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The radiation environment observed by Liulin-Photo and R3D-B3spectrum-dosimeters inside and outside Foton-M3 spacecraft

Mario Damasso a,*, Tsvetan Dachev b, Giuseppe Falzetta c, Maria Teresa Giardi a,Giuseppina Rea a, Alba Zanini c

a Institute of Crystallography-National Research Council, Dept. of Agrofood, via Salaria km 29,300, 00016 Monterotondo Scalo (Roma), Italyb Solar-Terrestrial Influences Laboratory, Bulgarian Academy of Sciences, Acad. G. Bonchev Str. Block 3, 1113 Sofia, Bulgariac National Institute for Nuclear Physics, Torino, Italy

a r t i c l e i n f o

Article history:Received 3 November 2008Received in revised form6 March 2009Accepted 9 March 2009

Keywords:Foton spacecraftSpace radiation dosimetryLow earth orbit space environmentGEANT4 simulations

a b s t r a c t

In this study we present the results obtained by the couple of active spectrum-dosimeters Liulin-Photoand R3D-B3 on board the Foton-M3 satellite, which flew in Low Earth Orbit from 14 to 26 September2007. During the Foton-M3 flight the two dosimeters measured in real-time the deposited energyspectrum by the incident ionizing particles, the particle flux and the absorbed dose behind differentshielding.By applying recently developed methods to the dosimeter measurements, an assessment of the ambientequivalent dose H*(10) is also performed, distinguishing the dose contributions of low- and high-LETradiation components and evaluating H*(10) in different sectors along the satellite orbit.The experimental data are compared with the results of simulations performed by using the GEANT4 MCcode and the SPENVIS and OMERE space environment tools for the free-space particle spectra modelling.The Foton-M3 mission took place during a continuous period of very low solar activity in the minimumphase of the 23rd solar cycle. The results presented in this study contribute to the assessment of theproperties of the Low Earth Orbit radiation environment during a well-defined and stable phase of thesolar activity and they will be also useful for all those experiments on board Foton-M3 which weresensible to the effects of the ionizing space radiation.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Two portable, active spectrum-dosimeters Liulin-Photo (L-P)and R3D-B3 successfully collected space radiation data on boardthe Foton-M3 (FM3) satellite. FM3 satellite belongs to the series ofunmanned Low Earth Orbit (LEO) space missions Foton-M, oper-ated by the partnership between the Russian Federal SpaceAgency and the European Space Agency. The satellite waslaunched on 14 September 2007 at 11:00 UT from the Cosmo-drome of Baikonur (Kazhakstan) and the recoverable capsulereturned to Earth on 26 September at 07:58 UT, after 12 days inorbit. The spectrum-dosimeters L-P and R3D-B3 were placedrespectively inside and outside the FM3 recoverable capsule, withR3D-B3 housed in Biopan-6. This shell-like exposure facility,provided by the European Space Agency, was opened once FM3was in orbit and all the experiments allocated inside were directly

exposed to the space environment. Fig. 1 shows the positionsoccupied by L-P and R3D-B3 in FM3, with the former placeddirectly in front of one of the capsule access doors. On board FM3were housed more than 40 experiments related to several scien-tific disciplines. For many of the experiments on board, like thebiological Photo-II experiment sponsored by the European SpaceAgency in conjunction with the Italian Space Agency, which alsoflew in 2005 on board Foton-M2 (Rea et al., 2008), the knowledgeof the space radiation properties is highly important for theinterpretation of the data collected during the mission. Then, theresults coming from R3D-B3 and L-P can be very useful to a largerscientific community than the one interested in space dosimetryand space radiation data.

The two devices have comparable characteristics and theycarry out the same kind of measurements (e.g. Dachev, 2008). Inthis work we perform a comparative study of the particle fluxesand doses measured simultaneously by the two instruments,discussing the principal features emerging from this comparisonthat give information about the effects of the capsule shielding.* Corresponding author. Tel.: þ39 339 1816786.

E-mail address: m.damasso@gmail.com (M. Damasso).

