Marenny, A.M. Measurement o / L E T spectra aboard cosmos 936 biological satellite, Rad. Prot. Dos. 1, 3, 169 (1981). Parnell, T.A., Watts J.W., Fishman G.J., Benton, E.V., Frank, A.L., Gregory, J.C. The measured radiation environment within spacelabs 1 and 2 comparison with predictions COSPAR (1986). Nguyen, V.D., Luccioni C., Parmentier N. , Average quality factor and dose equivalent meter based on microdosimetry techniques, Rad. Prot. Dos., 10, 1,277 (1985). Nguyen, A dose equivalent meter based on the tissue-equivalent proportional counter, and problems encountered in its use, Rad. Prot. Dos. 9, 3,223 (1984). Bouisset, P., Nguyen V.D., Comet B., Bourrieau, J., Le Grand, J., Roux, Y., Parmentier N., Calcul de la dose induite aux organes critiques par les particules charg~es en orbites basses circumterrestres (to be published). Nguyen, V,D., Bouisset, P., Akatov, Y.A., Petrov, V.M., Siegrist, M., Zwilling, J.F., . Preliminary studies on the use o f C I R C E for manned space mission dosimetry, Workshop on implementation of dose equivalent meters based on microdosimetrie techniques in radiation protection, Schloss Elmau (RFA) (1988). Janni, J., S K Y L A B 2 radiation dosimetry systems and flight results, Report AFWL-TR-73-222, 1976. Akatov, Yu. A., et al., Thermoluminescent dose measurements on board Salyout type orbital stations, Adv. Space Res., 4(10) : 77 (1984). Kovalev, E.E., Benton, E.V., Marenny, A.M., Measurement o f L E T spectra aboard C O S M O S 936 biological satellite, Rad. Prot. Dos., 1, (3) : 169 (1981). Janni, J., Spacecraft cabin radiation distributions for the 4th and 6th Gemini flights, Aerosp. Med. 40 (12) : 1527 (1969). Ing, H., The status o f the bubble damage polymer detector, 13th International Conference on Solid State Nuclear Track detectors, Rome (1985).
Period .
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33.3
Absorbed dose D (pGy.h -~)
13.4
18.8
7.7
7.4
1.9
1.8
High LET quality factor Total Quality factor ......
.
¥-~'b3~'l"'--ff~Zn'valZZ~m~gre-d-I;~'~fli~if~Z
on board Soviet orbital station "MIR"
........
8th IAA Man in Space Symposium
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Orbit parameters Dose equivalent Absorbed dose (mSv/day) (mGy/day)
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0.071 .
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0.322
0.799
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(9)
0.256 .
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(1)
0.146
0.617 .
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(1)
0.208
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.
(8)
0.544
350 km 51'.5
MIR (March/ April 1989) .
.
419 x 224 km 62°.8
MIR (Dec. 1988) .
.
350 km 51°.5
Cosmos 936 (1983) .
.
380 km 28°.5
Salyout 6 (1980) .
.
297 x 454 km 28°.5
Atlantis (1985) .
.
435 km 50 °
Discovery (1985) .
221
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0.023 .
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(10) .
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present work
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0.451
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present work
51°'5 .
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Table 2 - Comparison of measured values averaged over the orbits of different spacecrafts
Mission .
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Orbit parameters Dose equiva|ent Absorbed ~ose Quality factor Ref (mSv.h -~) (mGy.h -~) .
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Skylab 2 (1976) .
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Gemini 4 (1966) .
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Table
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3
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Table
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4
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1.050 .
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(8) .
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(11) .
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0.755
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Dose equivalent (mSv/orbit) .
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1.4 .
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Absorbed dose (mGy/orbit) .
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0.039
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Present work .
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2.1
0.052 .
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1.9
0.015
0.081 .
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Quality factor
0.020
0.031
Over orbits crossing SAA .
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0.126
Over orbits out of SAA .
.
0.828
Over all orbits .
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Comparison of maximum dose rates measured on the South Atlantic Anomaly aboard different spacecrafts
-
.
.
350 km 51°.5
Route configuration .
.
296 x 166 km 32°.5
MIR (Dec. 1988) .
.
435 km 50*
.
.
1.6 .
.
Estimated average values related to one orbit aboard Soviet orbital station "MIR" (350 km, 51°.5, 90 minutes)
222
V . D . NGUYEN et al.
