SpacePart Conference

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GLAST Large Area Telescope Silicon-Strip
Tracker Robert P. Johnson Santa Cruz Institute
for Particle Physics Physics Department University
of California at Santa Cruz LAT Tracker
Subsystem Manager Representing the LAT
Collaboration johnson_at_scipp.ucsc.edu
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Gamma-ray Large Area Space Telescope

• GLAST Mission
• High-energy gamma-ray observatory with 2
  instruments
• Large Area Telescope (LAT)
• Gamma-ray Burst Monitor (GBM)
• Launch vehicle Delta-2 class
• Orbit 550 km, 28.5o inclination
• Lifetime 5 years (minimum)

• GLAST Gamma-Ray Observatory
• LAT 20 MeV and up
• GBM 20 keV to 20 MeV
• Spacecraft bus

LAT
Routine Data
GBM
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GLAST Science Opportunities

• Active Galactic Nuclei
• Isotropic Diffuse Background Radiation
• Endpoints of Stellar Evolution
• Neutron Stars/Pulsars
• Black Holes
• Cosmic Ray Production Sites
• Gamma-Ray Bursts
• Dark Matter
• Solar Physics
• DISCOVERY!
• ?40 increase in sensitivity over the previous
  gamma-ray telescope EGRET on the NASA Compton
  Gamma Ray Observatory (1991).

EGRETs view of the universe, in galactic
coordinates.
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Pair-Conversion Telescope

• Heavy metal foils (e.g. tungsten) convert
  high-energy gamma rays into electron-positron
  pairs.
• Detectors interleaved with the converter foils
  track the charged particles. The gamma-ray
  direction is reconstructed from the tracks.
• A calorimeter absorbs the electromagnetic shower
  and records the gamma-ray energy.
• Veto counters reject background from the
  predominant charged cosmic rays (electrons,
  protons and heavy ions).

Multiple-scattering limits angular resolution
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GLAST LAT Overview
Si Tracker 8.8?105 channels 185 Watts
CsI Calorimeter 8.4 radiation lengths 8 12
bars
3000 kg, 650 W (allocation) 1.8 m ? 1.8 m ? 1.0
m Effective area 1 m2
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Silicon-Strip Detectors

• 80 m2 of PIN diodes, with P implants segmented
  into narrow strips.
• Reliable, well-developed technology from
  particle-physics applications.
• A/C coupling and strip bias circuitry built in.
• gt2000 detectors already procured from Hamamatsu
  Photonics. Very high quality
• Leakage current lt 2.5 nA/cm2
• Bad channels lt 1/10,000
• Full depletion lt 100 V.

8.95 cm square Hamamatsu-Photonics SSD before
cutting from the 6-inch wafer. The thickness is
400 microns, and the strip pitch is 228 microns.
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Solid-State Advantages

• Thin detectors, placed immediately following the
  converter foils to minimize errors from multiple
  scattering.
• Nearly 100 efficiency for MIPs, with very low
  noise
• Tracker can self trigger. No need to be followed
  by additional trigger counters that would
  constrict the field of view.
• Angular resolution is optimized by guaranteeing a
  measurement in the first detector plane following
  the gamma conversion (minimize the lever arm from
  the multiple scattering).
• Very fine segmentation yields detailed
  information near the conversion vertex, to aid in
  rejection of background and identification of
  poorly measured events.
• Fast readout (tens of microseconds) prevents loss
  of data during gamma-ray bursts.
• No consumables except for electrical power!
• Robust and reliable low voltage, no gas system,
  long life.

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Electronics Packaging
Carbon composite side panels
Tested SSDs procured from Hamamatsu Photonics
4 SSDs bonded in series.
19 trays stack to form one of 16 Tracker
modules.
10,368
2592
Electronics and SSDs assembled on composite
panels.
Tray
342
342
18
648
Kapton readout cables.
Electronics mount on the tray edges.
Chip-on-board readout electronics modules.
Composite panels, with tungsten foils bonded to
the bottom face.
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Electronics Packaging

• Dead area within the tracking volume must be
  minimized.
• Hence the 16 modules must be closely packed.
• This is achieved by attaching the electronics to
  the tray sides.
• Flex circuits with 1552 fine traces are bonded to
  a radius on the PWB to interconnect the detectors
  and electronics.

