ISSN 0010�9525, Cosmic Research, 2010, Vol. 48, No. 3, pp. 206–210. © Pleiades Publishing, Ltd., 2010.Original Russian Text © A.E. Lishnevskii, M.I. Panasyuk, V.V. Benghin, V.M. Petrov, A.N. Volkov, O.Yu. Nechayev, 2010, published in Kosmicheskie Issledovaniya, 2010, Vol. 48,No. 3, pp. 212–217.

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INTRODUCTION

The radiation monitoring system (RMS) has been inoperation on the service module of the ISS almost unin�terruptedly since August 2001. The data produced by itare used for every�day estimation of the radiation envi�ronment onboard the station. In the middle of 2008 anincrease in the daily mean dose rate recorded by theRMS detectors was observed. According to data of theGOES�11 satellite (http://spidr.ngdc.noaa.gov/spidr/query.do?group=GOES), throughout the year 2008 thesolar activity remained very low, and no solar protonevents were observed. Therefore, all variations of theonboard dose rate can be caused by only two sources:galactic cosmic rays (GCR) and the Earth’s radiationbelt (ERB). In this paper the contributions of these twosources to the considered increase of the daily dosagerate are analyzed.

INSTRUMENTATION

The system of radiation control whose data areunder analysis was described in several papers [1, 2, 3].The RMS includes dosimeter R�16, four dosimetricunits DB�8, and two blocks of digital data processing. Inthis paper we consider the results obtained using high�sensitivity dosimetric units DB�8. All four units areidentical. Each of them has two fully independentchannels. Each channel consists of a silicon semicon�ductor detector, preamplifier, zoom amplifier, analog�to�digital converter, and a subsequent scheme of digitalsignal processing. One of two detectors composing eachunit DB�8 is shielded by a lead layer 2.5 mm thick. In

what follows another detector having no additional leadprotection is referred to as unshielded.

Points where the DB�8 units are placed were chosen sothat to provide for different shielding conditions of RMSdetectors by the station devices. The unit DB�8 no. 1placed in the ISS Service Module in the small�diametercompartment near the Servise Module central controlis the least protected. The unit DB�8 no. 4 placed in thelarge�diameter section near the working table is themost protected. The data of precisely these two units areconsidered below in this paper.

The data of RMS measurements are stored in theblocks of digital data processing and then are transmit�ted to the Earth through the ISS telemetry system.

METHOD OF DATA PROCESSING

The data transmitted are of a rather complex struc�ture. For the analysis in this paper we have used theabsorbed dose values measured every 10 min. In orderto calculate the mean daily dose rate the absorbed dosevalue recorded at 0 h 00 min of the day under consider�ation was subtracted from the absorbed dose valuerecorded at 0 h 00 min of the next day. The plot of thusobtained values is presented in Fig. 1 where the data forthe year 2008 are presented for the least protected unitDB�8 no. 1 and the most protected unit DB�8 no. 4.

One can see that the daily mean dose rate hasincreased in August 2008. All other detectors of RMSdemonstrated a similar behavior (readings of these detec�tors are within the interval between the plots presented inFig. 1). A comparison has been made with the data ofneutron monitors (stations Apatity and Moscow) taken

Variations of Radiation Environment Onboard the ISS in the Year 2008

A. E. Lishnevskii1, M. I. Panasyuk1, V. V. Benghin2, V. M. Petrov2,A. N. Volkov3, and O. Yu. Nechayev1

1 Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, Russia2 Institute of Biomedical Problems, Russian Academy of Sciences, Moscow, Russia

3 “Energiya” Rocket and Space Corporation, Korolev, Moscow oblast, Russiae�mail: ael@srd.sinp.msu.ru

Received June 4, 2009

Abstract—Using the data of high�sensitivity dosimetric units DB�8, variations of the radiation environmentonboard the International Space Station (ISS) during the year 2008 are analyzed. Very low level of solar activ�ity was observed throughout this time, and no proton events occurred. It is shown that the variations of themean daily dose rate during this period were caused by variations in the height of the ISS flight.

