**MArtian Radiation EnvIronment Experiment (MARIE)**

Gautam D. Badhwar

Mail Code SN

Earth and Solar System Exploration Division

NASA Johnson Space Center, Houston, Texas 77058-3696

Phone: 281-483-5065 Fax: 281-483-5276 Email:
Gautam.d.badhwar1@jsc.nasa.gov

## Abstract

Space radiation presents a very serious hazard to the crews of
interplanetary human missions. The two sources of this radiation are the
galactic cosmic rays (GCR) and solar energetic particle (SEP) events.
The GCR provides a steady source of low dose rate radiation that is
primarily responsible for stochastic effects, such as cancer, and can
also affect the central nervous system. The dose contribution is
inversely correlated to the solar activity, the higher the activity the
lower the dose rate. The SEP events, on the other hand, are directly
correlated with solar activity, are a sporadic source of high dose rate
radiation, which can lead to acute effects, such as nausea and even
death. The radiobiological effectiveness (RBE) of GCR particles, due to
the contribution of high charge (Z) component, is nearly twice as large
as that from the solar events, which are mostly protons. Nuclear
interactions of these components with the Martian atmosphere produce
substantial flux of neutrons with high RBE. Because of the much higher
energies of GCR particles compared to SEP, they are difficult to shield
against, and for a given level of acceptable risk to the crew, small
uncertainties in the knowledge of radiation received by the crew, lead
to nearly exponential increase in the required shielding mass. The
uncertainty in the knowledge of many fragmentation cross sections and
their energy dependence required by radiation transport codes,
uncertainties in the ambient radiation environment, and knowledge of the
Martian atmosphere, lead to large enough uncertainties in the knowledge
of calculated radiation dose in both free space (cruise phase), in
Martian orbit, and on Martian surface. Direct measurements of radiation
levels, the relative contributions of protons, neutrons, and heavy ions,
and Martian atmospheric characteristics is thus a pre-requisite for any
human mission. A set of two spectrometers, one for the Lander and
another for the Orbiter, were proposed for these studies. The Orbiter
spectrometer is designed to measure the energy spectrum from 15 to 500
MeV/n, and when combined other space based instruments, such as the
Advanced Composition Explorer (ACE), would provide accurate GCR spectra.
Similarly, observations of solar energetic particles will be combined
with observations at different points in the inner heliosphere from, for
example, the Solar Heliospheric Observatory (SOHO), to gain information
on the propagation and radial dependence in the Earth-Mars space. The
Lander spectrometer was designed to measure the absorbed dose rate, dose
equivalent dose rate, and the linear energy transfer (LET) spectra with
the capability of separating the relative contribution of these
quantities due to protons, neutrons, and high Z particles. However,
since the Mars'01 Lander mission was dropped, such information cannot be
obtained till some future flight. We plan to compare the Orbiter
measurements with the best available radiation environment and transport
models to both improve these models for subsequent use, and to provide
key inputs for the engineering of required spacecrafts to better protect
the human crews for interplanetary missions for space radiation. Studies
of SEP events would be a critical part of the investigation.

**Introduction**

The two sources of radiation in free space, galactic cosmic rays, and
solar energetic particles lead to different results of radiation
effects. The steady low dose rate from GCR particles leads to stochastic
effects and it is the long-term risk that is of main concern. Passage of
heavy nuclei through the brain can lead to central nervous system (CNS)
damage that may result, for example, in memory loss, and impact a human
Mars mission. There are other effects, such as gastrointestinal, that
are also of concern. The GCR contains fully stripped ions from hydrogen
to uranium and span an energy range from \~ 1 MeV/n to 10^20^ eV.
However, the flux of ions above nickel is too small to be of concern and
energies above \~ 10 GeV/n do not contribute much to the radiation dose.
The intensity of GCR is inversely related to solar activity, but this
source of radiation is always present and because of the high energy of
particles, difficult to shield against. Solar energetic particles (SEP)
are sporadic events leading to sharp short-term increase in dose and
dose rate. The Mars'01 mission will be launched around the peak of the
current 24^th^ solar cycle. The analysis of previous SEP data (Feynman
et al., 1990) shows that almost all of the SEP events fall about 3 years
prior and 4 years past the solar maximum in the \~ 11 year solar cycle.
Nymmik (1999) showed that the number of SEP events, N year, with a
fluence, F, greater than 10^5^ protons cm^-2^, is given by: N =
0.3W^0.75^, where W is the smoothed Wolf number. A number of events in
the current solar cycle have been observed (November 1997, July 2000,
and November 2000) in earth orbit and contributed to the exposure of the
astronauts in the Mir and the International Space Station, where the
earth's magnetic shielding provides a great reduction in radiation
exposures. Such events would, of course, generate considerably higher
doses in free space. There are no agreeable radiation exposure limits
for interplanetary missions. Low-earth orbit (LEO) limits are given in
Table 1 and are used for guidance for Mars mission design studies. The
stochastic limits are based on 3% excess mortality due to radiation.

