Download Definition of a Twelve-Point Polygonal SAA boundary for the GLAST

Definition of a Twelve-Point
Polygonal SAA Boundary for the
GLAST Mission
Sabra Djomehri
SULI 2007
Stanford Linear Accelerator
8/15/2007
Overview
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Background Information
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Methods used
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Results of project
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Conclusion/Summary + Outlook
Background
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Gamma-Ray Large Area Space Telescope (GLAST) – aims
at locating gamma ray sources throughout the universe and
studying their properties.
What makes GLAST so special?
 huge energy range of gamma rays detected (20MeV300GeV)
 more sensitive than previous gamma ray telescopes
 determines positions of gamma ray sources with high
precision.
GLAST’s Large Area Telescope (LAT)
 16 towers, each consisting of a tracker and calorimeter.
GLAST’s Anti-Coincedence Detector (ACD)
 consists of 89 plastic scintillating tiles for the purpose of
vetoing charged particles.
These tiles are sensitive to cosmic rays, but not gamma rays.
Cosmic rays vastly outnumber gamma rays.
Background
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GLAST’s altitude ~ 550 km
This coincides with a region of the
inner radiation belt called the
South Atlantic Anomaly (SAA).
Van Allen Radiation Belts – high
flux regions due to energetic
charged particles trapped by
geomagnetic field.
SAA – region of high flux protons,
portion of inner belt which comes
closest to Earth’s surface.
SAA high flux region degrades
sensitivity of ACD’s PMTs, sensors
must be turned off.
Objective: to define an optimal
SAA boundary
Methods
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Operational Constraints:
 Latitudes from -26 ° to 26°, longitudes from -180° to 180°
 Convex SAA boundary
 12 sided polygon of minimal area
Models of Trapped Radiation:
 AP8 – standard model maps wide range of particle fluxes
(proton fluxes in the energy range 0.1-400 MeV)
 PSB97 – more restricted model, maps low altitude proton
fluxes with energies 18-500 MeV
Evaluates SAA boundary taking all points in a grid of longitudes
and latitudes having flux > 1
Methods
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Convex Hull – the smallest
convex set that includes a
given set of points.
Shape of hull determined by
outer (extreme) points.
QuickHull Algorithm – runs in
O(nk) time.
Determines the lower and
upper hull by triangulation.
Can be used to define the
SAA’s convex hull.
Results
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The SAA flux region as described by both models
Results
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Two concerns in defining the SAA boundary are:
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What happens at solar maximum conditions?
How severe are the shifts over a large period of time?
Results
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Comparison between n-sided/12-sided SAA hull
Area lost < 0.01%
The SAA area relative to the total area of GLAST’s orbit zone is ~12.5%
Results
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Defined 12-point SAA boundary with respect to both models
Conclusion/Summary + Outlook
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Passing through the SAA will degrade ACD
efficiency, so GLAST shouldn’t be in survey
mode when crossing this region.
The determined 12-Sided SAA boundary
encompasses SAA flux regions of both models.
Solar maximum conditions are neglected since
this flux region never exceeds fluxes at solar
minimum.
Outlook – generate an appropriate safety margin
for this boundary.
Acknowledgments
Markus Ackermann (mentor)
 Doug Applegate (grad student)
 Office of Science, DOE
 SULI program coordinators
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