exomars iras (dose) radiation measurements

Mem. S.A.It. Suppl. Vol. 11, 178c© SAIt 2007

Memorie della

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EXOMARS IRAS (DOSE) radiationmeasurements

C. Federico1, A.M. Di Lellis2, S. Fonte3, C. Pauselli1, G. Reitz4, and R. Beaujean5

1 Dipartimento di Scienze della Terra, Universita degli Studi di Perugia, 06100 Perugia -Italy

2 AMDL Srl, Viale Somalia 133, 00199 Roma - Italy3 INAF - Istituto di Astrofisca Spaziale e Fisica Cosmologica, via del Fosso del Cavaliere

100, 00133 Roma, Italy; e-mail: [email protected] German Aerospace Center Institut of Aerospace Medicine, 51147 Koeln - Germany5 Universitaet Kiel, IEAP/Extraterrestrik, Leibnizstr. 11-19, 24118 Kiel - Germany

Abstract The characterization and the study of the radiations on their interaction with or-ganic matter is of great interest in view of the human exploration on Mars. The IonizingRAdiation Sensor (IRAS) selected in the frame of the ExoMars/Pasteur ESA mission isa lightweight particle spectrometer combining various techniques of radiation detection inspace. It characterizes the first time the radiation environment on the Mars surface, andprovide dose and dose equivalent rates as precursor information absolutely necessary todevelop ways to mitigate the radiation risks for future human exploration on Mars. TheMartian radiation levels are much higher than those found on Earth and they are relativelylow for space. Measurements on the surface will show if they are similar or not to thoseseen in orbit (modified by the presence of “albedo” neutrons produced in the regolith andby the thin Martian atmosphere).IRAS consists of a telescope based on segmented silicon detectors of about 40 mm diameterand 300 �m thickness, a segmented organic scintillator, and of a thermoluminescencedosimeter. The telescope will continuously monitor temporal variation of the particle countrate, the dose rate, particle and LET (Linear Energy Transfer) spectra. Tissue equivalentBC430 scintillator material will be used to measure the neutron dose. Neutrons are selectedby a criteria requiring no signal in the anti-coincidence. Last, the passive thermolumines-cence dosimeter, based on LiF:Mg detectors, regardless the on board operation timing, willmeasure the total dose accumulated during the exposure period and due to β and γ radiation,with a responsivity very close to that of a human tissue.

Key words. Thermoluminescence – dosimeter – IRAS – TLD

1. Introduction

The Galactic Cosmic Rays (GCR atomic nu-clei, stripped of their electrons) and the Solar

Send offprint requests to: S. Fonte

Particle Events (SPE protons and heavier ions)are of particular interest for astronauts voy-ages. Particularly for Mars’s ambient, that isnotable unlike from Earth.

Mars lacks a global magnetic field, GCRparticles arrive isotropically and are more en-

C. Federico, A.M. Di Lellis: EXOMARS IRAS (DOSE) 179

ergetic than those originating from Solar activ-ity. SPE particles can occur randomly duringthe Solar cycle although they tend to occur nearthe peaks of Solar activity.

Sun radiates in all wavelengths, from radio(low energy) to X-rays and γ-rays (highest en-ergy) through visible and infrared; the X-rayemission is highly variable. X-rays and γ-raysincrease when Sun increases its activity (1-2orders of magnitude).

There are two kinds of SPE: the Solar Windand the Solar Energetic Particle (SEP). Thefirst ones are charged particles continuouslyexpelled from the Sun: electrons, protons andalpha particles (Helium nuclei) with low speedand low energy. They are carried along solarmagnetic field and modulated by solar cycle(max at solar max), with an energy of about1 keV/nucleon.

While the second are charged particles ex-pelled from the Sun at high speed and highenergy, related to extreme events on solar sur-face. They are sporadic and unpredictable, withan anisotropic distribution (following magneticlines). They tend not to happen at solar min;their energy is about of 10-100 MeV/nucleonand it can be greater.

