Instrument Overview
   ===================
 
     The Radiation Assessment Detector (RAD) investigation is an
investigation to detect and analyze the most biologically-significant
energetic particle radiation on the Martian surface as a key element of
the Mars Science Laboratory (MSL) mission. Fully characterizing and
understanding the radiation environment on Mars is fundamental to
quantitatively assessing the habitability of the planet, and is essential
for future crewed Mars missions. RAD also addresses significant aspects
of the MSL investigation, including the radiation effects on biological
potential and past habitability, as well as keys to understanding the
chemical alteration of the regolith due to impinging space radiation.
 
 
   Scientific Objectives
   =====================
 
 
  1)  Characterize the Energetic Particle Spectrum on Mars
 
 
  Galactic cosmic rays (GCRs) are high energy (100 MeV/nuc to 10 GeV/nuc
and above) particles thought to be produced by supernovae shocks outside
the heliosphere and are composed of roughly 89% protons, 10% alpha
particles (He), and 1% heavier nuclei (Reedy and Howe, 1999). GCRs are
modulated by the solar wind and the 11-year solar cycle, with roughly 30%
higher flux at solar minimum, and show variability with respect to
elemental composition and energy. Specifically, near solar minimum,
substantially higher fluxes of lower-energy particles can access the
inner heliosphere compared to times near solar maximum. Because of their
high energies and continuous nature, GCRs are the dominant source of
background radiation at the Martian surface, and are responsible for the
production of secondary particles via complex interactions in the
atmosphere and regolith. The radiation dose from these secondary
particles is comparable to that from the primary GCR. The Earth's
magnetic field (magnetosphere) and deep atmosphere (~ 1000 g cm**-2)
effectively shield us from most of the hostile interplanetary radiation
environment. No primary cosmic rays reach the surface of the Earth.
However, this is not the case for Mars. Mars has no significant
magnetosphere and only ~1-2% of the atmospheric mass of Earth. In the
thick terrestrial atmosphere, most of the GCR energy deposition, and
secondary particle production, occurs in the top 20 km of the atmosphere,
while in the thin Martian atmosphere this occurs at ground level.
 
  The secondary particles generated within the atmosphere include
neutrons and gamma rays that, due to their lack of electric charge,
penetrate the remaining column of the Martian atmosphere rather freely.
The gamma rays do not contribute significantly to the radiation dose at
the surface, but the neutrons do.  Also, some secondary neutrons
generated within the regolith backscatter to the surface, where they
contribute to the dose. GCR heavy ions also collide with carbon and
oxygen in the atmosphere and regolith to produce a flux of energetic
charged nuclear fragments at the surface. Some GCR heavy ions, such as C,
N, and O are relatively abundant and have a significant probability to
survive traversal of the atmosphere intact. Despite their relatively
limited range in matter, these particles have high quality factors (a
measure of biological effectiveness) and therefore need to be considered
in radiation risk assessments.
 
  Solar energetic particles (SEPs) are produced by the Sun as a result of
shocks from coronal mass ejections (CMEs) associated with large solar
storms and flares; they are dominantly protons. Although most SEPs have
energies lower than 100 MeV/nuc, the flux of SEPs is highly variable and
can vary by more than 5 orders of magnitude on time scales of hours to
days, (Posner and Kunow, 2003) reaching energies as high as several GeV.
About 140 MeV/nuc of kinetic energy is needed for protons and helium ions
to penetrate the average column depth of the Martian atmosphere. Typical
SEP events produce a flux composed of 98% protons, 1% alpha particles and
1% heavier nuclei (Reedy et al., 2001). Because most SEPs have energies
below 100 MeV, much of their flux does not reach the Martian surface,
although even non-penetrating SEPs can create secondary neutrons that do
reach the surface. Also, some events produce a significant SEP flux at
energies above 100 MeV/nuc. Thus, although sporadic, SEP events may
overwhelm the background GCR radiation at the Martian surface.
 