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Radiation Measurements

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1350-4487/$ – see front matter � 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.radmeas.2009.03.007

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As a part of a more complete study about the characterization ofthe space radiation environment on board FM3 (Damasso et al.,2008), by using GEANT4 Monte-Carlo Code a simulation of theparticle transport through the capsule shielding is performed andthe predicted results are compared with the experimental data.Finally, an evaluation of the ambient equivalent dose H*(10) isperformed for different sectors of the spacecraft orbit and, by usinga conversion method recently developed especially for Liulin typedosimeters (Spurn�y et al., 2007), the absorbed dose determined byL-P and R3D-B3 is transformed to the biologically more meaningfulquantity H*(10).

A list of the most frequent acronyms used in this paper is addedas an Appendix at the end of the manuscript.

2. Materials and methods

2.1. Overview of the R3D-B3 and Liulin-Photo devices

R3D-B3 and L-P are mobile dosimetry units (MDUs) and theirmain purpose was to monitor in real-time the doses and particle

fluxes along FM3 orbit (R3D-B3 measured also the solar UV radia-tion). In past space missions Liulin type dosimeters successfullyflew from 1988 to 1994 on board the Mir space station (Dachevet al., 1998, 1999; Shurshakov et al., 1999) and from 2001 on boardthe International Space Station (ISS; Dachev et al., 2006; Nealyet al., 2007; Wilson et al., 2007). In 2005 the R3D-B2 spectrum-dosimeter, the previous version of R3D-B3, monitored the spaceradiation environment on board the Foton-M2 satellite (Dachevet al., 2005; Hader et al., 2009).

Each unit contains one semiconductor detector (PIN Si-diode)with a surface area of 2 cm2 and 0.3 mm of thickness. The spec-trometers–dosimeters measure the amplitude of the preamplifiedelectrical pulse produced by an ionizing particle incident on thedetector. The amplitude is proportional by a factor of240 mV MeV�1 to the energy loss in the detector. The two dosim-eters can detect an energy loss up to 20.83 MeV or 69.44 keV m�1. Bya 12 bit ADC converter the pulse measurements are digitized andorganized in 256 channels, producing an energy spectrum for eachdata sampling period that is recorded into a flash memory. Theabsorbed dose and flux are calculated automatically during the data

Fig. 1. The locations (indicated by white circles) occupied by the R3D-B3 (a) and Liulin-Photo (b) dosimeters in Foton-M3. R3D-B3 was housed in the Biopan-6 shell-like facility,mounted on the external wall of the capsule. The movable lid of the Biopan facility can be seen at left side of the picture a, while the bottom part (the one that is screwed onto theFoton capsule) is visible in the right side. Liulin-Photo was located inside the capsule, behind one of the access doors.

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retrieval from the memory. The dose (in Gray) is evaluated usingthe expression D ¼ Kð

P256i¼1 CiEiÞ=MD, where Ci is the number of

counts in the channel i, Ei is the energy loss (in Joule) in the channeli, MD is the mass of the detector (in kg) and K is a coefficient. In R3D-B3 and L-P the detector is situated inside of a box that providesshielding (from inside to outside the box) by means of 0.2 mmplastic þ 0.1 mm copperþ 2 mm airþ 1 mm aluminium.

2.2. R3D-B3 and Liulin-Photo in Foton-M3

For the FM3 flight R3D-B3 and L-P were set with a sampling rateof 60 s and collected a complete set of data for the whole missionduration.

L-P, mounted inside FM3 capsule, was placed above the Photo-IIautomatic fluorimeter as a complementary part of this experiment,which aimed to study the space radiation effects on the develop-ment and survival of some photosynthetic microorganisms. L-P washoused straight behind one of the access doors of the capsule(Fig. 1b), which in that position provided a shielding of w5 g cm�2.

R3D-B3 was placed in the bottom part of the Biopan-6 exposurefacility (Fig. 1a) which was mounted on the external wall of thecapsule. When the Biopan lid was open R3D-B3 was exposeddirectly to the space radiation behind 0.81 g cm�2 of shieldingallowing protons in the energy range 24–500 MeV and electrons inrange 1.4–20 MeV to encounter the detector. When the Biopan lidwas closed, an additional shielding of 9.1 g cm�2 was provided tothe R3D-B3 device.