The GCR are charged particles from super-nova explosions at very high speed acceleratedby galactic magnetic field; they are composedof protons (85%), α (12.5%), heavier atoms(1%) and electrons (1.5%) with an isotropicdistribution. Their intensity are modulated bysolar cycle (∼11 years); Min at solar max.Their energy is about of 50-500MeV/nucleon(which means 10sof GeV for heavy ions).

The interplanetary space is unshielded, fullof GCR and solar particles, while near theEarth, most of solar particles are shielded bythe Earth’s magnetic field, adding effects tothe trapped particles (mostly protons and elec-trons) in Van Allen belts, from 4000 km to10000 km in altitude. On the surface, the parti-cles are shielded by atmosphere. On Earth, wehave an average of 2.4 mSv/year, due to space(17%) and natural radiactivity (83%).

In the Mars Odyssey mission (that is part ofNASA’s Mars Exploration Program), the cal-culations of MARIE, the Martian RadiationEnvironment Experiment for studying the ra-

diation environment, estimate about 1 Sv/yearon Mars surface.

Another radiation comes from Mart’s rock.Martian crust has a bulk composition equiva-lent to large-ion lithofile (LIL) and heat pro-ducing element (K, Th, U) enriched basalt,with a potassium content of about 0.5% andMartian Dose Rate = 240 nGy/h (max esti-mation), see McLennan (2001).

All these radiations interact primary withthe spacecraft structure and after with the as-tronauts tissues, producing secondary radia-tions and particles.

The absorption of all this energy by cellu-lar molecules causes several effects (cell death,molecular breakage and cell genetic mutation)leading to an increase in the cancer developingprobability.

Every tissue show a different tolerance todose. Usually, three organ types are consid-ered: the skin (outside part, unshielded), theeyes (specially sensitive organs) and the BloodForming Organs (BFO, bone marrow): deeperorgans, highly self-shielded.

There is in the ExoMars/Pasteur missionan instrumentation to estimate the risk of in-teraction between the radiation and the hu-man’s tissues. It combines a principal sen-sor: Ionizing RAdiation Sensor (IRAS) andan optional Thermo Luminescence Detectorwith the annexed reader: TLD Reader (for-merly named DOSE). IRAS, whose leadershipis German, is the instrument actually selectedfor Exomars/Pasteur mission.

2. Ionizing Radiation Sensor (IRAS)

The Ionizing RAdiation Sensor (IRAS) se-lected in the frame of the ExoMars/PasteurESA mission is a lightweight particle spec-trometer combining various techniques of ra-diation detection in space. Its design from her-itage components makes it a powerful exper-iment to deliver at minimum risk outstandingscientific data.

It characterizes for the first time the envi-ronment radiation on the Mars’s surface, andprovides dose and dose equivalent rates as pre-cursor information absolutely necessary to de-

180 C. Federico, A.M. Di Lellis: EXOMARS IRAS (DOSE)

velop ways to mitigate the radiation risks forfuture human exploration on Mars.

The silicon detector telescope sensor of theinstrument uses segmented silicon detectorsand it will continuously monitor temporal vari-ation of the particle count rate, the dose rate,particle and LET spectra. During operation twotypes of measurements will be performed:

a) single detector mode for dose measure-ments in the individual detectors;

b) coincidence mode for the measurement ofparticle and Linear Energy Transfer (LET)spectra in the range 2 to 2000 MeV/cm.

Both measurements are an integration oversoftware defined time periods resulting in areasonable data reduction. LET spectra will bededuced from the energy deposit of coincidentevents in the silicon detectors. Since the inci-dence angle of the particles is not measured,the energy deposit is converted into LET in sil-icon, as energy deposit/t, where t is the meanpath length in the detector.

