| DESCRIPTION |
Scientific Objectives and Overview
==================================
Mars is substantially exposed to the harshest elements of space
weather. Unlike Earth, which sits inside a protective magnetic field
called the magnetosphere, Mars does not have a global magnetic field
to shield it from solar flares and cosmic rays. Another factor is
the lack of atmosphere. Mars' atmosphere is less than 1% as thick as
Earth. These two factors make Mars vulnerable to space radiation.
The Marie instrument was designed to measure the amount of harmful
radiation in the Mars environment.
The particles which are thought to be most harmful to humans fall
mostly in the energy range of 30 MeV to thousands of MeV per
nucleon. These are the particles with enough energy to damage human
DNA. The MARIE instrument is designed to measure particles in the
range of 15 MeV to 500 MeV/n. The data gathered in several detector
elements is combined to identify the species of the incident
particles and their energies in this range. The MARIE Instrument was
developed by NASA Johnson Space Center. The development process was
a coordinated effort of NASA/JSC, Lockheed-Martin and Battelle.
Battelle developed the CPU, power boards, A detector, B detector and
C detector boards. Lockheed was tasked with development of the
position sensor devices (PSD), the instrument packaging, system
integration, software development and certifying the instrument for
flight. NASA/JSC provided the project management and coordination of
the contractors.
If a particle enters the MARIE detector telescope within the 60
degree cone defined by the A1 and A2 detectors, and has enough
energy to reach the A2 detector, it is considered a coincident
event. On coincident events, all detector boards are polled by the
CPU and the data for this event is recorded. The readout for each
detector records a pulse height that is proportional to the amount
of energy deposited in the detector. The PSD's also record the
position of the strike within the detector.
The minimum proton energy required to form an A1A2 coincidence
corresponds to a proton with range greater than the sum of the
thickness of A1, PSD1, PSD2, and a minuscule part of the A2
thickness. This adds up to 0.374 g/cm2 of Si and corresponds to a
proton energy of 19.8 MeV. So protons above this energy will be
recorded by the telescope; more energy per nucleon is required for
higher-charged particles. If one takes into account the thin
aluminum case that surrounds MARIE, the minimum proton energy is 30
MeV. The angular response functions are calculated for those
particles that give an A1A2 coincidence and also pass through PSD1
and PSD2 detectors, since they are the only particles that can
provide the incidence angle of the charged particle. Note that not
all particles that give rise to A1A2 coincidence pass through PSD1
and PSD2 because the position sensitive detector are slightly
smaller in size. A new particle identification algorithm is being
developed to determine incident angles for particles that miss the
PSDs or have their positions mis-reported by the PSDs. (The latter
are common, owing to spurious detector noise.)
If a particle hits only one of the A-detectors, the event is
discarded because the angle of impact and energy loss in the other
detector boards is not known. Also, any particle entering the bottom
of the telescope will not register an event on the C-detector due to
the directional properties of the C-detector.
The chassis box of MARIE is made from machined aluminum with an
alodine coating. The exterior surfaces are painted white. Input
voltage to MARIE is 28 VDC and power requirements are 3 watts for
survival mode and 7 watts for nominal operation.There are no
external controls. All control is from the orbiter through an RS-422
interface.
Calibration
===========
Data obtained during the cruise phase of the 2001 Mars Odyssey
mission have been used to calibrate the data. Pulse height spectra
in the range 0 to 4096 have been scaled to yield distributions of
apparent charge, Z. Calibration factors for each detector were
determined by forcing the obvious high-energy proton peak in each
distribution to have its center at Z = 1.
Operational Considerations
==========================
During Odyssey's daily DSN session, MARIE is off for 1-2 hours,
causing small gaps in coverage. When the recorded data volume
approaches the capacity of MARIE's local storage, data acquisition
is halted until the next download opportunity. When all data have
been downloaded, the storage area is erased and the instrument
reset. This sequence of events causes relatively long outages, on
the order of 1 to 2 days.
Detectors
=========
Each of the two A detector assemblies contains a 25.4 x 25.4 x 1 mm
ion-implanted silicon solid state detector, detector signal
amplifiers, detector high voltage supply and the interface circuitry
between the detector and the MARIE CPU. The MARIE CPU controls the
interface circuitry including high voltage control, collecting
digitized signal amplitude data and controlling signal coincidence
timing sources. The two A-detectors are used to define a coincidence
event. These detectors are operated near 160 V.
Each of the four B-detector assemblies contains a 5 mm thick
lithium-drifted silicon solid state detector, detector signal
amplifiers, detector high voltage supply and the interface circuitry
between the detector and the MARIE CPU. The MARIE CPU controls the
interface circuitry including high voltage control and collecting
digitized signal amplitude data. These detectors are operated near
350 V.
The C detector consists of a Schott-glass Cherenkov detector and a
Hamamatsu photo multiplier tube (PMT). When a charged particle with
a velocity greater than [velocity of light / glass refractive index]
hits the Cherenkov detector, the detector releases a photon light
burst proportional to the energy of the particle which struck it.
The photo multiplier tube receives the light pulse and translates it
into an electronic pulse which is amplified by the tube and read by
the electronics on the C-detector board. The C-detector assembly
contains the PMT, detector signal amplifiers, detector high voltage
supply and the interface circuitry between the detector and the
MARIE CPU. The MARIE CPU controls the interface circuitry including
high voltage control and collecting digitized signal amplitude data.