 
  2) Determine the Radiation Dose Rate for Humans on Mars
 
 
  Presently, no radiation exposure limits are established for surface
Mars missions, but limits for low Earth orbit (LEO) provide a reasonable
baseline from which to compare astronaut safety and risk.  The LEO limits
are classified into short (30-day), annual, and career durations, and are
also a function of the exposed organs.  Astronauts conducting Martian
surface operations would be exposed to continuous GCR radiation, and
potentially large bursts of SEP radiation. Although the GCR flux is less
at solar maximum, the probability of large SEP events is greater, and the
combined dose equivalent can easily approach annual exposure limits for
blood forming organs, particularly at high elevations where the
atmospheric column above is minimal.  Thus, it is critical to quantify
through direct measurement the total radiation environment, including the
baseline GCR flux and the secondaries it produces, as well as the range
of episodic SEP radiation at the surface of Mars well in advance of any
future manned missions in order to properly assess the safety risks and
to develop potential mitigation strategies. RAD will provide the
precursor measurements necessary to fully characterize GCR and SEP
radiation, assess potential risks, and enable mitigation strategies to be
adequately designed in preparation for future manned Mars missions.
 
 
 3) Transport of HZE (Heavier than Helium) Particles through the Martian
    Atmosphere
 
 
  The lack of direct observations necessitates the use of radiation
transport models of the Mars atmosphere (e.g., HZETRN, SIREST).  However,
radiation transport models that provide input to dosimetry models are
static, driven by radiation inputs at the top of the atmosphere and Mars
Orbital Laser Altimeter (MOLA) topographic data only.  Unfortunately,
the true nature of the surface radiation environment is still highly
uncertain.  The model outputs need to be tested (Wilson et al., 1999),
and compared to observational ground truth in order to be validated and
considered complete to the point of ensuring astronaut safety on future
manned missions. Measurements from RAD will be compared to output from
existing models of these interactions.  Disagreement between observations
and model results elucidate weaknesses in the model physics, or the
understanding of the modeled interactions, and will be used as feedback
for improvement.
 
 
  4) Characterize the Radiation Hazard for Extant Life on Mars
 
 
  The radiation hazards for indigenous Martian life forms are unknown,
but most current studies assume that life elsewhere will be based on
polymeric organic molecules (Pace, 2001), and will in an overall sense,
share with terrestrial life the vulnerability to energetic radiation.
Thus the risks to extant organisms are assumed to be analogous to the
risks to future human explorers.  Energetic charged particles ionize
molecules along their tracks.  This ionization creates OH and other
damaging free radicals which can in turn break DNA strands within cells.
Double strand breaks are most significant, as they may be mis-repaired,
leading to mutagenesis. Surviving cells may become cancerous.  While
Martian life may not be based on DNA, most astrobiologists assume that it
will require some system of heredity based on large polymeric organic
molecules.  Thus it will likely have similar vulnerability to energetic
radiation.
 
RAD will quantify the flux of biologically hazardous radiation at the
surface of Mars today, and measure how these fluxes vary on diurnal,
seasonal, solar cycle and episodic (flare, storm) timescales.  Through
such measurements, we can learn how deep life would have to be today for
natural shielding to be sufficient.  This depth can be compared to the
calculated diffusion depth of strong oxidants which will destroy organic
molecules in the near surface environment of Mars today (Bullock et al.,
1994), and thus learn whether radiation or oxidizing chemistry will
determine the minimum depth needed to drill to look for extant life on
Mars today.
 
Much attention has been given to the possibility of life in subsurface
voids (caves) that will be protected from the surface radiation
environment (Boston et al., 1992).  It has been noted that the cave
environments likely to exist on Mars could possibly facilitate the
evolution of macroscopic life in the subsurface, as opposed to merely
microbial life (Boston et al., 2001).  The shielding required to make
such an environment suitable for life will depend on the surface
radiation.  Measurements of the surface radiation will allow us to
determine how deeply buried such voids must be to be safe from the
high-energy radiation environment at the surface.  This, in turn, will
directly impact future strategies involving drilling and digging to
search for subsurface life.
 