2.3. Orbital parameters of Foton-M3 satellite

FM3 is a LEO satellite that orbited the Earth with a period of89.9 min, an inclination of 63� with respect to the Earth’s equator(highly inclined orbit), and with an altitude above the Earth surfacein the range 263 O 302 km. During the mission the satellitecompleted a total of 190 orbits. Considering the orbital parameters,the major contributions to the overall radiation exposure for FM3were expected to be given by Galactic Cosmic Rays (GCR) andrelativistic electrons and protons trapped in the outer and innerVan Allen radiation Belts (VAB). In the inner belt, FM3 encounteredthe trapped particle population in the South Atlantic magneticAnomaly (SAA) region, almost entirely represented by high-energyprotons (up to w400 MeV).

In this study the orbital parameters used were calculated by thesoftware KADR-2 (Galperin et al., 1980).

3. Data analysis and results

3.1. Dose and flux measurements

FM3 mission took place in the minimum phase of the 23rd solarcycle, and the satellite flew during a period characterized by verylow solar activity, without the occurrence of any highly energeticsolar phenomena as flares, Coronal Mass Ejections and SolarEnergetic Particle events. Then, the space radiation environment atEarth orbit can be considered with stable properties during themission, in particular the fluxes of GCR and VAB particles. The maindose measurements of R3D-B3 and L-P are showed in Table 1(accuracy 10%). The lowest dose rates were measured at low lati-tudes close to the Earth equator. R3D-B3 recorded the highest doserates while the satellite was crossing the outer electron belt, whilefor L-P this occurred crossing the SAA. The R3D-B3 data refer toprior the closure of the Biopan lid, occurred on 24 September at14:00 UT. After the lid closure, the total shielding increased fromw0.81 g cm�2 up to a value of w9.9 g cm�2, which was determinedby the contribution of the Biopan lid structure (w5.0 g cm�2) plus

the shielding provided by the experiment package covering theR3D-B3 device (which is estimated to be w4.1 g cm�2).

In Fig. 2 the R3D-B3 and L-P dose rate (mGy h�1) and integral flux(particle cm�2 s�1) measurements, and the calculated dose/flux ratio,or specific dose (nGy cm2 particle�1), are plotted as a function of theMcIlwain’s L-parameter, that corresponds to the equatorial radius ofa magnetic drift shell in the case of a dipole field. The plots show thedifferent particle populations and how they are distributed in thespace around the Earth. By organising the data in this way it ispossible to see immediately the effects of the different shieldingsurrounding the two dosimeters. The specific dose evaluated for eachdata point provides indication about the type and energy of theparticles that mostly contribute to the counts recorded for oneminute in the detector, according to a well-defined dependencebetween the energy of the incoming protons and electrons and thedose-to-flux ratio (Heffner,1971). A space environment region mostlycomposed of 24–300 MeV protons should be characterized bya specific dose in the range 1.12 O 3.2 nGy cm2 particle�1 (the highestratio values corresponding to the lowest proton energies), whilea population of relativistic electrons with energy 1–10 MeV is char-acterized by a specific dose 0.4–0.45 nGy cm2$particle�1 (e.g. Dachev,2008). For bremsstrahlung X-rays the dose-to-flux ratio is<0.3 nGy cm2 particle�1. By using a different data representation,plots in Fig. 3 allow to distinguish the radiation components observedby the instruments in terms of combined dose and flux. In particularit can be seen that the measurements corresponding to the lessenergetic protons from the inner VAB, observed only by R3D-B3behind lower shielding, are characterized by a dose ratew10 O 100 mGy h�1 and a flux of w3 O 20 particle cm�2 s�1. Themain features observed in Figs. 2 and 3 are discussed in detail in thenext section.

3.1.1. The inner Van Allen proton beltAn increase in the dose rate and flux with respect to the GCR

background is observed in both R3D-B3 and L-P data starting atL¼ 1.1. This is produced by protons trapped in the inner VAB and inthe SAA region in particular. The outer borders of this region aredifferent when observed in each of the two devices. For R3D-B3 theinner VAB terminates at L¼ 2.5 while L-P observes a narrower beltending at L¼ 2.

