Analysis of the absorbed dose measurements outside the MIR space station in June–July 1991

~

Pergamon

Radiation Measurements, Vol. 26, No. 3, pp. 521-526, 1996

PII:S1350-.4487(96)00051-0

Copyright ,© 1996ElsevierScienceLtd Printedin Great Britain. All rights reserved 1350-4487/96 $15.00+ 0.00

ANALYSIS OF THE ABSORBED DOSE MEASUREMENTS OUTSIDE THE MIR SPACE STATION IN J U N E - J U L Y 1991 YU. A. AKATOV*, V. A. SHURSHAKOV*, P. SCHMIDTt, H. SCHTROIBELt, T. HANt and H. HARTMANNt *State Scientific Center--Institute of Biomedical Problems, Khoroshevskoye shosse 76-a, Moscow 123007, Russia; and tInstitute of Radiation Protection Physics, Dresden, Germany Abstract--The paper contains data about measurements of the absorbed dose attenuation curve with thin thermoluminescent detectors in a flight of the MIR Space Station in June-July 1991, i.e. in the period of existence of a "new" belt of trapped radiation that supposedly emerged on 24 March 1991. For X < 0.03 g/cm2, dose rate was shown to exceed l Gy/day conforming with calculations using model descriptions. Some methodical aspects associated with the use of U.S. and Russian models of trapped radiation are discussed. Comparison of experimental data concerning absorbed doses with calculations based on the model descriptions of the radiation environment of near-Earth space reveal a 2-3-fold excess of the experimental measurements over the theoretical values obtained for AE8 at 0.03 < 0.3 g/cm2 and for the Russian model at 0.05 < 0.15 g/cm2. This difference may be related to the input of a "new" belt of quasi-trapped particles disregarded in current models. Copyright © 1996 Elsevier Science Ltd

2. EXPERIMENTAL TECHNIQUE AND RESULTS

1. INTRODUCTION Distribution of the absorbed dose in a thin layer of the matter on the external surface of a spacecraft (SC) is one of the basic parameters of the radiation environment of outer space. Experimental data allow testing of models of trapped radiation which are used to predict the radiation environment in space and to assess the radiation burdens on the top layers of materials and structures. In Akatov et al. (1989, 1990) measurements of absorbed doses on the outer surface of the low-orbiting recoverable Cosmos satellites are cited. These data, obtained around the solar minimum, are in good agreement with the results of calculations using model descriptions (Akatov et al., 1990). In 1991 these measurements were conducted for the first time on the outer surface of the MIR Space Station within an international experiment. The present paper presents preliminary results of measurements in June-July 1991, i.e. in the period of existence of a "new" trapped radiation belt, that supposedly emerged on 24 March 1991 (Mullen et al., 1991; Blake et al., 1992; Petrov et al., 1993, 1994). Stacks of thin thermoluminescent detectors exposed on the outer surface of the station were used in the experiment. Also, the authors discuss some methodical aspects associated with the U.S. and Russian models of trapped radiation in an effort to compare experimental data and calculated absorbed dose values based on model descriptions of radiation environment in near-Earth space.

The detectors were thin disks of thermoluminescent glass with about 0.025 g/cm 2 thickness (made in Russia), and German thin films with impregnated crystals CaF2:Mn (CaF2:Min-PTFE-TLD, 30 m% CaF2), about 0.010 g/cm-' thick. Stacks of detectors were allocated in the channels of special aluminum packages (Akatov et al., 1989, 1990). The tops of these stacks were shielded by thin, 0.00114 g/cm 2 thick, aluminum foil. The detectors were exposed on board the MIR Space Station in the following stages: 4-24 June 1991 (20 days)--inside the transport spaceship docked to MIR Station; 24 June-28 July 1991 (35 days)--on the outer surface of the space station after scheduled extra-vehicular activity; 28 July-10 October (74 days)--inside the Station. Then they were returned to Earth for treatment. Inside the modules of the transport vehicle and the MIR Space Station the effective thickness of shielding was 5-15 g/cm 2 of aluminum. The orbit parameters of the MIR Space Station during external exposure of the dosimetric package varied within the following ranges: apogee altitude 413-408 km, perigee altitude 391-387 km, perigee argument 42-62 °, inclination of the orbit plane 51.62 °, i.e. the orbit parameters were smoothly changing and no orbit correction was undertaken in that period. Experimental results are shown in Tables 1 and 2 The good agreement between the data from the Russian and German detectors is noteworthy. 521

YU A. AKATOV et al.