DOI: 10.1134/S0010952510030020

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from their websites http://cr0.izmiran.rssi.ru/mosc/main.htm (Moscow) and http://pgia.ru/cosmicray/(Apatity), and with the data of GOES�11 satellite [http://spidr.ngdc.noaa.gov/spidr/query.do?group=GOES]. Cor�responding plots are presented in Fig. 2 and Fig. 3.

One can see that the above�mentioned increase ofthe daily mean dose rate on the ISS is not associatedwith either increasing GCR fluxes of with SCR streams.This, naturally, brings up the question about a cause ofthis enhancement. The task of the subsequent data pro�cessing was to separate contributions of the ERB andGCR to the daily mean dose rate. As is known [3], alongthe ISS flight trajectory the ERB dose is accumulatedonly during passages through the South Atlantic Anom�aly (SAA) region. Therefore, the strategy of processingincluded determination, based on the ballistic data, ofall time intervals during a day, when the ISS passesthrough the SAA zone, and then summation of allabsorbed doses accumulated over these time intervals.

When realizing such an approach, one needs todetermine the instants of entering the SAA region andexiting it. For this purpose, its boundaries should bespecified. To represent the SAA region in the form of arectangle in latitude and longitude would be too roughan approximation for this problem. We have used forcalculations of the SAA region boundaries the empiricalapproximation of dose rate measured by the RMS. Theform of this approximation taken from paper [4] isquasi�Gaussian. As an example, Fig. 4 presents the doserate distribution in the SAA region measured by theleast protected unshielded SPC detector in September

2004 with a corresponding approximation in the formof quasi�Gaussian distribution.

One can see that this approximation is fairly reason�able for the formulated problem of determining thetime intervals of passages through the SAA region.

The following algorithm of calculations was used inthis paper.

—For a day under consideration all time intervals ofpassages through the SAA region were calculated.

—For each such interval the dose was determined,measured by the RMS at the end of 10�min interval pre�ceding the instant of entering the SAA. Then, the dosewas determined at the end of 10�min interval duringwhich an exit from the SAA region had occurred. Thedose increment between these two instants was consid�ered as a dose obtained in the ERB. The GCR contri�bution to the dose was assumed to be insignificant inthis period.

—The ERB contribution to the daily dose was cal�culated as a sum of contributions of all passages throughthe SAA region during a day under consideration.

—The GCR contribution to the daily dose was cal�culated as a difference between the total daily dose andthe ERB contribution to it.

Sometimes, one of the time intervals of passingthrough the SAA region turned out to be on the dayboundary. The data of such a day were excluded fromthe analysis. Also excluded were the days in which gapsin data measurements were observed, falling on theintervals of passages through the SAA region.

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Fig. 1. The mean daily dose rate in 2008 according to the data of unshielded blocks DB�8 (curves 1 and 2 are for the least protectedblock DB�8 no. 1 and the most protected DB�8 no. 4, respectively).

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The algorithm described above was realized as a pro�gram written in the language National InstrumentsLabView 8.2 (http://www.ni.com/labview/).

RESULTS

Figure 5 presents the time behavior of contributionto the mean daily dose rate of the ERB (middle panel)and og GCR (bottom panel) for the most protected andthe least protected detectors of the RMS. Also pre�sented (in the top panel) is the time behavior of the alti�tude at which the ISS passes through the SAA region.

Comparing the plots in the middle and bottom pan�els of Fig. 5 one can see that the GCR contribution tothe dose rate is almost invariable throughout the entiretime interval under analysis, and the total variation ofthe mean daily dose rate is caused by variations of theERB contribution.

When the plots of ERB contribution to the meandaily dose rate (middle panel of Fig. 5) are compared tothe plot of the altitude of passing through the SAA (thetop panel of Fig. 5), the peculiarity marked in Fig. 5 by arectangle colored in gray engage our attention. Withinthis peculiarity the time intervals of increase anddecrease of analyzed parameters are seen to coincide.