In the inner VAB both the instruments recorded the highestvalues for the dose rates in the sector L¼ 1.4 O 1.5. For R3D-B3 themaximum is 482 mGy h�1, recorded when FM3 was passing over thepoint of geographical coordinates 36.32�S 36.7�W, while L-Pmaximum is 292 mGy h�1 (32.98�S 39.13�W).

The highest values for flux were observed in the sectorL¼ 1.2 O 1.4. The corresponding maxima are 67.2 cm�2 s�1 forR3D-B3 (geog. coord. 30.05�S 52.91�W) and 45.6 cm�2 s�1 for L-P(32.17�S 52.95�W). All the differences mentioned between the twoinstruments are due to the different shielding surrounding thedetectors.

In Fig. 4 the deposited dose spectra for both instruments areshown corresponding to the crossovers of the SAA region. Eachspectrum was obtained as an average over 20 single spectrarecorded simultaneously by the dosimeters. For L-P these spectracorrespond to dose peaks higher than 100 mGy h�1, which are thehighest values for this device. As for all the R3D-B3 spectra theywere recorded before the closure of the Biopan lid. The averagedspectra refer to data recorded in the L-range 1.24 O 1.63 and theycorrespond to a satellite altitude range 279 O 285 km and toa narrow region centered in the point of geographical coordinates32�S 41�W. The dose spectra are calculated as a function of thekinetic energy of the incident particles on the detector (IE).Focusing the analysis to the SAA region, we can reasonably assumethat the dose is deposited mostly by the protons trapped in the

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inner belt. Under this hypothesis it is possible to convert thedeposited energy (DE) to IE using a power law expression obtainedfrom calibration curves and GEANT code predictions (Dachev et al.,2002, 2006). The plot shows that the spectra have a similar shapefor both dosimeters. The spectrum is higher in R3D-B3 than in L-Pdue to lower shielding. In R3D-B3 the maximum of the dose isdeposited by 56 MeV protons, while it moves to higher value of66 MeV for L-P. This shows a dependence of the peak value on thethickness of the shielding surrounding the instrument, asconfirmed by the comparison of spectra obtained from 5 differentLiulin type devices discussed by Dachev (2008).

In the range L¼ 1 O 3 we analyzed how the counts per singleDE channel distribute according to the L-parameter (Fig. 5). Thisanalysis can provide an estimation of the minimum energyrequired for a free-space proton to penetrate the capsule shielding(or to produce secondary particles detected by the device). Fig. 5shows the count distributions only for 10 selected channels amongthe 256 representing the complete set of spectra measured by thedetectors. The first plot contains the counts in the channel corre-sponding to DE¼ 2.64 MeV. The deposited energy decreasesmoving down on the plot, and from the left panel to the right one,so that the last plot on the right shows counts corresponding to thelowest deposited energy (DE¼ 0.16 MeV). Besides, the plots arearranged according to increasing kinetic energy of the incidentprotons, which are indicated in each plot and evaluated accordingto Dachev et al. (2002, 2006). It is observed that the outer border ofthe inner VAB increases with decreasing IE of the protons due tothe natural shielding provided by the geomagnetic field, whichallows the particles with increasing rigidities to penetrate to loweraltitude above the Earth surface. The VAB profiles as seen by bothinstruments are very similar for the last two channels (IE up to185 MeV) despite they are behind different shielding, indicatingthat protons with such energies could penetrate the capsule wall.In the lowest channel (IE¼ 305 MeV) R3D-B3 counts are actuallyhigher than for L-P in all the L-range, and this is due to thecontributions from electrons and g-rays which deposit the smallestamount of energy corresponding to that channel. For decreasing IEchannels R3D-B3 records more events than L-P in the L-range inthe inner VAB and the upper boundary of this region becomesmore and more different for the two devices. R3D-B3 observesalways a wider VAB behind the lowest shielding. The missingcounts in L-P for L>w1.7 could be explained assuming that forIE< 130 MeV the capsule shielding becomes effective againstprotons. This experimental evidence is supported by resultsobtained using Monte-Carlo simulations, as discussed in theSection 3.3.