Each of the two position sensitive detector (PSD) assemblies
contains 25.4 X 25.4 mm position sensitive detector. These are
double-sided silicon strip detectors with 24 strips on each side,
with a 1 mm pitch. The strips on one side are oriented so as to be
orthogonal to the strips on the other side. The active area of these
detectors is 24 mm x 24 mm. Hits from all four strip planes define
the particle's incident angle.
From the front of the device, particles entering the detector pass
through detector A1, PSD1, PSD2, A2, B1, B2, B3, B4, and C.
Depending on the angle of incidence and scattering within the
detector, some of the downstream detectors (B's and C) may be missed
on any given event.
Electronics
===========
The Central Processing Unit (CPU) board has an Intel 80C188
microprocessor, detector interface circuitry and data communication
hardware for transferring data to the spacecraft from the 80 MB
flash memory. The flash memory holds the program code and any data
which has not been transferred to the spacecraft. The power from the
spacecraft is nominally 28 volts. The Marie instrument has
Interpoint DC-DC converters to convert the power to a usable level.
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 power, mode control and data download of the MARIE instrument
are controlled by the Odyssey spacecraft. Commands are sent from the
ground to the spacecraft central processing unit (CPU) to power on
MARIE and to change modes.
Location
========
The MARIE instrument frame is illustrated by this diagram:
_______________ HGA
\ /
.. `._________.'
Science || ._______________.Science deck
Orbit || | ^+Xsc |
Velocity || | | |
^. || | | ^+Xmarie .' MARIE FOV
`. || | |+Zsc /|| .' (68 deg cone)
`. ||@| <-----o ..'._|_. .'
|| +Ysc / | | |.'
|| | _.' <----o o--------> MARIE FOV
|| | _.' +Ymarie _.`. boresight
Solar || ..'_____________. `.
Array .. Bottom Deck `.
`.
/
/ -------->
/ Aerobraking
V Nadir Velocity
Actual keywords defining MARIE instrument frame and incorporating
MARIE mounting alignment information are provided in reference [1].
The MARIE FOV (field of view), as defined in [2], is a 68-degree
cone centered around the -Y axis of the MARIE instrument frame.
The set of keywords in the data section above defines MARIE FOV
as a circle with a half-angle of 34 degrees and boresight direction
along the -Y axis of the MARIE instrument frame.
The following data for the FOV geometry were extracted from the
SPICE instrument kernel for MARIE, provided by the NAIF Node of
the Planetary Data System [3]. (The text of this section also was
adapted from that SPICE kernel.) These data are included here for
the benefit of those familiar with the use of SPICE kernels.
INS-53040_FOV_FRAME = 'M01_MARIE'
INS-53040_FOV_SHAPE = 'CIRCLE'
INS-53040_BORESIGHT = (
0.0000000000000000
-1.0000000000000000
0.0000000000000000
)
INS-53040_FOV_BOUNDARY_CORNERS = (
0.0000000000000000
-0.8290375725550400
+0.5591929034707500
)
Operational Modes
=================
The instrument has only two modes, Science Mode and Survival Mode.
Science Mode: When placed in Science mode, the MARIE acts as an
autonomous data acquisition device. Data is collected until the
spacecraft issues a mode change command to move to survival mode.
Survival Mode: From survival mode, the spacecraft can issue commands
to download data, change parameters, power down or return to
Science Mode. During the data download, the spacecraft controls
the download process and downlinks the data to the ground.
Measured Parameters
===================
The detector is composed primarily of three types of silicon
detectors: the A detectors, which are square in cross-section (25.4
mm on a side) and 1 mm in depth; the B detectors, circular, 63.5 mm
diameter and 5 mm thick; and the PSDs, or position-sensitive
detectors. The PSDs are square double-sided strip detectors with 24
1 mm strips on each side (the strips on one side are orthogonal to
those on the other side), and have a thickness of 0.3 mm. There are
two A detectors, A1 and A2; sandwiched in between them are PSD1 and
PSD2; behind A2, there are the B detectors, B1 through B4.
Downstream of B4 is a circular piece of quartz, 10 mm thick, that
radiates photons (Cerenkov radiation) generated by the passage of
high-velocity particles through it. The photons are reflected by a
45 deg mirror into a photo multiplier tube that sits out of the path
of particles that hit the detectors.
MARIE is triggered by a coincidence of hits in detectors A1 and
A2. Once triggered, the data acquisition system records 12-bit
digitized outputs which are proportional to the energies deposited
in the A and B detectors. A two-byte data word is stored for each of
these channels. The pulse height from the phototube is similarly
digitized in 12 bits and stored.
Readout of the PSDs is more complex. Each PSD has two orthogonal
sides, referred to as columns and rows. The following description
applies to each side of each detector. Onboard hardware analyzes the
signals from each of the 24 strips and finds the two largest pulse
heights. For each, the pulse height is digitized in 8 bits (256
channels) and stored, along with the strip number. The largest pulse
height and position are referred to as 'event1', the second-largest
as 'event2.' The event2 data are usually noise. Four quantities are
stored for each side of each detector, so that a total of sixteen
words (thirty-two bytes) of PSD data are stored on each event. The
eight-bit pulse heights are referred to as 'magnitudes', the
positions are valid only when in the range 1 to 24.
References
==========
1. M'01 Frames Definition Kernel (FK), latest version as of March 6,
2001
2. 'MARIE ICD', MSP01-98-0016, June 23, 1999
3. MARIE Instrument Kernel (TI), March 6, 2001
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