The detector telescope of the IRAS willwork in one mode only. In this mode datafor count and dose rates will be integratedover pre-selected time intervals, a typical in-terval may be 10 minutes. These time intervalsare software dependent and can be adjustedwithin reasonable counting statistics. Data pro-vided are time resolved count and dose rates(separate measurement of charged and neu-tral components) and the measurement of theLET spectrum. LET values in water from about2 MeV/cm (minimum ionisation for singlycharged particles) to 2000 MeV/cm are cov-ered.

All data will be stored temporally in aninternal histogram memory. Data need to bedownloaded and transferred to ground. Themaximum data rate is 1 Mb/day.

The coincidence requirement between thetwo detector signals of the telescope selectsparticles with restricted pathlength in the pla-nar detectors (the angle of the acceptance coneis about 120◦). The LET(Si) values are de-duced from the measured energy, deposited inthe individual silicon detectors in coincidencemode, using a mean pathlength of 364 µmat adensity of 2.33 g/cm3. This procedure under-

estimates the LET value of short range parti-cles stopping inside the detector. In order toaccount for the difference in stopping powerof Si and water, the LET(Si)-value is multi-plied by a factor of 1.23 to get the LET(H2O)-value. This mean conversion factor neglects thedependence on the particles energy, the uncer-tainty is estimated to be 3%.

Figure 1. LET spectra for three components: aGCR particles, solar energetic particles during theApril 15, 2001 event (SEP) and radiation belt parti-cles (SAA).

In Figure 1 shows the measured LET spec-tra converted to water for the three radiationfields components which could be separated.The shape and gradient of the LET spectrumdefines the mean radiation quality factor Q cal-culated with Q(LET)-values. It is obvious thatthe GCR spectrum has the lowest gradient andthus the highest Q-value.

The IRAS consists of three segmented pla-nar silicon PIN-detectors (300 µm thickness,outer diameter 42 mm) and a segmented solidorganic scintillator (BC430), with an outer di-ameter of 50 mm and a thickness of 15 mm.BC430 is an orange-emitting scintillator thathas a nominal wavelength peak emission near580 nm. BC430 is conventionally used in radi-


Figure 2. The BC430’s emission spectrum in thered region, credit Saint-Gobain Crystals

ation detector applications because of its highlight output and because of the high quantumefficiency of PMTS in this part of the spectrum.

Two additional silicon sensors serve formeasurements of particle count rates and doserates in the two remaining directions. Tissueequivalent BC430 scintillator material will beused to measure the neutron dose. The scintil-lator is surrounded by an anticoincidence de-tector, build up from the telescope and a scin-tillator ring. Neutrons are selected by a criteriarequiring no signal in the anti-coincidence de-tectors. The energy spectrum of recoil protonsfrom neutron interactions will be measured inthe range 1 to 30 MeV.

Container

Lid

(segmented)Silicon Detector

(segmented)BC 430 Detector

(segmented)Silicon Detector

(light tight)Cover Foil

Circuit BoardPower−/Data−

Connector

Figure 3. The IRAS instrument plan consists ofthree segmented planar silicon PIN-detectors and asegmented solid organic scintillator

The outer segment of the scintillator andpart of the silicon detectors act as anticoinci-dence shield for the inner part of the scintillator

(26 mm diameter, 15 mm height, see Figure3). The detector telescope sensor is built up bythe inner segments (26 mm diameter) of twoplanar silicon detectors. The third Si-detectoris in optical contact with the scintillator andprovides for the light detection.

5V Analog Voltage70V Detector Voltage

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Solid State Detectors(Dose, LET)

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H/K ControlExternal InterfaceData Processing+StorageS/H+PHATrigger Logic

PC Boards

Internal Power Supply Unit

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CSA

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(optional)TLD Reader

Figure 4. IRAS Electrical design

Two additional PIN diodes are added inthe electronic part beneath the detector headto provide measurements in x and y direction.The size of the detector system is 58 mm indiameter and 55 mm in height, the mass isabout 500 g with a power consumption of600 mW. The operation of the system is man-aged by a microcontroller which controls theoperation mode by switching the power anddata lines to the individual sensors via multi-plexer on the main board, data sampling, in-ternal intermediate storage and data transfer tothe external main storage. The instrument usesa RS 422 interface. Similar designs are usedfor MATROSHKA sensors and for the detec-tor telescope DOSTEL on EuTEF.