While the idea that life exists today on Mars is controversial, the idea
of life on Mars in the past is much less so. The recent discoveries by
the Mars Exploration Rovers (MER) and Mars Express of evidence for
abundant surface liquid water in the past reinforce the widespread view
that Mars, in the past, may have been a habitable planet.  In seeking to
understand the limits of surface habitability in the past on Mars, it is
important to be able to characterize the radiation environment during
past epochs when surface water existed, the climate was more moderate,
and presumably the atmosphere was substantially thicker than at present.
Radiation is an important source of biological mutations, and as such may
have been the dominant source of genetic diversity in the past on Earth
and presumably on any planet (perhaps including Mars) where life is based
on a genetic code (which is part of most definitions of life). How would
the thicker past atmosphere, required for a warm, wet early Mars, modify
the radiation environment?  According to accepted models of atmospheric
evolution, how have the dose rates of radiation capable of doing tissue
damage, and radiation-induced mutation, varied throughout Martian
history?  How would extreme radiation events (solar flares, gamma ray
bursts) have affected evolution of past organic life on Mars?  For any
effort to understand this past radiation environment of Mars, the
starting point must be a more thorough understanding of the role that the
current atmosphere plays in modulating and altering the radiation from
space. Understanding how radiation interacts with the contemporary
atmosphere permits the extrapolation of this interaction with the
ancient, thicker atmospheres.
 
  5) Chemical and Isotopic Effects of Radiation on the Martian Surface
  and Atmosphere
 
  Space 'weathering' is a well-known but fairly poorly understood
phenomenon that alters the chemistry and appearance of the surfaces of
airless bodies (Hapke 2001, Chapman 2004).  It usually consists of two
components, that due to micrometeorite bombardment, and that caused by
the impingement of charged particles on the surface of asteroids and
airless satellites. An enormous fluence of high-energy charged primary
and secondary particles has interacted with the Martian regolith
throughout its history. The annual dose rate at the surface, and in the
first several tens of cm of regolith, is expected to be on the order of
0.1 Gy/year. Extremely large doses accumulate over the eons. There is
thus reason to believe that radiation contributes significantly to the
unique chemistry of the Martian surface.  The unique space weathering on
Mars can only be understood and quantified with direct observations of
energetic particles at the surface.
 
  One of the primary objectives of the MSL mission is to emplace mobile
analytical chemistry instruments at the surface of Mars, including those
that can quantify light elements. A detailed analysis of the makeup of
both bulk rocks and their surfaces will pave the way for a far greater
understanding of the weathering and alteration processes active on Mars.
RAD will supply the basic input to chemistry models that up to now has
been lacking - the space radiation environment of the surface of Mars.
Together with the analytic chemistry experiments on MSL, RAD will provide
real constraints on how primary rocks weather to their current, highly
altered state.
 
 
   Calibration
   ===========
 
  Because RAD has limited storage and telemetry, much of the data is
stored in the form of histograms. These depend on real-time calibration.
The parameters that control this calibration are loaded into RAD via a
configuration table. In addition, the RAD electronics, and some of the
detectors, are temperature sensitive.  Prior to a RAD observation, the
temperature of the detectors is taken and recorded.  The initial
temperature is used to identify an additional onboard table of correction
parameters.  Therefore, observational data is received on the ground
having gone through onboard calibration.
 
 
   Operational Considerations
   ==========================
 
 The RAD experiment requires long time integration to capture the
statistics of GCR radiation and frequent observations to capture the
random and nearly unpredictable SEP events.  A minimum of 15 minutes of
observation per hour every hour is sufficient to achieve RAD science
objectives.
 
 
   Detectors
   =========
 
  Details of the RAD instrument are given in Hassler et al (2012).
RAD's particle detection capabilities are achieved with a solid-state
detector (SSD) stack (A, B, C), a CsI(Tl) scintillator (D), and a plastic
scintillator (E) for neutron detection, as shown below. The D and E
detectors are surrounded by an anticoincidence shield (F), also made of
plastic scintillator. All scintillators are optically coupled to silicon
diodes which convert scintillation light to electrons.
 
