Radiation Measurements 43 (2008) 427–431www.elsevier.com/locate/radmeas

TriTel-S: Development of a complex dosimetry instrument for a satellite ingeostationary transfer orbit

A. Hirna,∗, T. Pázmándia, S. Demea, I. Apáthya, L. Bodnárb, A. Csokea

aHungarian Academy of Sciences KFKI Atomic Energy Research Institute, P.O. Box 49, H-1525 Budapest, HungarybBL-Electronics, Sport u. 5, H-2083 Solymár, Hungary

Abstract

One of the many risks of long-duration space flights is the excessive exposure to cosmic radiation, which may have serious consequencesparticularly during solar flares and higher solar activity. Since space radiation mainly consists of charged heavy particles, the equivalent dosediffers significantly from the absorbed dose. The objectives of this project, which began in the KFKI Atomic Energy Research Institute of theHungarian Academy of Sciences several years ago, are to develop a three-dimensional silicon detector telescope (TriTel) and to develop softwarefor data evaluation of the measured energy deposition spectra. A version of TriTel will be installed onboard a European satellite (ESEO) in ahighly eccentric orbit crossing, the Van Allen belts. The instrument will encounter high fluxes of trapped electron radiation in a considerablepart of the orbit. In order to give a rough estimate of the expected fluxes and spectra of protons and electrons in orbit, calculations were madewith the Space Environment Information System (SPENVIS) online tool. The number of electron coincidences and the deposited energy spectraof electrons were calculated with the Monte Carlo simulation-based software MUlti-LAyered Shielding SImulation Software (MULASSIS).

The description of the instrument, the expected environment in orbit and the evaluation of the results of the preliminary simulations arediscussed in this paper.© 2008 Elsevier Ltd. All rights reserved.

Keywords: Silicon detector telescope; Space dosimetry; Dose equivalent; Electron coincidences

1. Introduction

One of the most important risk factors associated withmanned space missions is the excessive exposure to cosmic ra-diation. Due to the lack of shielding provided by the atmosphereof our planet, the dose rates measured onboard the Interna-tional Space Station (ISS) are at least two orders of magnitudehigher than those on the Earth’s surface (Deme et al., 2006).Moreover, during solar flares, the dose to the astronauts on aninterplanetary mission leaving the geomagnetic shielding ofthe Earth may exceed the career dose limits or may even leadto deterministic effects in the absence of appropriate shielding.

Due to significant spatial and temporal changes in the cos-mic radiation field, radiation measurements with advanced

∗ Corresponding author. Tel.: +36 13 922 222; fax: +36 13 959 293.E-mail addresses: hirn@aeki.kfki.hu, hirn@sunserv.kfki.hu (A. Hirn).

1350-4487/$ - see front matter © 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.radmeas.2007.12.043

dosimetric instruments onboard space vehicles are extremelyimportant. To characterise the stochastic biological effective-ness of cosmic radiation, the concept of dose equivalent was in-troduced in the recommendation of the International Commis-sion on Radiological Protection (ICRP Publication 60, 1991).Since dose equivalent was defined in terms of a linear energytransfer (LET)-dependent quality factor, determining the LETspectrum and the quality factor of cosmic radiation is neces-sary. For this reason, the development of a 3D silicon detectortelescope with almost uniform sensitivity got underway in theAtomic Energy Research Institute (MTA KFKI AEKI) severalyears ago. The instrument comprising three mutually orthogo-nal axes (fully depleted PIPS detector pairs) will be capable ofproviding the LET spectrum and average quality factor of pro-tons, the alpha particles and heavier ions, the average qualityfactor of the radiation as well as the absorbed dose and doseequivalent (Pázmándi et al., 2006).

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2. Instrumentation onboard ESEO

The silicon detectors with a thickness of 300 �m and a sen-sitive area of 150 mm2 are mounted at a distance of 8.9 mm,forming three orthogonal telescopes with a geometric factorof 287 mm2 sr. The analogue signal section (with 1 �s pulseshaping time constant) together with the 16-bit ADC allowsa pulse height analysis of the detector signals from 80 keVup to 85 MeV, which corresponds to a LET range of about0.2.120 keV/�m in water.

In addition to several versions of TriTel planned to be oper-ated onboard the European module and the Russian segment ofthe ISS, a compact version of TriTel (TriTel-S) incorporating aGeiger–Muller counter accompanying the three telescope axeswill be installed on the European Student Earth Orbiter (ESEO)satellite. The size of the instrument is 8 × 8 × 14.8 cm3. Thetotal mass is 1 kg; the maximum power consumption is 7.2 W.