Detector signals, 100 V bias, and ground
reference are brought around the 90 corner by a
Kapton circuit bonded to the PWB.
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Readout Electronics

• Based on 2 ASICs developed exclusively for this
  project
• 64-channel amplifier-discriminator chip (GTFE)
  24 per module.
• Readout controller chip (GTRC) 2 per module.
• Two redundant readout and control paths for each
  GTFE chip (left or right) makes the system
  nearly immune to single-point failures.

• Programmable channel masks and threshold DACs.
• Internal, programmable charge-injection system.
• Trigger implemented from OR of all channels/layer.

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Mechanical Structure

• Carbon-fiber composite used for radiation
  transparency, stiffness, thermal stability, and
  thermal conductivity.
• Honeycomb panels made from machined carbon-carbon
  closeouts, graphite/cyanate-ester face sheets,
  and aluminum cores.
• High-performance graphite/cyanate-ester sidewalls
  carry the electronics heat to the base of the
  module.
• Titanium flexure mounts allow differential
  thermal expansion between the aluminum base grid
  and the carbon-fiber tracker.

Bottom Tray
Flexure Mounts
Thermal Gasket
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Performance

• The LAT silicon tracker performance has been
  studied in several ways
• Detailed Monte Carlo simulation.
• Beam tests and cosmic-ray studies with prototype
  detector assemblies.
• A high-altitude balloon flight.
• Data from the prototypes have been used to tune
  and validate the simulation model.

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1997 Beam TestVerify Simulation Model
Small-aperture first prototype Operated in a
tagged ? beam at Stanford
Data
Monte Carlo
Published in NIM A446 (2000), 444.
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Beam Test of a Complete Module

• Full-scale Tracker module with 51,200 readout
  channels operated in positron, photon, and hadron
  beams at Stanford Linear Accelerator Center.
• The Tracker power, noise, and efficiency
  requirements were met
• 99 efficiency with lt10?5 noise occupancy.
• Only 200 ?W of power consumed per channel.

NIM 457, 466, 474
Operating Point
Hit efficiency versus threshold for 5 GeV
positrons.
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Carbon-Composite Mechanical Prototype
Bottom tray panel, electronics side
Bottom tray panel, orthogonal side

• First full-scale carbon-composite tracked module
  mechanical structure.
• Thermal cycling, vacuum testing, and random
  vibration testing have been carried out at the
  tray and tower-module levels.
• Results were satisfactory except that the joint
  between the corner flexures and the bottom tray
  failed at the highest vibration levelswork is in
  progress to reinforce the joint.

Full module instrumented for thrust-axis vibration
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LAT Tracker Status and Schedule

• January 2002 NASA PDR DOE Baseline Review.
• Present complete the Engineering-Model tracker
  module
• Complete mechanical-thermal module with dummy
  silicon detectors.
• 4 fully instrumented and functional trays.
• Winter 2003 Critical Design Review follows
  Engineering-Model testing.
• First 2 of 18 tracker modules completed and ready
  for qualification testing by the end of 2003.
• Final tracker modules completed by September
  2004.
• LAT Integration and Test until mid 2005.
• Launch in 3rd quarter of 2006.

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Conclusions

• Solid-state detector technology and modern
  electronics enable us to improve on the previous
  generation gamma-ray telescope by well more than
  an order of magnitude in sensitivity.
• The LAT tracker design uses well-established
  detector technology but has solved a number of
  engineering problems related to putting a 900,000
  channel silicon-strip system in orbit
• Highly reliable SSD design for mass production
• Very low power fault-tolerant electronics readout
• Rigid, low-mass structure with passive cooling
• Compact electronics packaging with minimal dead
  area
• We have validated the design concepts with
  several prototype cycles and are now approaching
  the manufacturing stage.
• Were looking forward to a 2006 launch and a
  decade of exciting GLAST science!