Доклади на Българската академия на науките
Comptes rendus de l’Acad´emie bulgare des Sciences
Tome 68, No 4, 2015
RADIATION ENVIRONMENT INVESTIGATIONS DURING
EXOMARS MISSIONS TO MARS – OBJECTIVES,
EXPERIMENTS AND INSTRUMENTATION
J. Semkova, T. Dachev, St. Maltchev, B. Tomov, Yu. Matviichuk,
P. Dimitrov, R. Koleva, I. Mitrofanov∗, A. Malakhov∗,
M. Mokrousov∗, A. Sanin∗, M. Litvak∗, A. Kozyrev∗,
V. Tretyakov∗, D. Golovin∗, S. Nikiforov∗, A. Vostrukhin∗,
F. Fedosov∗, N. Grebennikova∗, V. Benghin∗∗ , V. Shurshakov∗∗
(Submitted by Corresponding Member P. Velinov on March 21, 2015)
Deep space manned missions are already a near future of astronautics.
Radiation risk on such a long-duration journey appears to be one of the basic
factors in planning and designing the mission. The paper relates to the scientiﬁc
objectives and experiments for investigation of the radiation environment to be
carried out during the ExoMars 2016 and 2018 joint missions of the European
Space Agency (ESA) and the Federal Space Agency of Russia (Roscosmos) to
Mars. The following topics are described: 1) The charged particle telescope and
the experiment Liulin-MO for measuring the radiation environment on board
the ExoMars 2016 Trace Gas Orbiter satellite as a part of the Fine Resolution
Epithermal Neutron Detector (FREND) and 2) Liulin-ML experiment and in-
strument for investigating the radiation environment on Mars as a part of the
active detector of neutrons and gamma rays (ADRON) on the Russian surface
platform for ExoMars 2018 mission. Liulin detectors will be used in combina-
tion with the neutron detectors to study the radiation conditions both from
charged particles and neutrons during the cruise phase, in Mars orbit and on
the surface of Mars.
Key words: space radiation, radiation risk, space radiation measure-
ments, interplanetary missions, ExoMars
This work is partly supported by Contracts 503/2-13 and 63/4-14 between SRTI, Bulgarian
Academy of Sciences, and SRI, Russian Academy of Sciences.
1. Introduction. The deep space manned missions are already a near future
of astronautics. Radiation risk on such a long-duration journey, a great part of
which will take place in the interplanetary space, appears to be one of the basic
factors in planning and designing the mission.
The estimation of the radiation eﬀects for a long-duration manned space
mission requires three distinct procedures: i) Knowledge and modelling of the
particle radiation environment; ii) Calculation of primary and secondary particle
transport through shielding materials; and iii) Assessment of the biological eﬀect
of the dose.
1.1. Sources of ionizing radiation in the interplanetary space. The
radiation ﬁeld in interplanetary space is complex, composed of galactic cosmic
rays (GCR), solar energetic particles (SEP), and secondary radiation produced
in the shielding materials of the spacecraft and in the biological objects.
The GCR-charged particles that originate from sources beyond the Solar
System are the dominant radiation component in the interplanetary radiation
environment. GCR represent a continuous radiation source and they are the most
penetrating among the major types of ionizing radiation . The distribution of
GCR is believed to be isotropic throughout the interstellar and interplanetary
space. The energies of GCR particles can reach 1020 eV/nucleon. Most of the
deleterious eﬀects with regard to health produced by this radiation are associated
with nuclei in the energy range from several hundred MeV/nucleon to a few
GeV/nucleon. The ﬂux and spectra of those particles show modulation that anti-
correlate with the solar activity. The GCR spectrum consists of 98% protons
and heavier ions (baryon component) and 2% electrons and positrons (lepton
component). The baryon component is composed of 87% protons, 12% helium
ions (alpha particles) and 1% heavy ions . The highly energetic particles in the
heavy ion component, typically referred to as high Z and energy (HZE) particles,
play a particularly important role in space dosimetry  and aﬀect strongly the
biological objects and humans in space . HZE particles, especially iron, possess
high linear energy transfer (LET) and are highly penetrating, which gives them
a large potential for radiobiological damage . The average dose rate from
GCR in the interplanetary space measured by the RADOM instrument on the
Chandrayaan-1 satellite  during low solar activity was 12–13 µGy h−1.
