Radiation Measurements 39 (2005) 225–228

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Radiationmeasurement on the International SpaceStation

A.B.Akopovaa,∗, M.M. Manaseryana, A.A. Melkonyana, S.Sh. Tatikyana,Yu. PotapovbaYerevan Physics Institute, 2Alikhanyan Brs str, YerPhI, Yerevan, Armenia

bResearch Center of Spacecraft Radiation Safety, Moscow, Russia

Received 8 March 2004; received in revised form 20 June 2004; accepted 29 June 2004

Abstract

The results of an investigation of radiation environment on board the ISS with apogee/perigee of 420/380 km and inclination51.6◦ are presented. For measurement of important characteristics of cosmic rays (particles fluxes, LET spectrum, equivalentdoses and heavy ions withZ�2) a nuclear photographic emulsion as a controllable threshold detector was used. The use ofthis detector permits a registration of the LET spectrum of charged particles within wide range of dE/dx and during last yearsit has already been successfully used on board the MIR station, Space Shuttles and “Kosmos” spacecrafts. An integral LETspectrum was measured in the range 0.5–2.2× 103 keV/�m and the value of equivalent dose 360�Sv/day was estimated.The flux of biologically dangerous heavy particles withZ�2 was measured(3.85× 103 particles/cm2).© 2004 Elsevier Ltd. All rights reserved.

Keywords:International Space Station; Radiation dose; Photoemulsion; Cosmic rays

1. Introduction

Space radiation poses significant risk to astronautsthroughout the stay and rotation cycle aboard the ISS, wherethe duration in space mission increases and is as great asseveral months and more. The problem of radiation safetyof the space vehicle crew is highly important and, hence,any accurate and reliable appraisal of the radiation hazardfor astronauts exposed to the action of charged particles(CP) is of great practical interest. The detailed knowledgeof this environment under varied spacecraft shielding ge-ometry is necessary to minimize the risk due to radiationexposure. That is why the aim of this work is to estimatethe total dose rate induced by charged and neutral CRcomponents on board the ISS.The radiation exposure was measured by other authors us-

ing passive thermoluminescent detectors (TLDs) and plastic

∗ Corresponding author. Fax: +3742-350030.E-mail address:akopova@lx2.yerphi.am(A.B. Akopova).

1350-4487/$ - see front matter © 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.radmeas.2004.06.013

nuclear track detectors (PNTDs). The PNTDs do not re-spond to radiation below about 5 keV/�m, the contributionof which to the absorbed dose amounts to nearly 60–70%.The full efficiency of these detectors is not attained till al-most 10–15 keV/�m (Benton and Benton, 2001). The ac-tive ionization chambers were flown on the Skylab mission(Parnell et al., 1986) and the Mir orbital station, but theyprovide data on the dose rate only and do not give the LETspectrum, which is a key to obtaining an effective doseand rough estimate of the dose equivalent (Golightly et al.,1994). Badhwar et al. (1994)have flown a 512-channel tis-sue equivalent counter and more recently—the tissue equiv-alent proportional counter on Space Shuttle flight STS 57and STS 65 (Badhwar et al., 1995). These measurements in-dicate that the estimates of radiation exposure using passivedetectors are low compared to those measured by means ofactive detectors. However, the active detectors are not uti-lizable in a wide variety of experiments.As regards the nuclear photoemulsions, their exposure in

space using traditional methods of development is compli-cated to a considerable extent because of extremely high

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Fig. 1. Nuclear emulsion microphotographs after developing under standard conditions (a) and by the selective method (b).

optical blackening produced by particles of weekly ioniz-ing radiation (seeFig. 1). In practice such exposed layerscannot be used in microscopic studies. This drawback ofNPE may be overcome by using a special treatment method(Akopova et al., 1987) that affords an opportunity to controlthe NPE layer threshold sensitivity in a broad range of LET(∼ 0.2–103 keV/�m) in the biological tissue. The emulsionis thereby transformed into a controllable tracking detectorthat has no world analogs up to now. This method has beensuccessfully tested in a number of spacecrafts in joint ex-periments with scientists from the Russian Research Centerof Spacecraft Radiation Safety as well as US scientists fromthe University of San Francisco and NASA Marshall SpaceFlight Center (Akopova et al., 1990; Badhwar et al., 1995).Nuclear photoemulsions have a unique combination of

features—small size and mass, zero power and robust de-sign. For this reason we hope that they are likely to continueto play an important role in the space radiation dosimetry.