Table 1: Recommended dose equivalent limits (Sv) for low-earth orbit
flights (NCRP-98, 1989)

  ----------------- ----------------- ----------------- -----------------
                    BFO               Skin              Ocular lens

  Career            1-4               6                 4

  Annual            0.5               3                 2

  30 day            0.25              1.5               1
  ----------------- ----------------- ----------------- -----------------

These 1989 developed career limits are age and sex dependent. New
recommendations from the National Council on Radiation Protection
(Sinclair, 2000) would require lowering the career limits by factor of
nearly 2, thus increasing the need for better dose estimation and
radiation protection. The 30-day limits apply to SEP events.

Interest in the likelihood of a human Mars mission in the next decade
has remained high. Studies on the radiation hazard of interplanetary
missions were first carried out by Letaw et al. (1989) and the National
Academy of Sciences (Stafford Report, 1993) reported that radiation
protection posed a significant challenge to the design of such missions.
With significant improvements in the development of models of the
galactic cosmic radiation environment (Badhwar and O'Neill, 1996,
Nymmik, 1996), improvements were made in predicting the dose rate during
the cruise phase (Badhwar, 1994) using the NASA Langley Research Center
developed radiation transport models BRYNTRN and HZETRN (Wilson, 1995).
Simonsen et al., (2000) have, using improved version of these models,
carried out more detailed analysis of expected dose rates for a
potential Mars mission. Table 2 gives the surface exposure and shows
that the dose equivalent is insensitive to the choice of the Mars
atmospheric model.

Table 2: Martian surface radiation exposure at solar minimum.

+-----------------------+-----------------------+-----------------------+
| Atmospheric Model     | Overhead areal        | Annual Dose           |
|                       | density               | Equivalent            |
|                       |                       |                       |
|                       | (g cm^-2^)            | (cSv)                 |
+-----------------------+-----------------------+-----------------------+
| COSPAR high density   | 21.9                  | 9.00                  |
+-----------------------+-----------------------+-----------------------+
| Clancy                | 19.1                  | 9.16                  |
+-----------------------+-----------------------+-----------------------+
| COSPAR Marsgram       | 17.3                  | 9.30                  |
+-----------------------+-----------------------+-----------------------+
| COSPAR low density    | 16.0                  | 9.43                  |
+-----------------------+-----------------------+-----------------------+

A mission with a 1-year transit time and 1.5 year surface stay has been
under consideration as a possible first human flight. Simonsen et al.
using the recently developed concept of a lightweight inflatable
habitable (TransHab) volume calculated the expected exposures on such a
mission. Table 3 gives their results.

Table 3: Dose equivalent (cSv) estimates for a potential Mars mission.
(Simonsen, 2000)

+---------------+---------------+---------+---------------+-----------+
| Source        | 1 yr Transit  | BFO     | 1.5 yr        | BFO       |
|               |               |         | Surface Dose  |           |
|               | Dose          |         | Equivalent    |           |
|               | Equivalent    |         |               |           |
|               |               |         | Skin          |           |
|               | Skin          |         |               |           |
+---------------+---------------+---------+---------------+-----------+
| Solar Maximum | 33.4          | 27.0    | 20.1          | 17.6      |
+---------------+---------------+---------+---------------+-----------+
| Solar Minimum | 93.8          | 72.7    | 46.5          | 40.7      |
+---------------+---------------+---------+---------------+-----------+
| August 1972   | 63.8          | 17.0    | 4.6           | 2.4       |
| SEP           |               |         |               |           |
+---------------+---------------+---------+---------------+-----------+

