SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting1 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 Gamma-ray Large Area Space Telescope

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting2 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, 28.5 o inclination  Lifetime: 5 years (minimum) GLAST Gamma-Ray Observatory: LAT ~20 MeV and up GBM 20 keV to 20 MeV Spacecraft bus Routine Data LAT GBM

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting3 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). EGRET’s view of the universe, in galactic coordinates.

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting4 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

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting5 GLAST LAT Overview e+e+ e–e–  Si Tracker 8.8  10 5 channels 185 Watts Grid (& Thermal Radiators) Data acquisition 3000 kg, 650 W (allocation) 1.8 m  1.8 m  1.0 m Effective area ~1 m 2 CsI Calorimeter 8.4 radiation lengths 8 × 12 bars ACD Veto Counters Segmented scintillator tiles

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting6 Silicon-Strip Detectors  ~80 m 2 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.  >2000 detectors already procured from Hamamatsu Photonics. Very high quality:  Leakage current < 2.5 nA/cm 2  Bad channels < 1/10,000  Full depletion < 100 V cm square Hamamatsu-Photonics SSD before cutting from the 6-inch wafer. The thickness is 400 microns, and the strip pitch is 228 microns.

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting7 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.

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting8 Electronics Packaging Kapton readout cables. Tested SSDs procured from Hamamatsu Photonics 19 “trays” stack to form one of 16 Tracker modules. Electronics and SSDs assembled on composite panels. 4 SSDs bonded in series. Composite panels, with tungsten foils bonded to the bottom face , Carbon composite side panels Chip-on-board readout electronics modules. Electronics mount on the tray edges. “Tray”

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting9 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. Composite Panel High thermal conductivity transfer adhesive PWB attached by screws Detector Readout IC Machined corner radius with bonded flex circuit.

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting10 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.

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting11 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. SSDs Bias Circuits Tungsten Panel MCM Flexure Mounts Thermal Gasket Bottom Tray

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting12 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.

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting Beam Test—Verify Simulation Model Small-aperture first prototype Operated in a tagged  beam at Stanford Data Monte Carlo Published in NIM A446 (2000), 444.

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting14 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 <10  5 noise occupancy.  Only 200  W of power consumed per channel. Hit efficiency versus threshold for 5 GeV positrons. Operating Point NIM 457, 466, & 474

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting15 Carbon-Composite Mechanical Prototype  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 levels—work is in progress to reinforce the joint. Full module instrumented for thrust-axis vibration Bottom tray panel, electronics side Bottom tray panel, orthogonal side

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting16 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  Final tracker modules completed by September  LAT Integration and Test until mid  Launch in 3 rd quarter of 2006.

SPIE 4784A-35 GLAST LAT Silicon Tracker Robert P. JohnsonSPIE 47 th Annual Meeting17 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.  We’re looking forward to a 2006 launch and a decade of exciting GLAST science!