Duration : 26 days . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Detector
Dose equivalent (roSy)
Absorbed dose (mGy)
Quality factor
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CIRCE
16.0
TLD
8.4
1.9
6 to 8
Bubble detector
0.6 (neutron only)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Table 5 - Dosimetric data related to "ARAGATZ" mission aboard Soviet orbital station "MIR" (Dec. 1988)
316 L stainless steel ]
A-150 plastic
]
polyethylene g 94
.403
1
V, f i l l
1
-} i
I
L
20 pm golden tungsten
Details of polyethylene cap
central wire Figure 1- Schematic geometry of the tissue equivalent proportional counter
i
8th IAA Man in Space Symposium
223
PULSE h
PROCESSOR
PULSE SEPARATOR PULSE PROCESSOR T,E. PROPORTIOtLAL COUNTER
F i g u r e 2 - M a i n features o f the dose equivalent m e t e r C I R C E
, . . . .
16052
16093
16082*
F i g u r e 3 - P r e d e t e r m i n e d orbits for f a s t measurements through the S A A . The underlined numbers indicate the orbit number, the arrows indicate the directions of the space station. * on this o r b i t no m e a s u r e m e n t could be carried out .
224
V.D. NGUYEN et air
DECEMBER 1988 RESULTS
W
0
20
4
40
60
80
100
++
120
0
a)
2
6
4
8
I0
12
40 40.
+. T
40
:,o. I0
I0 I(~ 0 Z
0
20
40
d:O
80
lO0
0
0.
0
120
2
4
~
:g
10
12
M A R C H - APRIL 1989 RESULTS
0,
20
?
dO
,
120
,
I
BO ,
I00 ,
I
~
120
0
120 8el
,
c)
4
2 "
'
'
'
~ '
r
8 .
t
.
' 80
d)
.. 2
~
~ '
20
Z
.
100
40
| o:
12
I0
~
~
20 "
0 0
20
40
~0
80
|00
0
120
0
2
4
g
~
|0
J2
SUPERPOSITION OF THE TWO DISTRIBUTIONS 0 2e
=
20 "
40 '
-
xO '
•
'
80 '
100 '
'
'
120 20
'
15. I0.
-
D
15
- I0
O20
40
~0
80
100
- 0 120
0 20!
2 •
4
-
Distributions
,
I
,
1
12 •
~ 20
',t
15 10
0
0-~ 0
8
4
H : Dose e q u i v a l e n t pSv.h - I D : Absorbed dose pGy.h -1
Figure
I
•
10
8
4 '
10
12
Q : Total quality f a c t o r QHL : High LET quality f a c t o r
of
doses
and
Soviet
orbital
station
quality
MIR
factors
measured
on
board
8th IAA Man in Space Symposium
225
1.2"
0.9m
°
H
0,~-
..,lllillll.li.,,,i,,, .....
..
"'~'1[1111
• ---"...,]i!II ..... 2~ •
290 •
3~)6=
320 •
335 •
:.."
3~-~)e
LONGITI.IDE
F i g u r e 5 - D o s e e q u i v a l e n t r a t e s a b o a r d t h e S o v i e t space s t a t i o n M I R on S A A N o r t h South r o u t e s . A b o v e ~0 }JSv.h -1 d i a g r a m s a r e d r a w n i n b l a c k .
0.9 m
O.
- IO
/
~ f l
0~,~-.-.--/-,o. ,,
/ ~ ! 1i.--. 'tw~,,'-. i
2713•
l
~= "r" - ~ " ~
290 •
I =
I
I
3OS•
/-~'e I
I
320 •
I
I
3-3Se
I
--50 °
3S0=
LONGI TLIOE
F i g u r e 6 - Dose e q u i v a l e n t rates aboard the S o v i e t space s t a t i o n M I R on S A A South N o r t h r o u t e s . A b o v e 40 p S v . h" l d i a g r a m s a r e d r a w n in b l a c k .
226
V . D . NGUYEN el a/.
1.2-
0,9
275 °
290 °
305 °
320 °
335 °
3S0 °
LONGITUDE
F i g u r e 7- D o s e e q u i v a l e n t r a t e s m e a s u r e d a b o a r d t h e S o v i e t s p a c e s t a t i o n MIR t h r o u g h SAA in t h r e e d i m e n s i o n r e p r e s e n t a t i o n .