Solar energetic particles (SEP) are randomly distributed events, but they
may deliver very high doses over short periods and that is why they could be
associated with the lethal equivalent doses. The SEP are mainly produced by
solar ﬂares, sudden sporadic eruptions of the Sun chromosphere. High ﬂuxes of
charged particles (mostly protons, some helium and heavier ions) with energies up
to several GeV and intensity up to 104particle cm−2s−1sr−1are emitted. The
time proﬁle of a typical SEP event starts with a rapid increase in ﬂux, reaching
a peak in minutes to hours. Although SEPs are more likely to occur around
solar maximum, such events are at present unpredictable with regard to their
486 J. Semkova, T. Dachev, St. Maltchev et al.
times of occurrence and it cannot be assumed that SEPs will not occur under
solar minimum. The most intense solar proton ﬂuencies observed were those in
August 1972 and October 1989. The ﬂare containing the largest peak ﬂux of
highly penetrating particles was in February 1956 [7–10]. On this basis the so-
called worst-case ﬂare is composed, which is thought to occur once a century, but
statistics are extremely poor.
1.2. Radiation environment on Mars. The radiation environment on
the surface of Mars is much harsher than that on the surface of the Earth for
two reasons: Mars lacks a global magnetic ﬁeld to deﬂect energetic GCR and
SEP, and the Martian atmosphere is much thinner (<1%) than that of Earth,
providing little shielding against the high-energy particles that are incident at
the top of its atmosphere. Both GCRs and SEPs interact with the atmosphere
and, if energetic enough, penetrate into the Martian soil, or regolith, where they
produce secondary particles (including neutrons and γ-rays) that contribute to
the complex radiation environment on the Martian surface.
The cosmic rays produce ionization in the ionosphere, atmosphere, hydro-
sphere, cryosphere and lithosphere of the planets [7–10]. The contribution of
cosmic rays to ionization of the outer planetary ionospheres and atmospheres
increases with the increment of the planetary distances from the Sun.
2. Implications for future human missions to Mars. The currently
adopted NASA ionizing radiation exposure limits allow for astronauts a maximum
annual dose of 0.5 Sv to the blood-forming organs (BFO) . The Russian
standards for manned space missions allow also 0.5 Sv a year, but not more
than 1 Sv for the cosmonaut’s career . The recommended ESA and Canadian
astronaut’s career dose limit is also 1 Sv.
To prepare future human exploration of Mars transport models developed
in ([13–17] and references therein) are used for prediction of the particle ﬂux,
absorbed dose, dose equivalent and ionization by GCR and SEPs.
Model estimations show that (see ): i) behind relatively thin shielding
the annual dose equivalent to the BFO from GCR is larger than the annual
limit of 0.5 Sv/year; ii) the dose equivalent is a slowly decreasing function of the
shield thickness. As it is pointed out in  the uncertainties and the possible
inaccuracies involved in the calculations could result in a potential shield mass
increase by up to a factor of 4. If the exposure is underestimated by a factor of
2, then the shield mass must be increased by an order of magnitude.
Model calculations  give a dose rate of about 0.42 Sv h−1to the unshielded
BFO for a standard solar proton event and up to 0.70 Sv h−1for the worst-case
solar proton event. Under these estimations the necessity of eﬀective shielding is
more than evident. Shielding against solar energetic particles is, at ﬁrst glance,
simpler than against GCR. However the beneﬁt of less penetration of the ﬂare
particles because of their softer spectrum is oﬀset to a large degree by their high
intensity. In addition, solar energetic particles also produce secondaries, which
Compt. rend. Acad. bulg. Sci., 68, No 4, 2015 487
build up in the shielding materials. In particular the commonly used aluminum
proves to be an insuﬃciently eﬀective shielding material [7–10].