3.1.2. Relativistic electrons in the outer radiation beltThe slot region (Lw2 O 3) between the inner and the outer belts

appears to be populated by GCR. The contributions of the trappedrelativistic electrons (1 O 10 MeV) to the dose rate and flux areobserved only by R3D-B3 and the corresponding data are clearlyvisible in Fig. 2 in the range L¼ 3.2 O 7 (outer VAB). The shieldingsurrounding L-P was too high for the relativistic electrons topenetrate in the Foton capsule. Evidence for their presence is seenon the middle panel of Fig. 2, where very small increase in the fluxis observed by L-P in the L-range corresponding to the outer beltmaximum. This is the contribution produced by bremsstrahlungradiation generated by relativistic electrons coming from outside.The maximum dose rate recorded at Lw4.4 by R3D-B3 is2315 mGy h�1 corresponding to a maximum in the flux(w1000 cm�2 s�1).

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The specific dose shown in Fig. 3 reaches the value ofw0.6 nGy cm2 particle�1 in the range L¼ 3.2 O 7, giving confirma-tion that the data recorded by R3D-B3 are due mostly to relativisticelectrons. This value is larger than the one predicted by theHeffner’s formula (Heffner, 1971) 0.4–05 nGy cm2 particle�1

because in the calculation of the dose-to-flux ratio the counts fromGCR are not excluded. These counts increase the doses andrespectively the ratio.

3.1.3. Galactic cosmic raysThe contribution from the GCR is represented by the thicker part

in the doses and flux plots of Fig. 2 containing the majority of thedata, showing a knee around L¼ 3. It can be seen that GCRcontribution is in practice equal in both instruments for all the

L-values. The dose values range from 10 to 15 mGy h�1 starting fromL¼ 3 and up to L¼ 25 (out of the scale).

3.2. Evaluation of the ambient dose equivalent H*(10)

Recently, a method based on Liulin type dosimeter measure-ments and applied to data collected on board the ISS and the Foton-M2 capsule was developed to calculate the ambient dose equivalentH*(10), more meaningful for radiation protection purposes than theabsorbed dose in the silicon detector. The method allows evaluatingthe contributions of different particles to the dose, distinguishingthe low- and high-LET components (Spurn�y et al., 2007; Spurny andDachev, 2009). It was developed comparing energy spectra recor-ded on board the spacecraft, aircraft and on calibration

Fig. 2. Dose rate, integral flux and dose/flux ratio (specific dose) observed by R3D-B3 (black dots) and Liulin-Photo (red dots) dosimeters. The data are plotted versus the McIlwain’sL-parameter.

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experiments in the CERF high-energy reference field facility atCERN. Comparisons between the calculated ambient equivalentdose on board the ISS with the phantom TORSO NASA TEPC data(Tissue Equivalent Proportional Counter) can be found in Dimitrovet al. (2008). Assuming that events with deposited energies belowEw1 MeV correspond to low LET radiation (the non-neutroncomponent: protons, electrons, bremsstrahlung radiation), whileevents with deposited energy greater than 1 MeV are due toneutrons and HZE particles, some relations can be used to evaluateH*(10) (in mSv h�1) depending on the orbital position where themeasure is made. For orbital sectors where GCR are the dominantparticle population, H*(10) values are calculated from the energyspectrum as HGCR*ð10Þ ¼ Kf

P14i¼1 CiEi þ 5

P256i¼15 CiEig=MD, while

for data collected in the SAA sector, H*(10) is evaluated using therelation HSAA*ð10Þ ¼ Kf1:3

P14i¼1 CiEi þ 1:3

P256i¼15 CiEig=MD.

Ci is the number of counts in the channel i, Ei is the energy loss(in Joule) in the channel i, MD is the mass of the detector (in kg) andK is a coefficient. Here, the channel i¼ 15 separates the low fromthe high-LET component.

