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

SPIE 4784A-35 GLAST LAT Silicon Tracker

Robert P. Johnson SPIE 47th Annual Meeting 1

GLAST Large Area TelescopeGLAST Large Area TelescopeSilicon-Strip TrackerSilicon-Strip Tracker

Robert P. JohnsonSanta Cruz Institute for Particle PhysicsPhysics DepartmentUniversity of California at Santa Cruz

LAT Tracker Subsystem ManagerRepresenting the LAT Collaboration

[email protected]

Gamma-ray Large Gamma-ray Large Area Space Area Space TelescopeTelescope



Gamma-ray Large Area Space TelescopeGamma-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

Routine Data

LAT

GBM



GLAST Science OpportunitiesGLAST 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.



Pair-Conversion TelescopePair-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



GLAST LAT OverviewGLAST LAT Overview

e+ e–

Si Tracker8.8105 channels185 Watts

Grid (& Thermal Radiators)

Data acquisition

3000 kg, 650 W (allocation)

1.8 m 1.8 m 1.0 m

Effective area ~1 m2

CsI Calorimeter8.4 radiation lengths 8 × 12 bars

ACD Veto CountersSegmented scintillator tiles



Silicon-Strip DetectorsSilicon-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.

>2000 detectors already procured from Hamamatsu Photonics. Very high quality: Leakage current < 2.5

nA/cm2

Bad channels < 1/10,000 Full depletion < 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.



Solid-State AdvantagesSolid-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.



Electronics PackagingElectronics 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.

2592

10,368

342

64834218

Carbon composite side panels

Chip-on-board readout electronics modules.

Electronics mount on the tray edges.

“Tray”



Electronics PackagingElectronics 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.



Readout ElectronicsReadout 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.

24 64-channel amplifier-discriminator chips for each detector layer

2 readoutcontroller chipsfor each layer

Con

trol

sig

nal f

low

Control signal flow

Data flow to FPGAon DAQ TEM board.

Data flow to FPGAon DAQ TEM board.

Control signal flow

Data flow

Nine detector layers are read out on each side of each tower.

GTRC

GTFEGTFE

GTRC

GTRC

GTRC

GTRC

GTRC

9-998509A22

Programmable channel masks and threshold DACs.

Internal, programmable charge-injection system.

Trigger implemented from OR of all channels/layer.



Mechanical StructureMechanical 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

MCMFlexure MountsThermal Gasket

Bottom Tray



PerformancePerformance

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.



1997 Beam Test1997 Beam Test——Verify Simulation ModelVerify Simulation Model

Small-aperture first prototypeOperated in a tagged beam at Stanford

101 102 103 104

Energy (MeV)

0.1

1

10

Con

tain

men

t S

pace

Ang

le (

deg)

68% Containment95% Containment

Data

Monte Carlo

Published in NIM A446 (2000), 444.



Beam Test of a Complete ModuleBeam Test of a Complete ModuleFull-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 <105 noise occupancy. Only 200 W of power consumed per channel.

Hit efficiency versus threshold for 5 GeV positrons.

Operating Point

NIM 457, 466, & 474



Carbon-Composite Mechanical PrototypeCarbon-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



LAT Tracker Status and ScheduleLAT 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.



ConclusionsConclusions

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!

Documents

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