Gamma-ray Large Area Space Telescope

1. Gamma-ray Large Area Space Telescope The Silicon Strip Tracker of the GLAST Large Area Telescope L. Baldini INFN - Pisa Nuclear Science Symposium 2004 Rome October 18, 2004

2. Outline GLAST Burst Monitor (GBM) 10 kev – 25 MeV Large Area Telescope (LAT) 20 Mev – 300 GeV Launch Vehicle Delta II – 2920-10H Launch Location Kennedy Space Center Orbit Altitude 575 Km Orbit Inclination 28.5 degrees Orbit Period 95 Minutes Launch Date February 2007 • Talk overview: • The science case for GLAST. • Instrument design. • Silicon Tracker construction. • Conclusions.

3. A brief history of the g-ray astronomy... • SAS-2 (1972 - 1973) • Energy range: 30 MeV – 1GeV • Energy resolution: ~ 100% • Peak effective area: 100 cm2 • Field of view: 0.25 sr • EGRET (1991 - 1996) • Energy range: 20 MeV – 30GeV • Energy resolution: ~ 15% • Peak effective area: 1500 cm2 • Field of view: 0.5 sr 1970 1980 1990 2000 Time Balloon flights, Small satellites… (… 621 photons above 50 MeV detected by OSO-3!) • COS-B (1975 - 1982) • Energy range: 30 MeV – 5GeV • Energy resolution: ~ 40% • Effective area: 70 cm2 • Field of view: 0.25 sr

4. The need for a high-energy g-ray detector • Broad spectral coverage is crucial for understanding most astrophysical sources. • Multiwavelenght campaigns: space based and ground based experiments cover complimentary energy ranges. • The improved sensitivity of GLAST will match the sensitivity of the next generation of ground based detectors filling the energy gap in between the two approaches. • Overlap for the brighter sources: cross calibration, alerts. • Predicted sensitivity to point sources: • EGRET, GLAST and MILAGRO: 1 year survey. • Cherenkov telescopes: 50 hours observation. (from Weekes, et al. 1996 – GLAST added)

5. The sky above 100 MeV: the EGRET survey • The heritage of EGRET: • Diffuse extra-galactic background (~ 1.5 x 10-5 cm-2s-1sr-1 integral flux). • Much larger (~ 100 times) background on the galactic plane. • Few hundreds of point sources (both galactic and high latitude). • Essential characteristic: variability in time.

6. Point source sensitivity and sky-map • 3rd EGRET catalog: • 271 point sources found. • Based on 5 years of data. GLAST 1 year sky survey: • Will discover thousands of new sources (based on the extrapolation of the number of sources vs. integral flux). • Key points: large FOV and effective area. Integral Flux (E>100 MeV) cm-2s-1

7. Unidentified sources • 170 point sources of the EGRET catalog still unidentified (no know counterpart at other wavelengths). • GLAST will provide much smaller error bars on sources location (at arc-minute level). • GLAST will be able to detect typical signatures (spectral features, flares, pulsation) allowing an easier identification with know sources. • Most of the EGRET diffuse background will be resolved into point sources. • Large effective area and good angular resolution are crucial! Cygnus region: 15o x 15o, E > 1 GeV Counting stats not included.

8. Active Galactic Nuclei • AGNs phenomenology: • Vast amount of energy from a very compact central volume. • Large fluctuations in the luminosity (with ~ hour timescale). • Energetic, highly collimated, relativistic particle jets • Prevailing idea: accretion onto super-massive black holes (106 – 1010 solar masses). • AGN physics to-do-list: • Catalogue AGN classes with a large data samples (at least ~ 3000 new AGNs) • Detailed study of the high energy spectral behavior. • Track flares (t ~ minutes). • Large effective area and excellent spectral capabilities needed!

9. Gamma Ray Bursts • GRBs phenomenology: • Dramatic variations in the light curve on a very short time scale. • Isotropic distribution in the sky (basically from BATSE, on board CGRO, but little data @ energies > 50 MeV). • Non repeating (as far as we can tell…). • Spectacular energies (~ 1051 – 1052 erg). • GRBs physics: • GLAST should detect ~ 200 GRBs per year above 100 MeV (a good fraction of them localized to better than 10’ in real time). • The LAT will study the GeV energy range. • A separate instrument on the spacecraft (the GBM) will cover the 10 keV – 25 MeV energy range. • Short dead time crucial! Simulated 1 year GLAST operation (Assuming a various spectral index/flux.)

10. Requirements on the instrument • High sensitivity, pointing accuracy: • Large effective area. • Large field of view. • Good angular resolution (Point Spread Function). Good spectral capabilities: • Good energy resolution. • Wide energy range. Fast alert: • Short instrumental dead time. Long observation time: • No consumables. Requirements connected with the operation in space: • Modularity, robustness, redundancy. • Severe power and mass budgets.