522

Table 1. Dose distribution in the German stack Detector Foil 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 !6 17 18 19

Thickness (#m)

Depth range (mg/cm2)

Dose (Gy)

53 53 64 59 55 58 53 56 58 55 41 48 50 50 52 110 112 111 109

1.4 1.4-14.2 14.2-27.0 27.0-42.5 42.5-56.7 56.7-70.0 70.0-84.0 84.0-96.8 96.8-110.3 110.3-124.3 124.3-137.6 137.6-147.0 147.0-158.6 158.6-170.7 170.7-182.8 182.8-195.4 195.4-222.0 222.0-249.0 249.0-275.0 175.0-302.0

-92.3 47.6 33.7 26.2 20.9 16.3 13.3 10.3 8.5 6.53 5.99 5.32 4.33 3.48 3.02 2.23 1.65 1.25 1.04

The absorbed dose attenuation curve from Table 1 at 0.005 < 0.3 g/cm 2 can be approximated within an accuracy of no less than 10% by the function D(X) = D0"exp( - X/Xo) where Do --- 1.6 Gy/day, X0 = 0.065 g/cmL 3. CALCULATION OF PROTON AND ELECTRON FLUENCES IN THE MIR SPACE STATION ORBIT To calculate the absorbed dose rate outside the space station as a function of the shielding thickness D(X), or the dose attenuation curves, it is necessary to determine the integral fluence of particles (electrons and protons) F( > E) in the orbit, and then to pass from fluences to doses. As is known, the major sources of radiation hazard in space station orbit are the Earth's radiation belts (ERB) and solar and galactic cosmic rays (SCR and GCR). There is a whole battery of standards for radiation safety of space crews adopted in Russia (Kovalev and Sakovich, 1990) and a set of U.S. space radiation models (Sawyer and Vette, 1976; Bilitza, 1987) which sufficiently well reflect scientific knowledge at the time of their development. Most of the absorbed radiation measured by the dosimetric stacks was caused by the ERB energetic electron fluxes and was obtained during the 35 day

period of the exposure on the outer surface of the MIR Space Station. Absorbed doses obtained when inside the transport spaceship or when inside the MIR Space Station were caused mainly by the ERB protons and were at least an order of magnitude lower than the doses obtained outside the space station. It is necessary to note that during the time the detectors were exposed inside the transport spaceship, there were several solar proton events (SPE), including two ground-level enhancements with protons in excess of I GeV (Solar Geophysical Data, 1991). Clearly these particles would have penetrated through the 5-15 g/cm2 of aluminum thickness to contribute to the total dosage when the spaceship was at high latitudes. Simple estimations show that the contribution of such SPEs would have been very small in comparison with the contribution of the ERB particles. Another major SPE with particles > 100 MeV on 7 July 1991 was registered in the period of outside exposure of the dosimetric stacks (Solar Geophysical Data, 1991). The radiation exposure of this SPE was, again, confined to the extremities of the high latitude portion of the orbit. In the overall measurements, it would have contributed little to the total exposure in comparison with the contribution of the low energy ERB particles.

Table 2. Dose distribution in the Russian stack Detector Foil 1 2 3 4 5 6

Thickness (#m)

Depth range (mg/cm-')

Dose (Gy)

100 100 100

1.14 1.14-25.0 25-50 50-75

-103 25.9 17.0 6.23 1.65 0.48

300

75-150

300 300

150-225 225-300

ANALYSIS O F THE ABSORBED DOSE M E A S U R E M E N T S

523

Table 3. Characteristics of the U.S. and Russian models of the radiation environment Parameter

U.S.A. (8, 9)