Uncertainties in the GCR model predictions and errors in the
fragmentation cross-sections and their energy dependence directly impact
the radiation dose and design of an appropriate shielding. A study by
Badhwar et al., (1994) indicated that approximately 17.5 g cm^-2^ of
aluminum equivalent material would be sufficient to provide just enough
shielding during a solar minimum transit phase to stay below the 50 cSv
annual BFO limit. New calculations, however, with improved version of
the HZETRN code show that almost 50 g cm^-2^ is required for the same
level of protection. This is due to the nearly asymptotic response of
dose rate for large aluminum shielding thickness (Simonsen, 2000). Thus,
the new challenge is to choose low atomic weight shielding materials and
to significantly reduce the errors in estimating radiation exposures.

In estimating crew radiation exposures, the contribution of the
secondary neutrons is currently not fully understood and not properly
accounted for. This is a more serious problem with aluminum type
shielding than with carbon based or more hydrogenated materials. It will
definitely be of concern for a Lander mission due to production of
neutrons in the Martian atmosphere and from the albedo neutrons. Since
only an Orbiter instrument will be part of the Mars'01 mission, the
focus will be to determine the galactic cosmic ray energy spectra during
the maximum of the 24^th^ solar cycle, and study the dynamics of SEP
events and their radial dependence, during the cruise phase and during
the orbit phase around Mars.

**[Orbiter Instrumentation]{.underline}**

Two flight instruments, one for the Orbiter and one for the Lander,
based on a common design of the backplane, the central processing unit
(CPU), power supply, and onboard data storage were designed and built.
The Orbiter instrument consists of an energetic particle spectrometer
that can measure the elemental energy spectra of charged particles of
the energy range of 15-500 MeV/n. These limits are charge dependent and
would extend to about 600 MeV/n for iron nuclei. The spectrometer is
mounted on the science deck (Figure 1) with an angular acceptance of 50.
As the spacecraft orbits Mars, the axis of this field of view sweeps a
cone of directions on the sky. During each orbit, the angle between the
axis of the spectrometer's field of view and the mean interplanetary
field direction varies from 90 to 180.

![](media/image1.png){width="5.7in" height="4.275in"}

Figure 1: Location of MARIE at launch configuration

Figure 2 shows a schematic of the proposed spectrometer. It consists of
a set of solid- state detectors and a high refractive index Cherenkov
detector. The basic telescope geometry is defined by two 25.4 x 25.4 x 1
mm thick ion-implanted silicon solid-state detectors A1 and A2 that are
operated at 160 V. In between A1 and A2 there are two 24 x 24 mm
position sensitive detectors PSD1 and PSD2, each with a 24 x 24 mm wire
grid, to define the incident direction of charged particle with respect
to the axis of the telescope. The A2 detector is followed by a set of 5
mm thick lithium-drifted silicon solid-state detectors (B1, B2, B3, and
B4), and a high refractive index Schot-glass Cherenkov (C) detector. The
telescope is built like a personal computer. Each detector has its own
card, with all of the electronics associated with the detector on it,
including a 12-bit analog-to-digital (ADC) converter, and Field
Programmable Gate Array (FPGA). The main power supply is a nominal 28 V
(16 -32) DC Interpoint unit. There is an 80 MB flash memory for data
storage. The CPU board has an Intel microprocessor, and data
communication hardware for transferring data through RS 422 and RS 232
ports. The memory device can store data for more than two weeks of
operations. The instrument is a dE/dx x E telescope for stopping
particles and dE/dx x C telescope for penetrating particles. Following
coincidence rates are also recorded: A1A2, A1A2B1, A1A2B1B2, A1A2B1B2B3,
A1A2B1B2B3B4, and A1A2B1B2B3B4C. The basic trigger is the A1A2
coincidence and requires a proton with energy \>15 MeV. Following such a
trigger all of the detectors are read out.

![](media/image2.png){width="6.747222222222222in" height="3.30625in"}

> Figure 2: Schematic of Orbiter Spectrometer (Dimension in Inches)

Table 4 gives the area-solid angle product for various coincidence rates
assuming an isotropic angular distribution of incident particles. Since
the PSD detectors are slightly smaller than the trigger A detectors,
about 15% of the particles miss these detectors and incident angle
information for such events is lost. These detectors have separate
thresholds for rows and columns.