The spacecraft will be injected into a highly elliptical Geo-stationary Transfer Orbit (GTO) with a perigee of 250 km, anapogee of 35 950 km and an inclination of 7◦. The launch of theorbiter is expected to take place in 2010. The orbit of ESEO isgoing to cross the Van Allen belts; therefore, in a considerablepart of the orbit, the orbiter will be exposed to high fluxes oftrapped electrons and protons. TriTel-S will be mounted ontoone of the shear panels of the spacecraft in such a way that the3D telescope and the ZP1301-type Geiger–Muller counter willextend over the lateral panel of ESEO. The measuring detec-tors of the telescope will face the outer space, and the axis ofthe GM tube will be parallel to the surface of the orbiter.

3. Measurements in the electron belt

Fig. 1 shows the expected fluxes and the electron fractionof the trapped radiation in the orbit of ESEO behind 2 mm ofaluminium shielding as a function of the orbital time during asolar minimum. Calculations were made with the AE8 and AP8

Fig. 1. Expected fluxes and the electron fraction of the trapped radiation in the orbit of ESEO behind 2 mm of aluminium shielding as a function of the orbitaltime (SPENVIS).

trapped electron and trapped proton models implemented inthe Space Environment Information System (SPENVIS) onlinetool. It should be noted that errors in the flux models can be aslarge as a factor of two (Heynderickx et al., 1996).

The maximum count rate (50 000 cps determined by the deadtime of the system) at which TriTel-S can be operated in its pulsemode is indicated in Fig. 1. For the other regions of the orbit,the operation of the instrument in its current mode was takeninto account. The preliminary calculations have shown that themaximum currents will be in the same order of magnitude asthe leakage current of the detector itself. If we add that thetemperature inside the instrument is to vary from −25 ◦C upto 30 ◦C, we can see that operating the detectors in the currentmode is inappropriate for measuring dose in orbit. From Fig. 1,it can be concluded that the instrument is going to providespectra for not more than 10% of the orbital time, mainly inthe vicinity of the perigee and between the inner and outerradiation belts. However, in the latter case, the detectors will beexposed to a considerable amount of trapped electron radiation.Consequently, the contribution of electrons to the depositedenergy spectra has to be taken into account.

The integral spectra of the primary electrons at four differentaltitudes are shown in Fig. 2. The curves indicated with 950and 1100 km correspond to altitudes in the lower sections of theinner radiation belt, 6900 km is between the two radiation belts,while the altitude of 12 600 km is representative of the outerVan Allen belt. According to the curves obtained, electrons withenergies of up to 3–4 MeV are expected to occur in the regionsof the orbit where TriTel-S will be capable of providing LETspectra.

3.1. Electron coincidences in the telescopes

Contrary to protons and heavier ions with a negligible de-flection from the original direction in the 300 �m thick siliconcrystal, electrons, due to their considerably smaller mass,

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Fig. 2. Integral spectra of primary electrons in the orbit (SPENVIS).

evidence a more complicated zigzag path in the material. Onthe one hand, even those electrons that have a vector of velocitylying outside the solid angle of the telescope may give rise toreal coincidence. On the other hand, electrons that are incidentperpendicularly to the surface of the detector can be scatteredto such an extent that the gating detector might not detect theparticles. These effects may limit the use of the coincidencespectra.

Calculations were made with the Monte Carlo simulation-based MUlti-LAyered Shielding SImulation Software(MULASSIS) in order to determine the probability of coinci-dences in the telescopes for electrons with given energy andangle of incidence. The number of coincidences caused byelectrons was expressed relative to the number of coincidencesthat might be caused by a hypothetical particle that has thesame range as the extrapolated range of the given electron andhas a straight path in the material. This ratio is indicated by qe

in the paper.Simulations were performed in planar geometry for one tele-

scope axis behind an aluminium shielding of given thickness.In Fig. 3, the values of qe at 0◦ are plotted for electron energiesof 700 keV, 1, 2 and 4 MeV as a function of the thickness ofthe shielding.

A necessary condition for a coincidence signal to appear isthat the energy deposited in the detectors exceeds 60 keV. Thisis the lower discrimination level, which is determined by thenoise of the system. The values in Fig. 3 indicate that increasingthe thickness of the aluminium shielding in front of the measur-ing detector is to decrease the number of coincidences for elec-trons that are incident perpendicularly or at small angles to thesurface of the detector. However, a thinner shielding would in-crease the total number of electrons to be detected. This wouldresult in a further increase in the dead time of the system. Forthe final TriTel-S geometry with 2 mm of aluminium shielding,the values of qe are plotted in Fig. 4 as a function of the angleof incidence for 2, 3, 4 and 7 MeV.