L E, MeV B, Gs

Protons

Electrons

Protons

1.1-11 0,04-7 B/Bo = 1-100

1.1-7.0 0.1-400 B/Bo = 1-100

1.2--6.6 0.04-4 0.005-0.62

1.2-6.6 0.1-1000 0.05-0.38

The contribution of G C R behind small shielding was considerably less than that of the low energy radiation trapped in the geomagnetic field. Therefore, we shall dwell on the models describing the radiation situation in ERB, and their practical application in order to calculate absorbed dose attenuation curves. Systemization of data on the energy spectra of particles in the Earth's magnetosphere was accomplished in the U.S. model descriptions AE8 and AP8 (Sawyer and Vette, 1976; Bilitza, 1987) and analogous Russian models (GOST, 25645. 138-86, 1986; GOST, 25645. 139-86, 1986). In these models the flux of electrons and protons, J, is determined for minimum and maximum of solar activity in relation to three parameters: particle energy, E, geomagnetic field induction, B, and the parameter of magnetic shell, L. The angular distribution of particles was taken to be isotropic and fluences are assumed to be independent of levels of geomagnetic disturbance. Features of the U.S. and Russian models of the radiation environment in ERB are compared in Table 3 (B0 is the magnetic field on a specified L-shell in the equatorial plane). As follows from the table, the U.S. model gives two somewhat broader descriptions of data by parameter L than the Russian model. It should also be noted that the models manipulate with different geomagnetic epochs for solar minimum and maximum (1986 and 1990 in the U.S. model and 1964 and 1970 in the Russian model). In the light of current knowledge, the latter gains a special significance since, as was shown in Konradi et al. (1987) and McCormack (1988), radiation characteristics derived from the U.S. model descriptions are dependent on the magnetic epoch used. The excess of calculated over experimental values may be as high as 6-10-fold when extrapolated to the beginning of the 21st centaury. Hence, direct extrapolation of modern model descriptions for long ( > 10yr) periods is not permissible. To ensure uniformity, for each model the magnetic epoch of 1980.0 was taken. The fluence of particles with energies > E for period T was calculated by integrating particle flux with respect to the time along the station trajectory: F( > E) = ~

Russia (1 I, 12)

Electrons

J( > E,B(t),L(t))dt

(1)

where J( > E,B(t),L(t)) is the particle flux determined from the corresponding model description. The multiplier 1/2 takes account of the fact that the

"bottom" hemisphere of the dosimetric stacks is screened by the body of the space station. To implement numerical integration according to the formula (1), geographical coordinates of the station were calculated and then coordinates B and L at t were determined by the method of Gusev and Pugacheva (1980). The energetic spectrum of particles J( > E,B(t),L(t)) was determined using the two-dimensional logarithmic interpolation in (B,L) space of the tabulated values of J at four "bordering" points. To provide adequate accuracy, the time interval T in equation (I) was set equal to 2 days, and the step of integration was At = 30 s. Increasing the T parameter or reducing the step of integration, At, changed the integra fluence F ( > E) both for electrons and protons by less than 2%. It should be pointed out that time interval T = I day, used in a number of works (Akatov et al., 1989, 1990) to integrate over orbit, does not provide the required accuracy of calculations because of the dependence of the integra fluence on the initial longitude of the ascending node. In some cases (Akatov et al., 1989, 1990), integration over the orbit overlooked specified values of the perigee argument which determines location of the perigee in the plane of orbit. This also leads to a grave error, particularly for low orbits of the Cosmos satellites, since the difference between their apogee and perigee altitudes may exceed several tens of kilometers. Figure 1 compares integral fluences of electrons in the space station orbit based on GOST (dashed lines) and the AE8 model (solid lines) during solar maximum (upper of the two curves) and minimum (lower curves). At electron energies in the range 0.04--3.5 MeV, the Russian model gives electron fluences 1.5-3-fold greater than AE8. At E > 3.5 MeV fluences calculated using AE8 are predominant. The range of variation of electron fluences with solar cycle is wider in the AE8 model. Figure 2 shows integra fluences of protons in the same orbit: GOST (dashed lines) and AE8 (solid lines) during solar maximum (the lower of the two lines) and minimum (the upper lines). Proton energies 30-300 MeV and both models give practically identical values, mostly owing to utilization of similar experimental data during their development for this interval of energies. Also, the high-energy protons are the most stable component of trapped radiation, which is best represented in the existing models. For E < 30 MeV the Russian model gives proton fluences in the orbit under study 20-30% higher than AE8.

524

YU A. AKATOV et al. l e l l --

tel0 ?