2. Experimental techniques

The threshold sensitivity control is based on the intro-duction of Br ions into an exposed emulsion layer by diffu-sion. The Br ions differently inhibit the induction period ofdevelopment depending on the degree of dispersion of la-tent image centers while providing conditions for good dis-crimination and selective development tracks of high LETparticles against the background of low LET ones. There-fore, the technique for finding the planar flux of particleswith LET>LETth reduced to counting the number of trackstraversing the NFE surfaces at different threshold LET value.Thus, in the biological tissue the LET spectra may be ob-

tained in a wide range from 0.5 to 5×103 keV/�m preciselyfrom six to eight values of thresholds.The calibration dependence between the values of the

detection thresholds and quantity of Br− was obtained using24MeV (ITEPh) beam protons and helium, carbon and neonof 9MeV/nucleon (JINR, Dubna).The dose of charged component was calculated based on

the method described inAkopova et al. (1990).The technique to measure the neutron energy spec-

trum within 1–15MeV using 400�m thick Bya type low

sensitivity photographic nuclear emulsions has been devel-oped and used byDudkin et al. (1990). This technique isbased on elastic interactions between neutrons and hydro-gen nuclei in the emulsion.Only the proton tracks with the ends located within the

volume of the emulsion have been measured. Due to a sig-nificant visual inefficiency of the short path length of re-coil protons(En�1MeV) and proton contamination from14N(n,p) interactions with the emulsion nitrogen, the neu-tron fluxes withEn�1MeV were not measured.This method provides higher accuracy measurements in

comparison with widely used TLD and CR-39 detectors andhas been applied aboard the Space Shuttle Missions, Mir andSalut-7 cosmic stations, as well as aboard many “Kosmos”-type satellites (Badhwar et al., 1994; Dudkin et al., 1990;Akopova et al., 1990).Note also that the energy spectra of fast neutrons have

been measured aboard the Mir orbit station using a num-ber of other techniques including the photographic nuclearemulsions, activation foils and two active scintillator-baseddetectors. The agreement between the data obtained bymeans of emulsions and the ISS model calculations ofthe fast neutron energy spectrum predicted for ISS un-der 20 g/cm2 shielding is exceptionally good (Armstrongand Colborn, 1998). That is why we may hope that theemulsions will continue to play an important role in spacefor fast neutron dosimetry.Having in view an extreme radiation hazard to biologi-

cal objects and microelectronic circuits on board the space-crafts, the flux of particles withZ�2 was calculated bymeans of the method of�-electrons counting. To this endthe emulsion layers of the same casting were calibrated by4He ions withE = 9MeV/nucleon on the Dubna nuclearreactions laboratory accelerator.

3. Results and discussion

The experiment was carried out on board the ISS with or-bit inclination of 51.6◦, apogee – 420 km, perigee – 380 km.The stacks of 200 and 400�m thick layers of BR and Byatype NPE enveloped with lighttight paper and aluminizedmylar films were placed behind the panel 429 inside the ISS

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Fig. 2. Initial part of charged particles integral LET spectra obtainedon board of Mir and ISS.