11.  Anticoincidence shield Conversion foils Particle tracking detectors e– e+ • Calorimeter Experimental technique • The instrument must measure the direction, energy and arrival time of high energy photons (from approximately 20 MeV up to 300 GeV). • Pair production is the dominating interaction process for photons in the GLAST energy range. • e+ e- pair provides the information about the g direction/energy. • e+ e- pair provides a clear signature for background rejection. • Pair conversion telescope: • Tracker/converter (detection planes + high Z foils): photon conversion and reconstruction of electron/positron tracks. • Calorimeter: energy measurements. • Anti-coincidence shield: background rejection (cosmic rays flux typically ~104 higher than g flux).

12.  e– e+ Design drivers • Science drivers • Background rejection: • Drives the ACD design. • Also impact on TKR/CAL design. • Effective area and PSF: • Drive the converter thicknesses and layout. • PSF also drives sensor performance, layers spacing and overall tracker design • Energy range/resolution: • Drive the thickness/design of the calorimeter. • Field of view: • Basically sets the aspect ratio (width/height). • Mission drivers • Allocated space on the launcher: • Forces the maximum possible lateral dimension (and geometric area). • 1.8 m for GLAST. • Power budget: • Restricts the number of readout channels in the tracker (i.e. strip pitch, number of layers). • 650 W for GLAST. • Mass budget: • Basically bounds the total depth of the calorimeter. • 3000 kg for GLAST TKR TKR CAL CAL

13. Overview of the Large Area Telescope • Overall modular design: • 4x4 array of identical towers - each one including a Tracker, a Calorimeter and an Electronics Module. • Surrounded by an Anti-Coincidence shield. • Anti-Coincidence (ACD): • Segmented (89 tiles). • Self-veto @ high energy limited. • 0.9997 detection efficiency (overall). • Tracker/Converter (TKR): • Silicon strip detectors. • W conversion foils. • 80 m2 of silicon (total). • 106 electronics chans. • High precision tracking, small dead time. • Calorimeter (CAL): • 1536 CsI crystals. • 8.5 radiation lengths. • Hodoscopic. • Shower profile reconstruction (leakage correction) Tower DAQ (TEM)

14. Triggering and On-board Data Flow • Level 1 trigger: • Hardware trigger, single-tower level. • Three_in_a_row: three consecutive tracker x-y planes in a row fired. Workhorse g trigger. • CAL_LO: single log with E > 100 MeV (adjustable). Independent check on TKR trigger. • CAL_HI: single log with E > 1 GeV (adjustable). Disengage the use of the ACD. • Cosmic rays in the L1T! 13 kHz peak rate. • Upon a L1T the LAT is read out within 20 ms. • On-board processing: • Identify g candidates and reduce the data volume. • Full instrument information available to the on-board processor. • Use simple and robust quantities. • Hierarchical process (first make the simple selections requiring little CPU and data unpacking). x x x • Level 3 trigger: • Final L3T rate: ~ 30 Hz on average. • Expected average g rate: ~ few Hz (g rate : cosmic rays rate = 1 : few). • On-board science analysis (flares, bursts). • Data transfer to the spacecraft.

15. GLAST vs. EGRET 1After background rejection. 2Single photon, 68% containment, on axis. 31s, on axis. 41s radius, high latitude source with 10-7 cm-2s-1 integral flux above 100 MeV. 51 year sky survey, high latitude, above 100 MeV.

16. g FRONT BACK e+ e- A view on the Tracker • Tracker/converter design determines the PSF: • Low energy PSF completely dominated by multiple scattering effects (~ 1/E). • High energy PSF set by hit resolution/lever arm. • Converter foils layout/detectors design/layers spacing determine the rollover energy and the asymptotic value of the PSF @ high energy. • Final design: • 19 trays structures providing the basic mechanical framework. • 18 x-y detection planes immediately following W converter foils. • Front section: 12 trays with 3% X0 converter. Excellent PSF. • Back section: 4 trays with 18% X0 converter. Increase effective area. • Bottom: 3 trays with no converter at all. • Compact and aggressive mechanics: • Less than 3 mm spacing between x and y layers. • Front end electronics on the four sides of the trays (90º pitch adapters). • Few mm gaps between adjacent towers.

17. The Tracker electronics system • Key features: • Low power consumption (~ 200 mW/channel). • Low noise occupancy (< 10-4 at the single channel level, mainly for the trigger). • Redundant architecture. • Complete digital zero suppression onboard. • Overall design: • 24 custom Front End chips + 2 Readout Controller chips handle one silicon layers. • Data can shift left/right to one of the Controllers (single dead chips can be bypassed without loosing data). • Zero suppression takes place in the controllers (data stream includes IDs of the strips fired + TOT of the layer OR, which also provides the trigger). • Two flat cables per side complete the redundancy.