The connecting lines in Fig. 4 are there to secure better visu-alisation of the results—they are not fitted curves. The toothed

Fig. 3. Relative number of electron coincidences (qe) at 0◦ as a function ofthe thickness of the aluminium shielding (MULASSIS).

Fig. 4. Relative number of electron coincidences (qe) as a function of theangle of incidence (MULASSIS).

lines can be attributed to the statistical uncertainties in theMonte Carlo calculations.

For electron energies that are expected during the productionof deposited energy spectra, the value of qe does not exceed 0.5for angles of incidence between 0◦ and 40◦. However, above40.50◦, the value of the quotient increases drastically due to theelectrons that are scattered towards the gating detector, whilethe number of coincidences of the hypothetical particles tendsto zero. This means that the relative number of electron coin-cidences increases whereas their absolute value decreases. Dueto their small range, electrons below 1 MeV cannot reach thegating detector; therefore, no coincidences will occur.

3.2. Contribution of electrons to the deposited energy spectra

Owing to the multiple and significant scattering in the mate-rial, the coincidence spectra of the electrons behind the 2 mm

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Fig. 5. The deposited energy spectra of electrons (MULASSIS, parallel beam, 0◦).

Fig. 6. The deposited energy spectra of electrons (MULASSIS, isotropic).

Table 1Estimates of the specific energy loss of electrons from the deposited energy spectra

Eel,primary (MeV) E/lavr (keV/�m) isotropic E/lavr (keV/�m) parallel beam (0◦) E/w (keV/�m) parallel beam (0◦)

1.3 0.68 ± 0.01 0.73 ± 0.01 0.63 ± 0.011.5 0.79 ± 0.01 0.82 ± 0.01 0.70 ± 0.012 0.78 ± 0.01 0.75 ± 0.01 0.64 ± 0.014 0.59 ± 0.01 0.45 ± 0.01 0.39 ± 0.01

of aluminium shielding were calculated with the total depositedenergy spectra of the electrons in the measuring detector(incident side). The spectra for 1.2, 1.5, 2 and 4 MeV electronenergies were generated with MULASSIS in its pulse heightspectrum mode. The results for a parallel beam with an angleof incidence of 0◦ and for the isotropic case are summarisedin Figs. 5 and 6, respectively.

In both cases the deposited energy spectra have their max-imum at around 100 keV; however, in the case of an isotropicfield, the contribution of the channels of higher energies is moreconsiderable than it is for the parallel beam.

The specific energy loss of electrons between 100 keV and4 MeV is in the interval from 0.35 to 0.40 keV/�m in silicon(Deme, 1971). An estimate of the LET spectra can be given

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by dividing the deposited energies by the average pathlengthin the sensitive volume of the detector (LET ∼ E/lavr). Theaverage E/lavr and the E/w (w is the detector thickness) valuescalculated from the deposited energy spectra are summarisedin Table 1.

Although the values obtained in this way appear to haveoverestimated the specific energy loss of the electrons by afactor of 1.3–2, the radiation quality factor remains 1. However,in the case of a more complex spectrum unfolding algorithm amore detailed analysis will be needed in the future.

Conclusions

The development of several versions of a 3D silicon de-tector telescope (TriTel) began in the KFKI Atomic EnergyResearch Institute of the Hungarian Academy of Sciences afew years ago. Within the framework of the European SURE(ISS: a Unique Research Infrastructure) program, TriTel will beoperated onboard the European Columbus module of the ISS,while in cooperation with the Institute of Biomedical Prob-lems, Moscow, another version of the instrument will performmeasurements in the Russian segment of the ISS. The TriTeldeveloped for a satellite in GTO is going to encounter a signif-icantly different radiation environment than the other versionsonboard the ISS.

Simulations have been performed in order to give an estimateof the radiation field in the orbit of ESEO and to determinewhere the silicon detector telescope can be operated in its pulsemode. The number of electron coincidences as a function ofelectron energy, the angle of incidence and shielding thicknesshave been analysed as well. The results have shown that elec-trons will have a significant contribution to the coincidencespectra, from which the average radiation quality factor of

cosmic radiation will be determined. The expected depositedenergy spectra for monoenergetic electrons of four differentelectron energies have been calculated as well. Albeit the spe-cific energy loss values calculated from the spectra appear tohave overestimated the specific energy loss of electrons, the ra-diation quality factors were proved to be the same as the valuesgiven in the tables.

For more complex spectrum unfolding techniques and formixed fields containing electrons, protons and heavier ions, amore detailed analysis will be needed in order to determinethe uncertainties in the average quality factor of the radiationcaused by electrons.

Acknowledgements

The authors wish to acknowledge the services providedby SPENVIS. Acknowledgement and thanks are also offeredhereby to the developers of MULASSIS.

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