~

le9

le8 __

le7

le6

le5

0

[ 1

I 2

I 3

I 4

Electron energy (MeV) Fig. 1. Integral fluences of electrons in the MIR Space Station orbit according to the Russian State Standard (dashed lines) and AE8 (solid lines) during solar maximum (the upper curves) and solar minimum (the lower curves)

4. COMPARISON OF EXPERIMENTAL AND THEORETICAL DATA To pass from electron fluences to absorbed dose attenuation curves, the U.S. technique (Watts and Burrell, 1971) based on the numerical approximation of calculations by the Monte-Carlo method was used (Berger and Seltzer, 1968). In Martynov et al. (1990) calculation of the absorbed doses form electrons

behind thin shielding was made with the technique including, according to the Monte-Carlo method, single acts of electron scattering (Makhmutov et al., 1989). Comparative analysis showed that both techniques (Watts and Burrell, 1971; Martynov et al., 1990) provide results varying by no more than 5%. Isotropic (from the hemisphere) impingement of electrons on the plane which describes exposure of detectors in orbit is being discussed. Calculation of

le9

le8

le7

o ~

le6

le5

le4

0.1

]

i

I

t

10

100

Proton energy (MeV) Fig. 2. Integral fluences of protons in the MIR Space Station orbit according to the Russian State Standard (dash lines) and AE8 (solid lines) during solar maximum (the lower curves) and solar minimum (the upper curves)

ANALYSIS O F THE ABSORBED DOSE M E A S U R E M E N T S

525

10 ---

1 I

0.1

0.01

0.001

I 50

I 100

I 150

] 200

I 250

[ 300

Depth (mg/cm 2) Fig. 3. Experimental data (histogram) and calculated curves of the absorbed dose attenuation produced by electron and proton fluxes behind small shieldings on board MIR Space Station (AE8, solid lines; Russian model, dashed lines; doses from electrons, two upper curves; doses from protons, two lower curves).

the absorbed dose attenuation curves was accomplished from the data of the Russian (dashed line) and U.S. (solid line) models for the phase of maximal solar activity. Figure 3 displays experimental (histogram) and theoretical values of absorbed doses from electrons and protons. Given the calculations, these are the electron fluence that makes a predominant contribution to the absorbed dose at X < 0.3 g/cm 2. For shielding with 0.005 < 0.03 g/cm 2 thickness both models give largely the same absorbed dose values which, within an accuracy of 20%, are equal to the experimental results. However, there is a 2-3-fold excess of experimental data over calculated absorbed doses at 0.03 < 0.3 g/cm 2 for the AE8 model and at 0.05 <0.15 g/cm 2 for the Russian model which is likely to be an outcome of the "new" belt disregarded by these models. Variation between experimental and calculated results appeared to be less significant when the Russian model was employed. Agreement (Akatov et al., 1989, 1990) of the data on the absorbed dose attenuation curves obtained during experimental exposure on board the Cosmos satellites (solar minimum) and using the AE8 model calculation is probably due to the absence of the "new" belt of trapped particles in that period. Besides, some methodical errors in calculation (Akatov et al., 1989, 1990), e.g. the chosen epoch of the geomagnetic field, perigee argument magnitude, etc. should not be exclude 25 possible explanations as for the good agreement.

In Gusev et al. (1992), according to the Cosmos-1886 data, fluences of trapped electrons at altitudes of 350--500 km were found to be 1.5-2-fold lower than in the AE8 model while fluenccs of quasi-trapped electrons were 3-6-fold higher when compared with the U.S. model. The reported excess of the "experimental" dose over "calculated" during comparison (Gusev et al., 1992) suggests a prevailing contribution of quasi-trapped electrons to the total absorbed dose behind small shielding on the flight in the M I R Space Station orbit.

5. CONCLUSION The paper presents values of the absorbed dose attenuation curves for shielding thickness in the range from 0.005 to 0.3 g/cm 2 measured aboard the MIR Space Station in June-July 1991 during the existence of a "new" belt of trapped radiation that supposedly emerged on 24 March 1991. For X < 0.03 g/cm 2 the measured dose rate was shown to exceed 1 Gy/day and to be in conformity with calculations using model descriptions. Comparison of experimental data and absorbed doses calculated with the help of model descriptions of the radiation environment of the near-Earth space reveals a 2-3-fold excess of the former at 0.03 < 0.3 g/cm 2 for AE8 and at 0.05 < 0.15 g/cm 2 for the Russian model. This may be attributable to the contribution of the "new" belt of quasi-trapped particles left out of the existing models.

Y U A. A K A T O V et al.

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