In Figure 4 is rappresented the IRAS elec-trical design, with an optional insertion of aTLD Reader.

The IRAS instrument is capable to char-acterize the radiation field concerning particlefluences, dose rates and energy transfer spectrafor ionising particles. In addition, it allows todetermine the dose contribution of secondaryneutrons from GCR-interactions with the at-mosphere and the soil.


E

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Valence Band

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Figure 5. Energy band model showing the elec-tronic transitions in TL material according to a sim-ple two-level model: (a) generation of electrons andholes; (b) electron and hole trapping; (c) electron re-lease due to thermal stimulation; (d) recombination.Solid circle are electrons, open circles are holes.Level T is an electron trap, level R is a recombi-nation centre, EF is Fermi level. See Bos (2001)

The measurement will be carried out onMars, during cruise and in orbit. The IRASinstrument just need to be powered for tenminutes each hour, during cruise permanentoperation would be prefered. After switch/onmeasurements are started automatically. Datatransfer to the DPHU of the rover/GEP shall bedone at the end of the measuring period. Themeasuring intervals provide enough informa-tion to draw a complete time dependence pro-file of the radiation environment.

3. The TLD Reader

The TLD Reader is designed to keep track ofthe radiation exposure field when the astro-nauts will pass through during both the surfacestay and during the transit period to and fromthe red planet.

The TLD Reader instrument is a passivedosimeter working without any powering orsupport after the annealing. It will not sufferfor any gap during measurements and will col-lect energy even in presence of huge SPE.

The TLD Reader will only measure the in-tegrated energy over some period of time (dif-

ferent phases of the cruise and of the surfaceexploration) and will be able to measure β andγ radiation dose. The TLD Reader has a re-sponsivity very close to that of a living organ-ism, Bos (2001).

The TLD Reader is based on doped ther-moluminescence lithium-fluoride passive de-tectors (pills of LiF:Mg,Cu,P). An explanationof the observed ThermoLuminescence (TL)properties can be obtained from the energyband theory of solids (with one trap-one cen-tre model), as noted by Bos (2001).

In Figure 5, it is rappresented a pattern ofthe one trap-one centre model. As result of in-teraction with the radiation, some electrons aretrapped in the T state inside the gap energy; itis a non-equilibrium state.

The relaxation rate for the T state is de-termined by Arrhenius’s probability, see Bos(2001). More high is the temperature, greateris the transition probability from T state to con-duction band, and greater is the recombinationin the R state.

The recombination produces a γ ray in thevisible range, so called thermoluminescencesignal. A thermoluminescence photons’ countestimates the absorbed dose by semiconductordosimeter. Usually to read the TL signal, theTL material is stimulated with a thermal pat-tern, for example, a linear stimulus; this stimu-lus produces a responce so called glow curve.This curve is composted of peaks.

These peaks rappresent a dopant (or a de-fect) of the semiconductor dosimeter. Thereis an evident peak, so called 4-peak, at about240 ◦C for the LiF:Mg,Cu,P. The integral ofglow curve gives the number of trapped elec-trons in the T state, then the dosimeter’s ab-sorbed dose.

The environmental monitoring of radiationdoses using with TL detectors is extremelyvaluable companion to environmental monitor-ing using systems equipped with active, on-linedosimeters, see Olko et al. (2004).

TLD are used for long term (about 3-12months) environmental dosimetry, around nu-clear installations. The so called high sensi-tive on LiF:Mg,Cu,P is about two orders ofmagnitude more sensitive than conventionalLiF(TLD-100), which enables short term (2


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Dosimeter N◦7

Figure 6. The photon’s impact dynamics on the conical collector, from central point of dosimetral. On theright the tests on detectable dose ranges

weeks, against 3 months exposure) radiometrysurveys of the environment. This high sensi-tivity also allows short-term (hourly or daily)monitoring of local changes of radiation dosesafter substantial contamination of the environ-ment, see Olko et al. (2004).