We applied this relation to evaluate H*(10) in four differentorbital sectors, identified by specific ranges of the McIlwain’sL-parameter. Formulas for each L-interval were applied to anenergy spectrum obtained by averaging several spectra recorded in

the range. For the SAA we considered the same spectra used toobtain Fig. 4. Results are shown in Table 2, where the dominantparticle component in each sector is also indicated. Starting fromthe sectors dominated by the GCR component, close to themagnetic equator H*(10) is in practice equal for R3D-B3 and L-P.Moving towards higher L-values the biggest differences betweenthe two devices are observed in the high-LET component of H*(10).The fact that L-P doses are higher than for R3D-B3 can be explainedconsidering that L-P was behind a higher shielding than R3D-B3and it measured more secondary, high-LET particles produced bythe interactions of the primary cosmic rays with the capsule wall. Itshould be noted that in the SAA sector the high-LET part of thespectra is dominated by trapped protons, that highly overcome thecontribution of neutrons and HZE ions. This is taken into account inthe expression used to calculate HSAA*(10), where equal multipli-cation coefficients (average quality factors) are utilized for the low-and high-LET components. When compared with TEPC data in theISS, the Liulin H*(10) estimations for the SAA showed the biggestdifferences, while they are good agreement concerning the GCR.

3.3. Comparison between experimental data andGEANT4 simulations

We use GEANT4 Monte-Carlo code for a first attempt to simulatethe transport of the incident, free-space particles through the wallof the satellite, in order to predict the radiation environment insidethe capsule. The simulation study was performed to evaluate theinternal proton and neutron components, comparing the resultswith the data coming from L-P and a set of Neutron BubbleDosimeters, as described in Damasso et al. (2008). Here we willdiscuss some results concerning the proton component.

3.3.1. Description of the simulation set upThe FM3 capsule was modelled as a sphere of external radius

1130 mm and composed of three concentric onion-like shells.These are respectively, from outside to inside: heat shield in Carbonfibre (5 cm thickness); equivalent Aluminium (Al-2219-T851;3.3 cm), to implement the satellite outer structure; equivalentKevlar (composition: C, 71%; O, 12%; N, 13%; H, 4%; 3.7 cm), to modelthe mission equipment. The components of the free-space

Fig. 3. Direct comparison between dose rates and fluxes observed by R3D-B3 andLiulin-Photo. The plots clearly show which particle populations were observed by thedosimeters and their relative contributions.

Fig. 4. Deposited dose spectra in the South Atlantic magnetic Anomaly region asa function of the kinetic energy of the free-space incident protons.

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radiation field used as input for the simulation are GCR (protonsand alpha particles) and trapped protons in the inner VAB. Thecorresponding differential fluxes (cm�2 s�1 MeV�1) were evaluatedusing the space radiation models CREME86 (for GCR), implementedin the OMERE toolkit (OMERE, 2008), and NASA/AP8-min (fortrapped protons during solar minimum), via the SPENVIS platform(SPENVIS, 2008). The input fluxes are represented as averagedspectra along one FM3 orbit.

3.3.2. Model predictionsFig. 6 shows the results of the radiation environment simulation

inside the capsule. Plot a shows the proton spectrum predictedinside the capsule, comprising both primary and secondary parti-cles, and the orbit averaged differential flux observed by L-P, whichis dominated by SAA trapped proton flux in the energy rangeconsidered. The GEANT4 predicted spectrum and the free-space

flux start to overlap at Ey80 MeV. The L-P spectrum remains lowerthan the predicted flux inside the capsule for E< 100 MeV, and thedifference between them constantly increases for E< 60 MeV. Areliable explanation for this discrepancy is that many othershielding sources (i.e. all the complex experimental equipment onboard) were actually placed around L-P, and it is obviously difficultto characterize with precision their equivalent shielding to use forthe simulation. Taking into account all the uncertainties related tothe definition of the input parameters for the simulation, webelieve that the model and the experimental results are in goodagreement. This seems confirmed also considering what weobtained by evaluating the minimum kinetic energy for a proton topenetrate the capsule shielding or to produce secondary protons.Four different simulations have been performed, limiting theenergy of the incident protons to the range 0 O 300 MeV. We use asinput the orbit averaged spectrum predicted by the AP8-min model

Fig. 5. Counts in 10 deposited energy selected channels for R3D-B3 (black dots) and Liulin-Photo (red dots) detectors, as function of the McIlwain L-parameter in the range L¼ 1 O 3corresponding to the sector with the inner proton radiation belt. The last plot in the right panel corresponds to the lowest deposited energy channel (0.16 MeV). The depositedenergy increases from bottom to up and from the right to the left panel the (the first plot in the left panel corresponding to the highest deposited energy here considered,DE¼ 2.64 MeV). Protons are the main responsible for the counts recorded in the VAB and their kinetic energy is indicated in each plot.