GCR and SEP events aﬀect the evolution of the climate of Mars, the op-
eration of satellites, and the human exploration of the planet. They can aﬀect
the chemistry on the surface. The energetic inputs also have an impact on the
planetary atmosphere evolution by modifying the escape rates and the chemistry
of the upper atmosphere (see  and references therein). The determination of
the Martian surface and sub-surface radiation environment is an ongoing eﬀort
that started with numerical simulations [20,21 ].
Present calculations with the available models show that radiation doses
expected on manned missions to Mars can easily exceed the suggested allowed
doses , but we must keep in mind that these estimations bear a lot of uncer-
tainties. Validation of the radiation model predictions available must be secured
before any human mission to Mars is undertaken . Therefore experimen-
tal measurements of radiation environment characteristics on unmanned missions
like ExoMars are of a great importance for the future planning of manned mission
in the interplanetary space and on the surface of Mars.
The most recent investigations during the sojourn to Mars and on Mars
surface performed aboard of NASA Mars Science Laboratory (MSL, Curiosity) by
RAD instrument estimate a total mission dose equivalent of ∼1.01 Sv for a round
trip Mars surface mission with 180 days (each way) cruise, and 500 days on the
Martian surface for the 2012–2013 weak solar maximum [23,24 ]. The bigger part
of that dose (662 ±108 mSv), which approaches two-thirds of the career exposure
limit recognized at NASA to carry a 3% increased risk of fatal cancer at the
upper 95% conﬁdence level, would be accumulated during the cruise phase. That
dose is also two-thirds of the career exposure limit recognized at Russia, ESA and
Canada. It was noted that only about 5.4% of the contribution to the estimated
total dose equivalent from both GCR and SEP events of 466 ±84 mSv during the
253 days MSL’s cruise to Mars was due to SEPs and it was surmised (given the
relatively low activity proﬁle of solar cycle 24) that the SEP contribution could
have been many times larger had it been measured in a diﬀerent time frame.
Since 1989 the Liulin type dosemeters-spectrometers have been conducting
measurements of the radiation environment characteristics on board a number of
manned and unmanned spacecrafts in low Earth orbits or in the interplanetary
space . New Liulin type instrumentation will be used for radiation investiga-
tions during the upcoming ExoMars missions to Mars.
3. ExoMars mission. ExoMars is a joint investigation of Mars to be car-
ried out by ESA and Roscosmos that has two launches foreseen, in 2016 and
2018. Establishing if life ever existed on Mars is one of the outstanding scientiﬁc
questions of our time. To address this important goal the ExoMars programme
has been established to investigate the Martian environment and to demonstrate
new technologies paving the way for a future Mars sample return mission in
488 J. Semkova, T. Dachev, St. Maltchev et al.
the 2020’s (http://exploration.esa.int/mars/46048-programme-overview).
Planned for launch in 2016, its ﬁrst element, the Trace Gas Orbiter (TGO) satel-
lite, will spend at least one Martian year orbiting the planet.