Table 4: Area-solid product for an isotropic and sea-level muon angular
distribution

+-----------------------+-----------------------+-----------------------+
| Coincidence           | Area-solid angle      | Area-solid angle      |
|                       |                       |                       |
|                       | (cm^2^ sr) --         | (cm^2^ sr)            |
|                       | isotropic             | --sea-level           |
+-----------------------+-----------------------+-----------------------+
| A1A2                  | 1.0                   | 1.0                   |
+-----------------------+-----------------------+-----------------------+
| A1PSD1PSD2A2          | 0.85                  | 0.85                  |
+-----------------------+-----------------------+-----------------------+
| A1A2B1                | 1.0                   | 1.0                   |
+-----------------------+-----------------------+-----------------------+
| A1A2B1B2              | 0.996                 | 0.997                 |
+-----------------------+-----------------------+-----------------------+
| A1A2B1B2B3            | 0.958                 | 0.966                 |
+-----------------------+-----------------------+-----------------------+
| A1A2B1B2B3B4          | 0.907                 | 0.922                 |
+-----------------------+-----------------------+-----------------------+

If two wires have signals above the threshold, the position of each wire
is noted; however, if more than two wires have signals above the
threshold, only the median position of the wires is recorded. In
addition, the magnitude of the total charge from the whole detector is
read-out through a 10-bit ADC. Thus the PSDs not only provide the (x, y)
coordinates of the passage of the particle through the detector, but a
measure of the ionization loss in the detectors also. Figure 3 is an
example of the angular distribution of sea-level muons measured by the
telescope. Based on the 1 mm wire spacing and separation of the two
detectors, the incident angle of the detector can be measured to better
than 3%.

> Figure 4 is a typical example of the pulse height distribution in
> various silicon detectors for sea-level muons. They have the
> characteristic Landau distribution. The width of the distribution
> arises from the quadratic sum of the detector noise, width due to
> fluctuations in the ionization energy loss, the Landau-Vavilov
> distribution, and due to the variation in path length in the detector.

![](media/image3.png){width="4.897222222222222in"
height="3.688888888888889in"}

Figure 3: Comparison of the measured and expected muon angular
distribution.

> ![](media/image4.png){width="5.190277777777778in"
> height="4.454166666666667in"}
>
> Figure 4: Typical pulse height spectra in three of the silicon
> detectors
>
> Using the incident direction derived from the PSD detectors this data
> can be corrected for the acceptance angle leading to better charge and
> energy resolution. Figure 5 shows an example for the sea-level muons.
> The full width at half-maximum for Landau-Vavilov distribution would
> decrease approximately inversely as a function of increasing charge
> and would help in good overall charge resolution of the instrument.
>
> ![](media/image5.png){width="4.879166666666666in"
> height="3.686111111111111in"}
>
> Figure 5: Calculated charge distribution using data for A1 and A2
> detectors for muons (Z=1)

The dose, D, and dose equivalent, H, rates needed for astronaut
exposures are given by:

D = (k/) L J(L) dL

H = (k/) L Q (L) J(L) dL

> Here J(L) is the differential linear energy transfer (L dE/dx)
> spectrum of the incident particles, Q the definition of the radiation
> quality factor as a function of the linear energy transfer and
> multiplier for appropriate definition of the radiation units. Using
> the two trigger detectors A1 and A2 the LET distribution can be
> measured directly assuming that particles traverse the detectors at
> the mean incident angle. A more accurate distribution can be obtained
> using the knowledge of incident angles from the PSDs. Thus, direct
> information for the human exploration missions can be obtained if just
> the two trigger function normally. An example of the LET distribution
> measured with a silicon based LET spectrometer in a 51.65^o^
> inclination orbit Space Shuttle flight (STS-91) around the time of
> solar minimum and comparison with HZETRN model calculations is shown
> in Figure 6.
>
> ![](media/image6.png){width="3.8097222222222222in"
> height="3.7083333333333335in"}
>
> Figure 6: Measured LET spectra and comparison with HZETRN model
> calculations (GCR Component).

Also, using the knowledge of energy loss in all the detectors can
determine the actual energy spectra and more accurate assessment by
individual charges can be made.