The capacity of these detectors to integratethe received energy from their last reset, wouldbe used to measure the possible collected dosesduring different phases of the mission: cruisephases, permanence on Mars.

The measure process is based on the pho-tons counting of luminescence radiation emit-ted during the detectors heating cycle.

TLD can’t measure the heavy component,but recent works of Cucinotta et al. (2002)and those of Saganti et al. (2002) have shownthat the heavy component on the total accumu-lated dose results to be a negligible part in theMartian environment.

The detector is placed in a silver oven,and kept in place by means of a mechani-cal fixture (no adhesives). The mini oven isheated by means of a thermofoilT M heaterwith mica insulation, able to withstand the240 ◦C temperature. The heater is clamped bymeans of a steel plate.

The collection of light is enhanced by aconical collector. The collector’s geometry op-timises the signal acquisition, providing an ef-fective shielding for radiative heating towardsthe photomultiplier tube.

In Figure 6, it is rappresented the impact onthe conical collector versus the photons’ anglesemission form central point. It is correspondedto each angle a particular impact: there is a di-

rect collection of photons on the counter (di-rect view), or a collection after one reflectionon side cylinder, or a collection after one re-flection on the side cone.

To optimise the collector’s geometry, theoptical ray impact on the conical collector onlyone time.

A filter is placed in front of the tube,limiting the useful waveband to the range of300÷500 nm, reducing the infrared radiationentering the tube itself.

In Figure 6, it is rappresented the responseof system detector to different doses absorbedfrom a 109Cd source.

The linear responce of the LiF:Mg,Cu,P isfrom 1 µGy to 10 Gy; the down to high dosesis caused from a slow responce of electrical cir-cuit used to read the TL signal from dosimeterpill (a laboratory multifunction reader). Whilethe low doses non linear responce probable isdued to a dosimeter’s sensibility. Nevertheless,the responce is good and maybe when thereader is optimizzed, these diffects will disap-pear.

In Table 3 is reported the weight (in grams)of the all system (IRAS+TLD Reader). It isimportant to note that the passive sensor eventhought is not able to provide an instantaneousmeasurement of the dose rate, it is converselyable to return a total dose measurement veryclose to that a human tissue could accumulateduring any phase of the mission with no powerrequest to the system, being completely passiveoperated.

The advantages of passive detectors forenvironmental monitoring are that they are:


Table 1. Mass breakdown of the all radiationdetector

Item Sub-Item Weight [g]Telescope Housing & conn. 120IRAS Detector head

-Scintillator 60-Silicon detectors 40-Frame 120Electronics 160

Subtotal 500TLD Collimator 12Reader Structure 91

Detector & Holder 15Phototube 15HV Block 35Electronics 35Harness & conn. 30

Subtotal 228Total 728

small, cheap, don’t require in situ electronic

power supply and can be used in a large doserange.

They provide measurements of the dose in-tegrated during the time interval (days months)which means that only an average dose ratevalue for this period can be estimated.

References

Bos, A. J. J. 2001, Nucl. Instr. and Meth. inPhys. Res. B, 184, 3

Chen, T.C. and Stoebe, T.G. 1998, Rad. Meas.,29, 39

Cucinotta, F. A. and Saganti, P. B. and Wilson,J. W. and Simonsen C. 2002, J. Radiat. Res.,43, 35

Olko, P. and Budzanowski, M. and Bilskil, etal. 2004, Nucl. Tech. and Rad. Prot, 19, 20

Saganti, P. B. and Cucinotta, F. A. and andWilson, J. W. 2002, J. Radiat. Res., 43, 119

Garlick, G. F. J. and Gibson, A. F. 1948, Proc.Phys. Soc., 60, 574

McLennan, S.M. 2001, Geophys. Res. Lett.,28, 4019

exomars iras (dose) radiation measurements

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