Two dosemeters and dosemetric experiments are envisaged for the ExoMars
mission. The ﬁrst one is the charged particle telescope and the experiment Liulin-
MO for measurement the radiation environment on board the ExoMars 2016
TGO. Liulin-MO is a part of the Fine Resolution Epithermal Neutron Detector
(FREND) on TGO. FREND instrument will measure thermal, epithermal and
high energy neutrons with energies ranging up to 10 MeV, whose variations are
an excellent signature of H bearing substances presence in the Mars regolith at
up to 1 m depth (http://l503.iki.rssi.ru/FREND-en.html). The FREND’s
dosemeter module Liulin-MO is another important part of the system providing
information for the radiation environment during the cruise stage and in Mars’
orbit. The second envisaged experiment is the Liulin-ML experiment for inves-
tigation of the radiation environment on Mars surface. The experiment will be
conducted with the Liulin-ML dosemeter as a part of the active detector of neu-
trons and gamma rays (ADRON) on the Russian surface platform for ExoMars
4. Liulin-MO and Liulin-ML scientiﬁc objectives. The main goal of
the Liulin-MO and Liulin-MO dosemetric experiments is investigation of the ra-
diation conditions in the heliosphere at distances from 1 to 1.5 AU from the Sun
and on Mars. More detailed objectives are to provide during the cruise phase, in
Mars orbit and on Mars surface:
•Measurements of the energy deposition spectra, dose rate and particle ﬂux
that allow calculation of the absorbed dose D. Measurements of the LET
spectra in silicon, that allow assessment of LET in water LET(H2O) and
then calculation of the radiation quality factor Qaccording to the Q(L)
relationship given in ICRP-60 , where Lstays for LET:
Q(L) is related functionally to the unrestricted LET or LET(H2O) of a
given radiation. The quality factor (Q) describes the diﬀerent biological ef-
fectiveness of the various radiation types. The biologically signiﬁcant dose
equivalent His obtained as the absorbed dose is weighted by the corre-
sponding quality factor H=D×Q.
•Estimate the contribution of electrons, protons, heavier ions of GCR and
SEP as well as of secondary charged particles and γradiation in the ab-
sorbed dose and the dose equivalent composition.
•Investigate the journal and seasonal variations of the radiation characteris-
tics on Mars’ surface.
Compt. rend. Acad. bulg. Sci., 68, No 4, 2015 489
The data from Liulin-MO on TGO and Liulin-ML on the ExoMars 2018
surface platform will allow comparison of the orbital and surface Mars radiation
The combined data from the dosemeters and neutron detectors of FREND
and ADRON will allow assessment of both the charged particle and the neutron
ﬂuxes and doses over broad energy ranges during periods of quiet Sun and during
Data obtained will serve for veriﬁcation and benchmarking of the radiation
environment models and assessment of the radiation risk to the crew members of
future exploratory ﬂights.
An additional goal of the Liulin-MO and Liulin-ML experiments is to increase
the accuracy of the neutron measurements by providing information about radi-
ation ﬂuctuations from charged particles that can have an impact on the signals
from the neutron detectors of the FREND and ADRON instruments.
5. Liulin-MO and Liulin-ML description. The Liulin-MO ﬂight and
spare models have been developed and space ﬂight qualiﬁed. Liulin-MO is a fur-
ther development of the Liulin-5 and Liulin-Phobos particle telescopes already
ﬂown in space [25, 27–30]. The Liulin-MO particle telescope contains two dosemet-
ric telescopes – D1&D2, and D3&D4 arranged at two perpendicular directions.
The functional diagram of the instrument is shown in Fig. 1. Every pair of the
dosemetric telescopes consists of two 300 µm thick, 20 ×10 mm area Si PIN pho-
todiodes. The detectors D1 to D4, the charge-sensitive preampliﬁers – shaping
ampliﬁers CSA1 to CSA4, the threshold discriminators and the voltage bias cir-
cuits are mounted in a separate detector’s volume inside the box of the Liulin-MO
instrument and are connected to printed circuit boards that contain pulse height
Fig. 1. Functional diagram of Liulin-MO particle telescope
490 J. Semkova, T. Dachev, St. Maltchev et al.
Fig. 2. Block diagram of Liulin-MO
analysis circuits, DC-DC converters, microcontrolers, and interfaces to FREND.
The entire package Liulin-MO has a mass of 0.7 kg and consumes 2.2 W. All
major units of Liulin-MO are duplicated in order to increase the reliability of
the dosemeter. The block diagram is shown in Fig. 2. All detectors D1 to D4
and their electronics, one microcontroler MC1 or MC2 and one power supply
unit operate at every moment. The picture of the FREND accommodation on
the TGO (Credit: Thales Alenia Space – France http://exploration.esa.int/
mars/48523-trace-gas-orbiter-instruments/?fbodylongid=2217) is shown
in Fig. 3A. FREND instrument with mounted on it Liulin-MO is shown in
Fig. 3B. The external view of Liulin-MO is shown in Fig. 3C.