Figure 7 plots the sea-level muon spectra in the Cherenkov detector, C,
when the detector is accepting particles from the top and when the
telescope has been inverted. Most of the photons coming out of the
Schot-glass follow the direction of the incident particle. If the
telescope is inverted, the photomuliplier tube can no longer view these
photons. Thus the signal in C is very week. This provides an ability to
distinguish the direction of the high-energy particles.

![](media/image7.png){width="5.18125in" height="4.290277777777778in"}

Figure 7: Pulse height spectra in Cherenkov detector in two
orientations.

# Data Analysis

Data from the spectrometer will be analyzed as follows. The data are
divided into three energy regimes: (1) particles that stop in any
detector from A2 to B4, (2) particles that go through B4 but do not give
a Cherenkov signal, and (3) particles that give a Cherenkov signal. The
angular information from the PSD detector would ensure that the particle
falls in the right geometry.

Case 1: If the range energy relation for protons in silicon is expressed
as a power law in energy with index, n, R = K E^n^, and E the energy
loss in the thin detectors (A1 or A2). If E is the total energy
deposited by the stopping particle, then

\[E x (E-E)\] (Z^2^ M^n-1^)

where Z is the charge number and M is the mass of the incident particle,
quantities that are element and isotope specific.

Thus, for each isotope, there exists a separate hyperbolic curve.
Charge, mass and energy of the stopping particle can be determined.
Figure 8 shows a plot of the energy loss in A1 versus the energy loss in
A2 for protons and helium nuclei from the flight of a similar telescope
(with no PSDs but an anti-coincidence counter).

Case 2: In this case, the particle energy is assumed to be greater than
the energy required to penetrate all of the detectors but less than the
Cherenkov threshold of \~ 180 MeV/n. The measurements of energy loss in
each of the six (and in most cases eight) silicon detectors are compared
with values calculated for a given energy. The energy is varied to find
the best fit by minimizing the merit function:

^2^ (E) = \[E~i~^cal^ - E~i~^obs^\]^2^

where i (=1,6) is the index for the detector and cal and obs refer to
the calculated - and observed-energy loss.

![](media/image8.png){width="4.0680555555555555in"
height="3.942361111111111in"}

Figure 8: Energy loss curves determined for various particles

Case 3: In this case, the light emitted in the Cherenkov detector, C, is
given by:

C = k Z^2^ (1 - ~o~^2^/^2^)

where ~o~ is threshold velocity (=1/real part of index of refraction),
and is the particle velocity and the energy loss, E, is

\(E\) (Z^2^/^2^)

These two equations are solved to obtain the charge and velocity (energy
per nucleon). Using Schot-glass we can cover the energy range from about
180 MeV/n to 500 MeV/n.

Thus, this technique can cover the entire range from the minimum energy
required to form the A1A2 coincidence, to the energy where the Cherenkov
response saturates. In addition, one can construct an integral spectrum
of SEP events, independently, from various coincident rates. An
additional advantage of such an instrument is that using the energy loss
in A1 and A2 and the angle of the incident particle, a *true* LET
spectrum of particles can be determined.

The area solid angle of the proposed instrument would require large time
integration. We plan to use our measurements of protons and helium GCR
particles and correlate them with measurements from the ACE instrument
(at L1) to obtain accurate GCR spectra in the Martian orbit.

These GCR spectra (which arrive isotropically) and the complete
descriptions of the SEP events measured on the Orbiter are the input
spectral functions to radiation transport models to predict the flux,
dose, and LET spectra on the Martian surface. Because the arrival
direction and distribution of SEPs can be measured, proper account can
be taken of the shielding presented by the Martian atmosphere and the
solid planet to different arrival directions. These predicted fluxes,
doses and LET spectra can be compared with measurements made by a future
Lander instrument to determine the radiation transport characteristics
of the Martian atmosphere and provide key radiation information for both
the cruise and landed phases of a future manned interplanetary mission.
Excellent recent predictions have been recently reported by Wilson et
al. (1999) and Reddy (1999).

[Conclusion]{.underline}

The Orbiter spectrometer would provide data on GCR and SEP from the
cruise phase to Martian orbit and in the Martian orbit. The information
can be used for future human missions to Mars.

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