Table 1Equivalent dose rate from CP on Mir and ISS

Station Orbit incl./ Shielding Flight H,�Sv/haltitude (g/cm2) duration

(days)

Mir 88 51◦, 312 km 18 8.8 23.42Mir 91 51◦, 400 km 40 132 6.7ISS 51.6◦, 20 152 15.4

420/380 km

under the shielding of∼ 20g/cm2. The duration of expo-sure was 152 days.The LET spectra of charged particles have been inves-

tigated by means of NPE method at 8 different values ofthreshold mainly with Bya type emulsions so that the distri-bution range were from 1.2 to 2.2× 103 keV/�m of tissue.In Fig. 2 the initial parts of LET spectra measured on

board the ISS and Mir station are shown. As the orbits ofISS and Mir orbital station were known to be close, the ISSexperimental data were compared with the data obtainedduring two Mir missions—from 29.08.88 to 07.09.88, andfrom June 1991 through July, respectively.The experimental values of integral LET distributions are

seen in the figure to be very close to each other in case,when the shieldings on stations Mir and ISS are similar(20 and 18.6g/cm2), and differ only when the thicknessof shielding increases up to 40 g/cm2 that is quite natural.We have determined the equivalent dose of radiation fromcharged particles on board the ISS and Mir. These data, aswell as the flight characteristics, are provided in Table1,where the change of radiation rate with flight altitude andshielding thickness is given.

Table 2Fluxes of charged particles(I0) and heavy nuclei(IZ) on boardof ISS and Mir

Station I0 IZ I0/IZMir-88 2.39E-2 3.45E-4 69.3Mir-91 6.91E-3 3.12E-5 221.5ISS 2.35E-2 2.69E-4 87.4

Fig. 3. Fast neutron spectra obtained on board Mir station.

Along with the LET values, on both the stations the fluxesof charged particles, starting with dE/dx = 1.2 keV/�mand heavy nuclei withZ >2, practically coincide at equalthickness (see Table2).As is evident from the table, at the doubling of shield-

ing thickness the particle flux is halved and the flux of bi-ologically dangerous heavy nuclei reduced by an order ofmagnitude.Badhwar carried out preliminary measurements of ra-

diation environment in the ISS orbit using various activedetectors on board the Space Shuttle STS-91 (Badhwar,2001). The total equivalent dose rate observed by him was24.4�Sv/h. His data turned to be in good agreement withour estimations of the dose keeping in mind that our dataallow for the charged particles only.It should be mentioned that for measurements of the to-

tal equivalent dose of mixed radiation fields on the cosmicstation, many different measuring devices are usually nec-essary for consideration of different components of the ra-diation field. In our case, the NPE was the single type ofdetector used on board the cosmic station for estimation ofcharged and neutral CR components. InFig. 3 the fast neu-tron spectra obtained on board the Mir station is shown.It is noteworthy that NPE gives exceptionally good agree-ment between the neutron energy spectra measured on boardthe Mir using NPE and ISS model calculation from 1 to10MeV (Armstrong and Colborn, 1998). However, it turnedout impossible to measure the fast neutrons fluxes in thelayers exhibited in ISS due to some technical difficultiesthat will be undoubtedly obviated in future ISS experiments,that is why made an effort to apply for the first time the

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threshold development method also for obtaining the fastneutrons spectrum. Unfortunately, there was a significantdecrease of recoil proton tracks, which results in the spec-trum distortion. The ways for obviation of abovementioneddifficulties have been already worked out by us, and wouldbe used in future experiments aboard the station. It will af-ford an opportunity to determine the neutron spectrum inemulsion when usual methods are inapplicable; for exam-ple, in layers with considerable track load because of longduration missions at altitudes 400 km and higher.

4. Conclusion

The following preliminary conclusions may be drawn:

1. Experimental values of integral LET spectra on the Mirstation and ISS with similar shielding thickness are veryclose. The LET spectrum for the Mir-91 mission is sig-nificantly lower due to high shielding(∼ 40g/cm2).

2. Increase in the shielding thickness leads to a reductionof total fluxes of charged particles and more significantlyof the ratioIO/IZ.

3. Equivalent dose rates from charged particles on boardthe Mir and ISS are close under similar shieldingconditions.

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