The main measured parameters are the amplitudes of the voltage pulses
at the CSA1–CSA4 outputs (see Fig. 1). The amplitude of a voltage pulse is
proportional to the energy deposited in the corresponding detector by a particle
or a photon crossing the detector, and to the respective dose. By an 8-bit ADC
these amplitudes are digitized and organized in a deposited 256 channels energy
spectrum for every one of the detectors.
The gains of Liulin-MO preampliﬁers CSA1 to CSA4 are a compromise be-
tween the conﬂicting goals of measuring gamma rays, electrons and high-energy
protons (which have very low LET and hence require high gains) and covering the
HZE spectrum (which requires low gains to measure highly ionizing particles such
as iron). As a result of the compromise one of the detectors in every telescope
measures and provides the energy deposition spectrum in the range 0.1–18 MeV
(detectors D2 and D3), and the other in the range 0.4–190 MeV (detectors D1
and D4). The energy deposition spectra of D2 in the range 0.1–18 MeV and
of D1 in the range 18.1–190 MeV are later summarized and used to obtain the
energy deposition spectrum in the direction of D1–D2. The same procedure is
used to obtain the energy deposition spectrum in the direction of D3–D4. In
that way each dosemetric telescope provides data in the energy deposition range
Compt. rend. Acad. bulg. Sci., 68, No 4, 2015 491
Fig. 3. A) FREND accommodation on the TGO (Credit: Thales Alenia Space – France http://
B) the FREND instrument with Liulin-MO mounted on it; C) external view of Liulin-MO
A coincidence technique for the associated with every dosemetric telescope
electric signals is applied to obtain the linear energy transfer (LET). The energy
deposition spectra measured in D1 and D2 detectors in coincidence mode are
recorded separately and used to obtain the LET spectrum in the direction of D1–
D2. That LET spectrum consists of low and high LET parts. The low LET part
is obtained from the D2 coincidence spectrum in energy range 0.1–18 MeV and
the high LET part is obtained from the D1 coincidence spectrum in energy range
18.1–190 MeV. Similarly the energy deposition spectra measured in the D3 and
D4 detectors in coincidence mode are recorded separately and used to obtain the
LET spectrum in the perpendicular D3–D4 direction. In addition the instrument
measures the energy deposition spectrum in D3 in coincidence with D2, allowing
492 J. Semkova, T. Dachev, St. Maltchev et al.
estimation of the dose rate and particle ﬂux in the D3–D2 direction as well. In
that way the output parameters of Liulin-MO are provided simultaneously for
The energy deposition is converted to energy lost per unit of path length
(dE/dx) in silicon as:
(2) dEi/dx =ELi/hD,
where dEi/dx (keV µm−1) is the energy lost per unit of path length in silicon
in channel i,ELi(keV) is the energy deposition in channel i,hD(µm) is the
thickness of the corresponding detector.
A constant factor is applied to relate dE/dx in silicon to LET in water. Ap-
plying the same average value of the ratio dE/dx in silicon and LET in water
of 1.6, with an associated uncertainty of ±15% for GCRs like for RAD instru-
ment , LET for water LET (H2O) is then found by the relation:
(3) LET (H2O) = dE/dx
By deﬁnition the dose D(Gy) is the energy in Joule deposited in 1 kg of
matter. The absorbed dose in a silicon detector DSi (Gy) is calculated by dividing
the summarized energy deposition in the spectrum in Joules to the mass of the
detector in kilograms:
(4) DSi =
where ELiis the energy deposition in channel iin Joule, MD is the mass of
the detector in kg. The energy deposition in MeV is proportional to the voltage
amplitude Aiof the pulses in channel iand respectively to the spectral channel
number i:ELi(MeV) = niAi(V)/k (V MeV−1), where niis the number of
pulses in channel i,k(V MeV−1) is a coeﬃcient dependent on the preampliﬁer’s
sensitivity. k1 (J) is a coeﬃcient converting the spectral channel number to the
energy deposited by a single particle or a photon.
niiproportional to the ﬂux and to the dose are
calculated for each detector from the corresponding energy deposition spectrum
measured for a given time and provided in the Liulin-MO output data.