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for the trapped protons, which are the dominant component inparticle flux within hundreds of MeV. In each simulation one singlespectrum was used, restricted to a different energy range for eachcase study: from 0 to 100, 150, 200 and 300 MeV. Fig. 6 (plot b)

shows the variations in the proton flux predicted inside the capsule(primaryþ secondary particles), corresponding to the input spectrawith upper limits of 200 and 300 MeV. The results for E¼ 100 and150 MeV are not shown because the simulations predicted no

Fig. 5. (continued).

Table 2Evaluation of the ambient equivalent dose H*(10) on board Foton-M3 (total, low- and high-LET particle components), as a function of the McIlwain L-parameter and thedifferent radiation components which dominate in each sector.

Orbital sector Nr. of averagedspectra

Instrument H*(10)Low LET

(mSv h�1)H*(10)High

LET(mSv h�1)H*(10)total

(mSv h�1)Dose in Silicon(mGy h�1)

GCR component around geomagnetic equator (0.9< L<1.1) 2927 R3D-B3Liulin-Photo

1.16 1.84 3.00 1.531.16 2.06 3.22 1.57

GCR component at L¼ 2.8 O 3 461 R3D-B3Liulin-Photo

5.81 13.76 19.57 8.565.99 15.36 21.36 9.07

GCR component at L> 9 567 R3D-B3Liulin-Photo

7.25 19.36 26.62 11.137.79 20.31 28.10 11.85

South Atlantic Anomaly trapped protons(L¼ 1.24 O 1.63; Liulin-Photo dose> 100 mGy h�1)

20 R3D-B3Liulin-Photo

104.58 184.22 288.8 222.1580.84 122.43 203.27 156.36

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proton flux inside the capsule. An internal proton spectrum startsto appear in the model outputs for larger energy ranges. This resultis in good agreement with the experimental observation discussedpreviously, according to which the capsule shielding seems to stopprotons with E<w130 MeV (Fig. 5).

4. Conclusions

In this work the space radiation data successfully collected bytwo portable, active spectrum-dosimeters Liulin-Photo and R3D-B3on board the Foton-M3 satellite are presented and analyzed. Theresults represent a new contribution to the space dosimetry and tothe characterization of the radiation environment in Low EarthOrbit (LEO) (Benton and Benton, 2001) during a period of very quietand low solar activity.

R3D-B3 and Liulin-Photo active spectrum-dosimeters success-fully monitored in real-time the dose and particle flux along theorbit for the whole 12-day mission period. The devices were located

behind different shielding, respectively outside and inside thesatellite capsule.

The main results obtained are:

� The mission took place during a period of very quiet solaractivity in the minimum phase of the 23rd solar cycle. On theSun no remarkable energetic phenomena (as flares and coronalmass ejections) were recorded during that period and nogeomagnetic storms occurred. These circumstances let us toconclude that the radiation environment along the orbit of thesatellite was not characterized by intense and sharp variations inits properties. For this reason, all the averaged dose values andthe averaged energy spectra calculated in this study should beconsidered well representative for the whole mission period.� The total dose measured on the Silicon detectors was 3.06�10%

and 1.85�10% mGy by R3D-B3 and Liulin-Photo respectively.� The inner Van Allen radiation belt (South Atlantic magnetic

Anomaly) crossovers were characterized by dose and fluxpeaks in both the dosimeters, up to 481�10% mGy h�1 for R3D-B3 and as high as 292�10% mGy h�1 for Liulin-Photo (19% lessdue to higher shielding). Peaks are due to trapped protons withenergy up to 300–400 MeV.� Relativistic electrons in the outer radiation Belt were recorded

only by R3D-B3, and doses were as high as 2314�10% mGy h�1.Liulin-Photo was behind a shielding too high to allow thetrapped electrons to penetrate.� Over a narrow region in the centre of the South Atlantic

magnetic Anomaly, characterized by the highest dose peaks,the average dose spectra show that in R3D-B3 the maximumdose is deposited by 56 MeV protons, while for Liulin-Photo themaximum is due to 66 MeV protons.� A detailed analysis of the energy spectra corresponding to the

