1. Sabra Djomehri SULI 2007 Stanford Linear Accelerator 8/15/2007
Definition of a Twelve-Point Polygonal SAA Boundary for the GLAST Mission
Sabra Djomehri
SULI 2007
Stanford Linear Accelerator
8/15/2007

2. Overview Background Information Methods used Results of project
Conclusion/Summary + Outlook

3. Background
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 (20MeV-300GeV)
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.

4. Background 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

5. Methods 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 MeV)
PSB97 – more restricted model, maps low altitude proton fluxes with energies MeV
Evaluates SAA boundary taking all points in a grid of longitudes and latitudes having flux > 1

6. Methods
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.

7. Results
The SAA flux region as described by both models

8. Results Two concerns in defining the SAA boundary are:
What happens at solar maximum conditions?
How severe are the shifts over a large period of time?

9. Results 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%

10. Results
Defined 12-point SAA boundary with respect to both models

11. Conclusion/Summary + Outlook
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.

12. Acknowledgments Markus Ackermann (mentor)
Doug Applegate (grad student)
Office of Science, DOE
SULI program coordinators