The parameters provided by Liulin-MO are: absorbed dose rate in the range
10−7–0.1 Gy h−1; particle ﬂux in the range 0–104particle cm−2s−1; energy de-
position spectrum in the range 0.1–190 MeV; LET(H2O) spectrum in the range
0.2–395 keV µm−1. The above parameters are provided for three directions si-
multaneously. The dose rates and the ﬂuxes are resolved every minute, while
Compt. rend. Acad. bulg. Sci., 68, No 4, 2015 493
the energy deposition spectra and the LET spectra are resolved every hour. The
telemetry data rate is 250 kByte/day.
The dosemeter Liulin-ML for ExoMars 2018 will be similar to Liulin-MO
dosemeter developed for ExoMars 2016 mission.
6. Liulin-MO calibration. The Liulin-MO dosemeter has been electroni-
cally calibrated using electrical test pulses through a small capacitor (1–2 pF) to
inject a test charge into the input of each CSA. The amplitude Aiof the voltage
pulses at the output of each CSA was measured and the corresponding spectral
channel iwas obtained. In that way the value of k1 in (4) that converts the
spectral channel number to the deposited energy by a single particle or a photon
was obtained for each of the Liulin-MO detectors.
Liulin-MO has been physically calibrated using standard γ-sources. Figure 4
presents the calibration diagram that shows the dependence of dose rate measure-
ments in detector D3 (in relative units) on the dose rate of γ-sources used (dose
rate is estimated at the centre of the parallelepiped formed by all four detectors).
The linear approximation of that dependence is also shown. Those calibrations
conﬁrm the large dynamic ranges of the ﬂux (up to 104particle cm−2s−1) and the
dose rate (10−7–0.1 Gy h−1) measurements that allow Liulin-MO to measure the
ﬂuxes and dose rates both of the relatively low-intensity GCR and the occasional
high-intensity powerful SEP events. The instrument has also enough sensitivity
to measure the natural radiation background on the Earth surface that is used to
control the proper operation of its detectors during the pre-ﬂight tests.
7. Conclusions. A new charged particle telescope Liulin-MO for measuring
the radiation environment on board the ExoMars 2016 Trace Gas Orbiter satellite
as a part of the Fine Resolution Epithermal Neutron Detector (FREND) has been
Fig. 4. Calibration curve of dose rate measurements in detector
D3. The measured dose is marked with circles. Dashed line with
triangles is the linear approximation
494 J. Semkova, T. Dachev, St. Maltchev et al.
developed, calibrated and space ﬂight qualiﬁed. The dynamic range of the LET
spectrum (in H2O) measured by Liulin-MO 0.2–395 keV µm−1allows for the
assessment of the contribution to the absorbed dose and to the dose equivalent
of the electrons, protons and the high energy particles in the heavy ion cosmic
component including highly ionizing particles such as iron, and the secondary
charged particles and gamma radiation. The large dynamic ranges of the ﬂux
(up to 104particle cm−2s−1) and the dose rate measurements 10−7–0.1 Gy h−1
allows Liulin-MO to measure the ﬂuxes and dose rates both of the relatively
low-intensity GCR and the occasional high-intensity powerful SEP events. The
launch of the ExoMars mission to Mars is planned for the beginning of 2016. A
similar Liulin-ML experiment and instrument for investigation of the radiation
environment on Mars as a part of the active detector of neutrons and gamma
rays (ADRON) on the Russian surface platform is proposed for ExoMars 2018
mission. The charged particle telescopes will be used in combination with the
neutron detectors of FREND and ADRON to study the radiation conditions
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Space Research and Technology Institute
Bulgarian Academy of Sciences
∗Space Research Institute
Russian Academy of Sciences
∗∗State Scientiﬁc Center of Russian Federation
Institute of Biomedical Problems
Russian Academy of Sciences
496 J. Semkova, T. Dachev, St. Maltchev et al.