We also emphasize the fact that the time intervals ofincrease and decrease of ERB contribution to themean daily dose rate coincide within the gray rectan�gle zone for the least protected and most protectedblocks of the RMS.

Vertical boundaries of the rectangle colored regionin Fig. 5 correspond to the time interval from the begin�ning of June 2008 to the middle of September 2008.

The performed analysis of experimental data allowsus to make a conclusion that the August increase of themean daily dose rate on the ISS was caused by increasedaltitude of the station orbit.

One should note that in [5] an analysis variations ofradiation environment on the Mir station was per�formed, based on the use of data of the R�16 instru�ment. However, the data of the R�16 instrument did notallow one to isolate unambiguously the contribution todaily dose rate caused by passages through the SAA,which made it impossible separate variations caused bythe ERB and GCR. Notice that the use of high�sensi�tivity detectors in the DB�8 blocks allowed us to makesuch an analysis and allowed the effect caused by chang�ing altitude of the station flight to be observed clearly.

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Fig. 2. Comparison of the mean daily dose rate in 2008 (the data of unshielded DB�8 blocks) with the data of neutron monitorsat Apatity and Moscow stations (curves 1 and 2 are for the least protected DB�8 no. 1 and the most protected DB�8 no. 4, respec�tively; curves 3 and curve 4 are the data of neutron monitors at stations Apatity and Moscow, respectively).

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Fig. 4. Distribution of the dose rate caused by galactic cosmic rays and by the South Atlantic Anomaly (September 2004). The leftpanel presents the measurements results obtained with the help of the least protected block DB�8 no. 1; the right panel shows anapproximation in the form of quasi�Gaussian distribution.

CONCLUSIONS

The cause of increased mean daily dose rate on theISS in 2008 has been analyzed. It is shown that theGCR contribution to the mean daily dose rate does not

vary. This is consistent with the data of monitoringGCR fluxes, which show the absence of significantchanges in the flux of GCR in the period under consid�eration. The increase of the mean daily dose rate isentirely determined by the increased contribution of

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ERB and is associated with an increased altitude of theISS flight since the middle of 2008.

ACKNOWLEDGMENTS

The authors thank S.G. Drobyshev who took thedata of calculation of the ISS flight in 2008 at their dis�posal.

REFERENCES

1. Panasyuk, M., et al., Description of the Space RadiationContol System for the Russian Segment of ISS ALPHA,12th IAA Man in Space Symposium, Washington, DC,June 1997.

2. Lyagushin, V.I., Volkov, A.N., Aleksandrin, A.P., et al.,Preliminary Results of Measuring Absorbed Doses withthe System of Dose Control of the Russian Segment ofthe International Space Station, Vopr. At. Nauki Tekh.,Ser. Fizika Radiats. Vozdeistviya na Radioelektr. Appa�raturu, 2002, no. 4, pp. 22–25.

3. Model’ kosmosa (A Model of Cosmos), Collection ofPapers, Moscow, 2007, vol. 1, ch. 3.10, pp. 642–667.

4. Panasyuk, M.I., Kuznetsov, S.N., Kutuzov, Yu.V., et al.,Radiation Dose Rates on Russian Segment of the Inter�national Space Station in October 2003: A Comparisonof Estimates Obtained Using Coronas�F Satellite Datawith the Results of Dose Control, Astron. Vestn., 2007,vol. 41, no. 5, pp. 458–465.

5. Badhwar, G.D., Shurshakov, V.A., and Tsetlin, V.V.,Solar Modulation of Dose Rate onboard the Mir Sta�tion, IEEE Trans. Nucl. Sci., 1997, vol. 44, no. 6.

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Fig. 5. The upper panel presents the time behavior of the altitude of passing through the SAA region. The plot in the middle panelis the time behavior of ERB contribution to the mean daily dose rate (curves 1 and 2 are for the least protected block DB�8 no. 1and the most protected DB�8 no. 4, respectively). The bottom panel presents the time behavior of GCR contribution to the meandaily dose rate (curves 1 and 2 are for the least protected block DB�8 no. 1 and the most protected DB�8 no. 4, respectively).