 
 
            |------------|	<--Si A Detector
            |            |
            |            |
            |            |
            |            |
            |------------|	<--Si B Detector
            |------------|	<--Si C Detector
           //  /|-----|\ \\     |------------|
          //  / /     \ \ \\
         //  / /       \ \ \\
        //  / /    D    \ \ \\
       //  / /    CsI    \ \ \\
      //  / /             \ \ \\ <-- anti-coincidence wrapper F1
     ||  --------------------  ||
     ||    ----------------    ||
     ||   |   Bicron 432   |   ||
     ||   |   E Detector   |   ||
     ||    ----------------    ||
    |----------------------------|
    |----------------------------|<-- anti-coincidence wrapper F2
 
 
 
 
  The D calorimeter stops protons with energies up to about 95 MeV and Fe
ions up to 270 MeV/n. At high energies, charged particles produce
ionization electrons in the detector material and undergo no significant
change in direction. Thus, the path of energetic ions through RAD is, to
a good approximation, along a straight line. The opening angle of 65
degrees, defined by trajectories that hit both A and B (i.e., the A*B
coincidence), provides for 1 cm^2 sr geometric factor of the instrument.
This gives a fairly narrow distribution of possible path lengths through
the detectors. The shape of the CsI scintillator conforms to the field of
view as defined by the A*B coincidence. Identification of ion species is
achieved with the dE/dx vs. E method (McDonald and Ludwig, 1964).
 
  The high-energy threshold for ions is determined by the trigger settings
for detectors A and B. Particles that penetrate the chain of detectors
ABCD (and possibly E) and leave the system through F2 are accepted and
counted in the 'penetrating particle' histograms. It has been determined
that the A*B trigger can and will be operated at a low threshold so that
RAD efficiently detects minimum-ionizing, singly-charged particles (e.g.,
GCR protons). Limited particle identification at energies beyond ~100
MeV/n is possible for these events. The penetrating particle histograms
extend the energy range of the instrument to accommodate up to ~500 MeV/n
ion spectra with elemental resolution based on multiple dE/dx analysis.
 
  The A detector has outer and inner segments, referred to as A1 and A2
respectively.  During high fluence events such as SEPs, the outer segment
(A1) can be disabled so as not to overwhelm the electronics with a high
event rate.
 
  The use of a scintillator anti-coincidence shield F1 and F2 is a
requirement for neutron detection. Since neutral particles do not
interact with electrons, which are the source for light emission in
scintillator materials, the detection requires other means of energy
transfer.  In the D/E scintillator system, neutrons of 2 to about 100 MeV
are detected indirectly by elastic scattering with protons in the plastic
detector E. Recoil protons carry on average half the kinetic energy of
the incoming neutron. The detector has a 4-pi field of view. High energy
neutrons may cause D/E coincidences. Neutrons that produce very
high-energy recoils can go undetected if the recoil leaves E or D and
strikes the F anti-coincidence shield. This process effectively sets the
upper limit of reasonable neutron detection efficiency at about 100 MeV.
 
  The lower energy range of neutrons detectable by RAD will be dominated
by emission from the RTG that powers MSL.  RAD thresholds will be raised
accordingly. It is not known exactly where these thresholds will be, but
a preliminary test with the RTG at the Idaho National Laboratory suggests
that thresholds will have to correspond to about 4-5 MeV of energy
deposited in both D and E to keep the event rate manageable.
Additionally, when active, the Dynamic Albedo of Neutrons (DAN) Pulse
Neutron Generator (PNG) will provide a source of neutrons.
 
  Gamma-rays are a by-product of high-energy nuclear interactions and can
penetrate the anti-coincidence shield without interacting, in which case
signals can be produced in E and (more likely) D via the photoelectric
effect, Compton scattering, and pair production. The high-Z CsI material
of D has a much higher sensitivity for the detection of gamma-rays than
the plastic of detector E. D can be used as a gamma-ray spectrometer,
albeit with poor resolution compared to the Odyssey Gamma Ray Spectrometer
(GRS) in orbit around Mars, above the detector noise level of several
hundreds of keV. However the RTG will produce a flux of gamma-rays in D at
energies up to a few MeV
 
  Solar flares and coronal mass ejections can accelerate electrons to
energies of several MeV. Observations of relativistic solar electrons are
vital for event onset timing studies. Electrons are low linear energy
transfer (LET) particles with modest ranges in detector materials.
High Energy Telescope with Neutrons (HETn) will measure electrons in
the range from 150 keV up to 15 MeV.  The distinction from ions can
be performed with dE/dx vs. E analysis. A distinct signature in the
detectors comes from positrons, which are by-products of flare processes
in the corona and also produced by interactions of high-energy particles
in the atmosphere. The annihilation of the positron with a detector B
electron generates characteristic 511 keV X-rays. A*B coincidences with
electron-type energy loss signals in coincidence with a light pulse in D
equivalent to the characteristic X-ray energy deposit define a positron
detection.
 