18. 342 Tracker construction work flow • SSD procurement and testing • Ladders assembly • Towers assembly 18 10,368 342 2592 • Trays assembly and test 648 • Panels fabrication • Readout electronics fabrication, test and burn-in

19. Silicon Strip Detectors procurement/testing • Standard design: • 6’’ wafers size. • 8.95 x 8.95 cm detectors. • 400 mm thickness. • 384 strips, 228 mm pitch. • Reliable, small rate of defects. • Aggressive specifications: • Depletion voltage < 120 V. • Breakdown voltage > 175 V. • Leakage current @ 150 V < 500 nA (< 200 nA averaged over 100 SSDs). • Rate of bad strips < 0.2%. • Procurement and testing: • Hamamatsu Photonics qualified producer. • 11500 SSDs delivered (10368 for 18 towers + spares + wastage). • All SSDs already electrically and geometrically tested. • Electrical test: • Performed at wafer level. • Leakage current: 110 nA on average (~ 1 nA/cm2). • Depletion voltage: 70 V on average. • Electrical test: • Error on wafer cut: ~ 2.5 mm. • 0.5% final rejection rate!

20. Ladders assembly/testing • Ladders assembly: • 4 wafers glued head to head. • Wafers are wire-bonded together as to form an unique 9 x 35 cm detector. • Wire bonds are encapsulated. • Ladders production/testing: • 1330 flight ladders produced (more than 50% of the total production). • 500 ladders under construction. • 1200 ladders geometrically/electrically tested. • ~2% rejection rate (including start-up problems). • 0.016% bad channels caused by wire bonding or probing.

21. Panels production/testing • Bare panels construction: • Aluminum internal honeycomb, carbon-carbon closeouts. • Carbon-fiber face-sheets. • W tiles (converters). • Bias circuits. • Bare panels production/testing: • Final design frozen, flight production started. • ESPI: acoustic excitation + interferometry of laser beams. Resonance frequencies measurement and defects search. • Reliable, non-destructive. Allows early identification of problems before the final tooling.

22. Tray assembly • Trays assembly: • Glue spots deposition with automatic dispenser. • Micro-bonding with automatic wedge bonder. • Good experience with pre-production panels. • 1 assembly chain ready, 5 under construction. • Maximum rate: 15 trays/week. • Foreseen assembly rate: 10 trays/week.

23. Tray testing • Tray testing: • Trays stored in aluminum shipping containers before leaving the production site. • All testing (electrical/functional, thermal cycles) is done without opening the box. • No contamination or accidental mishandling. • Boxes opened right before the tower assembly. • Stacked trays: • Functional tests/CR burn-in for a whole tower in parallel. • External trigger capability.

24. Assembly of tower 0 • The history of tower 0: • Some issues in the production of flight trays recently identified. Few design changes needed. • First full tower (originally ‘Tower A’, then renamed as ‘Tower 0’) assembled anyway. • Not all the trays instrumented with silicon detectors. • All the hardware handled as flight hardware. • E2E test and validation of the assembly procedure. • First data collected in Tower configuration.

25. Assembly of tower 0 • Tray is positioned and the handlers are removed • Trays are slit into the assembly jig by means of service handlers • The stack grows upside-down and is completed with the bottom tray

26. Assembly of tower 0 • Each side is completed with two readout flex cables (to be connected with the TEM) and the tower is covered with a dark box for a CPT.

27. Tower 0 testing • Noise measurement: • Single strip noise occupancy (averaged over each layer) measured as a function of the threshold on discriminators. • Plateau @ high threshold consistent with the expected rate of accidental coincidences with cosmic rays. • Noise occupancy measured for each strip at the nominal threshold. Design specification Nominal DAC setting (1/4 MIP) • Detection efficiency: • Measured by means of external scintillators on the stack. • Detection efficiency (within the active area) found to be typically > 99.5% @ the nominal threshold setting.

28. Assembly of tower 0 (II) • Completion of the assembly procedure: • Sidewalls are put in place (> 700 screws per side!). • Tower craned to the CMM and fixed on the marble for the alignment procedure and verification. • ~200 mm maximum shift in one direction, ~60 mm ‘leaning’ (within specs). • Ready for electrical tests/data taking.

29. First events in tower configuration... XZ view YZ view

30. Online distributions • Online monitoring: • All relevant quantities (hit-map, hit multiplicity, TOT distribution) monitored online for all the layers.

31. Preliminary offline analysis

32. Conclusions • GLAST will survey the sky in the 20MeV~1TeV g-ray band, where the most energetic and mysterious phenomena in nature reveal their signature. • GLAST is equipped with state-of-the-art particle detectors, resulting in an order of magnitude improvement in sensitivity and resolution with respect to previous missions • The GLAST LAT Tracker is the largest Si tracker ever built for space applications (> 10K SSDs, >80 m2, ~106 channels). • A highly modular design was chosen for a simpler, cost-effective and more reliable construction. • Construction is organized in a well-defined sequence of increasingly complex operations; wafers testing completed, ladders assembly well on the way (50% produced and tested). • Final production phase just begun, delivery of the last tower to I&T next year.

34. Extra - Showers

35. Extra - Showers

36. Extra – detection efficiency