South Atlantic magnetic Anomaly transits seems to indicatethat the Foton capsule provided an effective shielding againstfree-space protons with energy E<w130 MeV.� By using a method recently developed for Liulin type dosime-

ters, an evaluation of the ambient equivalent dose H*(10) onFoton-M3 was made. We performed H*(10) calculations fordifferent sectors along the spacecraft orbit, distinguishingbetween zones dominated by Galactic Cosmic Rays and byenergetic protons in the inner Van Allen belt, consideringseparately the low LET, non-neutron (deposed energyE<w1 MeV) and the high-LET, neutronsþHZE particles(deposed energy E>w1 MeV) components. Differencesbetween R3D-B3 and Liulin-Photo assessments were foundmostly in the high-LET component: for L> 2.8 in the sectorsdominated by Galactic Cosmic Rays, where Liulin-Photo dosesare higher, and in the South Atlantic magnetic Anomaly, whereH*(10) is higher for R3D-B3.� We performed a preliminary study with GEANT4 Monte-Carlo

code to simulate the particle transport through the wall of thecapsule and to predict the internal radiation environment. Asinput the orbit averaged, free-space radiation spectra fromNASA/AP8-min (SPENVIS) and CREME86 (OMERE) modelswere used. The capsule was modelled as a 3-layers sphere ofdifferent materials. The comparison between experimental andcode predicted fluxes showed encouraging good results con-cerning the proton component of the radiation environmentinside the capsule, as for the neutron component discussed inDamasso et al. (2008). In particular, the model predictsa complete shielding of the capsule against incident protonswith energy E< 150 MeV, because protons (primaryþ secon-dary) begin to appear inside the capsule if incident particles areshooting with kinetic energy E> 200 MeV. This is in goodagreement with experimental observations.

Fig. 6. Plot a – Comparison between the GEANT4 predictions for the proton flux insidethe capsule (primaryþ secondary, red squares) and the orbit averaged spectrumobserved by Liulin-Photo (green squares). The primary spectrum used as input for thesimulation is shown in black squares. Plot b – Assessment of the minimum energynecessary for a free-space proton to penetrate the Foton capsule by using GEANT4code. Protons (primaryþ secondary) start to appear inside the capsule only when theenergy range of the primary spectrum is extended up to 200 MeV.

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Acknowledgements

This work was performed in the frame of a) MoMa project ‘‘FromMolecules to Man: Space Research Applied to the improvement ofthe Quality of Life of the Aging Population on Earth’’, funded by theItalian Space Agency, and of b) the Bulgarian Science Fund projectNo.HI-1511/2005.

The flight opportunity on board Foton-M3 was provided by ESA.The authors are grateful to R. Demets and A. Verga (ESA/ESTEC)

for their work concerning the Foton-M3 mission; to B. Tomov, Pl.Dimitrov and Yu. Matviichuk (STIL-BAS) for the work on thedevelopment and manufacture of R3D-B3 and Liulin-Photo instru-ments; to M. Briccarello (Thales Alenia Space), M. Bellucco (SofiterSystem Engineering) and F. Longo (INFN-Trieste) for their contri-bution to the GEANT4 simulation study; to V. Cotronei and J. Sab-bagh (ASI) for their support in exploiting the Photo-II experiment toM. Storini (IFSI-INAF) for useful discussions.

Appendix A. List of acronyms

ASI – Agenzia Spaziale Italiana (Italian Space Agency)DE – Deposited EnergyESA – European Space AgencyFM3 – Foton-M3 satelliteGCR – Galactic Cosmic RaysIE – Incident EnergyISS – International Space StationL-P – Liulin-Photo spectrum-dosimeterLEO – Low Earth OrbitSAA – South Atlantic magnetic AnomalyVAB – Van Allen radiation Belts

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