  Quantitative assessment of energy spectra depends on detailed response
functions derived from calibration data and from Monte Carlo
simulations. All particle detectors, and some of the analog electronics,
are contained in the RAD Sensor Head (RSH). Additional analog
electronics, and all of the digital electronics, are contained in the RAD
Electronics Box (REB).
 
 
 
   Electronics
   ===========
 
  The silicon diodes used both for direct detection of charged
particles (A, B, and C) and for detection of scintillation light (D, E,
F) are in all cases connected to charge-sensitive preamplifiers and
shaping amplifiers in the sensor head. These are of a standard design,
optimized for low noise and wide dynamic range. There are seventeen
analog signals at the output of the RSH; these are split into 34
redundant signal pairs at the input of a mixed signal Application
Specific Integrated Circuit (ASIC) known as the VIRENA (Voltage-Input
Readout for Nuclear Applications). The VIRENA provides, for each
channel, an additional amplification stage, two adjustable threshold
comparators, and a peak-hold circuit. The VIRENA is a 36-channel
device, of which 34 channels are used as described above to read out
the 17 RSH signals. The firmware requires that 32 of the 34 channels
be selected to be used in the onboard analysis, i.e., 2 of the 34
channels are not used. The choice of which 32 channels are used is
configurable. Furthermore, depending on which Level 2 (L2) trigger fired
to initiate the event readout(a process explained in more detail below),
different sets of channels may be read out, ranging from a few to the
full 32.
 
  The VIRENA output signals are multiplexed into a single 14-bit
analog-to-digital converter. For events with a valid L2 trigger, the
appropriate set of pulse heights is read out and kept in local memory for
analysis.
 
 
   Triggers
   ========
 
  RAD has two trigger levels, Level 1 (L1) and Level 2 (L2). Level 1
triggers are initiated by the VIRENA 'fast' discriminators. These are
enabled for one channel each for the A1, A2, B, C, D, and E detectors.
When the firmware recognizes an L1 trigger, the 'slow' discriminator
outputs are examined to see if any L2 trigger patterns (which are for the
most part coincidence conditions) are matched. If they are, the pulse
height readout commences according to the readout mask for the specific
L2 trigger that was matched.
 
 
   Real-time Analysis
   ==================
 
  Valid events are analyzed in RAD's Level 3 (L3) firmware. Events are
sorted as to whether they are caused by penetrating particles (those that
go all the way through the RAD detector stack), stopping particles (those
that hit at least A and B and possibly others, but not reaching F2), or
neutral particles (those hitting only D and/or E). All events that meet
selection criteria are entered into the appropriate histogram. A subset
of these are stored in the form of full pulse height records (sometimes
referred to as 'list data' format), but storage space for the latter is
quite limited.
 
 
   Location
   ========
 
  The instrument sits within the body of the rover.  The telescope
extends upward and sits nearly flush with the rover deck.  The telescope
is covered by a kapton window.  The field of view is unobstructed subject
to the variable position of the robotic arm.  RAD is sensitive to the
neutron emissions from the RTG.
 
 
   Operational Modes
   =================
 
  RAD uses a simple operational strategy.  Nominally, the instrument
wakes once per hour, takes a fifteen minute observation, and then returns
to sleep mode.  In the absence of specific changes to this sequence, RAD
will operate in this wake-sleep cycle without commanding.
 
  Upon waking, RAD takes a 'pre-observation' to check if event counts are
above a threshold indicative of a solar event.  In the event of solar
event, the outer ring of the A detector is disabled.