GLAST LAT Project Gammaray Large Area Space Telescope

GLAST LAT Project Gamma-ray Large Area Space Telescope DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 GLAST Large Area Telescope: Instrument Design Steven M. Ritz Goddard Space Flight Center LAT Instrument Scientist ritz@milkyway. gsfc. nasa. gov S. Ritz 1

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 From Science Requirements to Design From LAT proposal Foldout D: S. Ritz 2

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Simplified Flow Science Requirements Document (SRD) Mission System Specification (MSS) Interface Requirements Document (IRD) Mission Assurance Requirements (MAR) LAT Performance Specifications Design Trade Study Space LAT Subsystem Requirements S. Ritz 3

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 LAT Performance Specification The LAT PS sets instrument performance requirements on: – energy range and effective area – energy resolution – single photon angular resolution (68% and 95% containment) – field of view – source location determination – point source sensitivity – single-event time accuracy – background rejection – dead time – gamma-ray source transient detection capabilities on board The LAT PS also includes the physical, operational and communications requirements. S. Ritz 4

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Experimental Technique • Instrument must measure the direction, energy, and arrival time of high energy photons (from approximately 20 Me. V to greater than 300 Ge. V): - photon interactions with matter in GLAST energy range dominated by pair conversion: determine photon direction clear signature for background rejection Energy loss mechanisms: - limitations on angular resolution (PSF) low E: multiple scattering => many thin layers high E: hit precision & lever arm Pair-Conversion Telescope anticoincidence shield conversion foil particle tracking detectors e+ S. Ritz e– calorimeter (energy measurement) • must detect -rays with high efficiency and reject the much higher flux (x~104) of background cosmic-rays, etc. ; • energy resolution requires calorimeter of sufficient depth to measure buildup of the EM shower. Segmentation useful for resolution and background rejection. 5

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Some Constraints (MSS & IRD) on Instrument S. Ritz • Lateral dimension < 1. 8 m Restricts the geometric area. • Mass < 3000 kg Primarily restricts the total depth of the CAL. • Power < 650 W Primarily restricts the # of readout channels in the TKR (strip pitch, # layers), and restricts onboard CPU. • Telemetry bandwidth < 300 kbps orbit average Sets the required level of onboard background rejection and data volume per event. • Center-of-gravity constraint restricts instrument height, but a low aspect ratio is already desirable for science. • Launch loads and other environmental constraints. 6

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Overview of LAT • 4 x 4 array of identical towers Advantages of modular design. • Precision Si-strip Tracker (TKR) Detectors and converters arranged in 18 XY tracking planes. Measure the photon direction. • Hodoscopic Cs. I Calorimeter(CAL) Segmented array of Cs. I(Tl) crystals. Measure the photon energy. • Segmented Anticoincidence Detector (ACD) First step in reducing the large background of charged cosmic rays. Segmentation removes self-veto effects at high energy. • Electronics System Includes flexible, highly-efficient, multi-level trigger. Systems work together to identify and measure the flux of cosmic gamma rays with energy 20 Me. V - >300 Ge. V. S. Ritz 7

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Choice of Detectors • TRACKER single-sided silicon strip detectors for hit efficiency, low noise occupancy, resolution, reliability, readout simplicity. • CALORIMETER hodoscopic array of Cs. I(Tl) crystals with photodiode readout for good resolution over large dynamic range; modularity matches TKR; hodoscopic arrangement allows for imaging of showers for leakage corrections and background rejection pattern recognition. • ANTICOINCIDENCE DETECTOR segmented plastic scintillator tiles with wavelength shifting fiber/phototube readout for high efficiency and avoidance of ‘backsplash’ self-veto. S. Ritz 8

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Pre-Proposal Trade Studies Summary S. Ritz 9

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Benefits of Modularity • Construction and Test more manageable, reduce costs and schedule risk. • Early prototyping and performance tests done on detectors that are full-scale relevant to flight. • Aids pattern recognition. • Good match for triggering large-area detector with relatively localized event signatures. Issue: demonstrate that internal dead areas associated with support material and gaps between towers are not a problem. Resolution: Detailed Monte-Carlo model of instrument, combined with beam-test data of prototype hardware, used to validate design performance. S. Ritz 10

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Design Performance Validation: LAT Monte-Carlo Model The current LAT design is based on detailed Monte Carlo simulations. Two years of work was put into this before any significant investment was made in hardware development. Ø Cosmic-ray rejection of >105: 1 with high gamma ray efficiency. Ø Solid predictions for effective area and resolutions (computer models now verified by beam tests). Current reconstruction algorithms are existence proofs -- many further improvements under development. Ø Practical scheme for triggering. ØDesign optimization. Simulations and analyses are all OO (C++), based on GISMO toolkit. S. Ritz gaps, dead areas included Zoom in on a corner of the instrument scintillators front scintillators module walls First TKR module plane The instrument naturally distinguishes most cosmics from gammas, but the details are essential. A full analysis is important. gamma ray proton 11

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Monte Carlo Modeling Verified in Detailed Beam Tests Experimental setup in ESA for tagged photons: X Projected Angle 3 -cm spacing, 4% foils, 100 -200 Me. V Data Monte Carlo GLAST Data (errors are 2 ) Monte Carlo See NIM A 446(2000), 444; and SLAC-Pub 8682 (submitted to NIM) S. Ritz 12

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 1999 -2000 Beam Test at SLAC Using beams of positrons, tagged photons and hadrons, with a ~flight-size tower, studies of • data system, trigger • hit multiplicities in front and back tracker sections • calorimeter response with prototype electronics. • time-over-threshold in silicon • upper limit on neutron component of ACD backsplash • hadron tagging and first look at response S. Ritz 13

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Beam Test Engineering Model Flight-scale tower constructed • Beam test at SLAC in December 1999 • Balloon flight in 01 CAL and TKR have prototype custom electronics, with all functionality for flight demonstrated. S. Ritz 14

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Background Rejection Analysis done thus far for two main reasons: (1) Necessary for realistic estimate of effective area. (2) A proof of principle, demonstration of the power of the instrument design. Capabilities demonstrated. Next: Optimize selections. Some science topics may require less stringent background rejections than others. • Evolving understanding of the fluxes, new sources of backgrounds included. Right now: Source Chime % rate Avg L 1 T Rate [Hz] 36 2019 albedo 4 196 Electron 1 30 Albedo p 59 3224 TOTAL 5470 • Work is ongoing: update background fluxes and selections. L 1 T rate estimate being revised. S. Ritz 15

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Performance Plots (after all background rejection cuts, being updated) FOV w/ energy measurement due to favorable aspect ratio Effects of longitudinal shower profiling Derived performance parameter: high-latitude point source sensitivity (E>100 Me. V), 2 year all-sky survey: 1. 6 x 10 -9 cm-2 s-1, a factor > 50 better than EGRET’s (~1 x 10 -7 cm-2 s-1). S. Ritz 16

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Instrument Triggering and Onboard Data Flow Level 1 Trigger Hardware trigger based on special signals from each tower; initiates readout Function: • “did anything happen? ” • keep as simple as possible x x x • TKR 3 x • y pair planes in a row** workhorse trigger OR • CAL: LO – independent check on TKR trigger. HI – indicates high energy event ground Upon a L 1 T, all towers are read out within 20 ms Instrument Total L 1 T Rate: <5 k. Hz> rates are orbit averaged; peak L 1 T rate is approximately 10 k. Hz. L 1 T rate estimate being revised. **ACD may be used to throttle this rate, if req. S. Ritz Level 3 Processing Level 2 Processing Function: • reject background efficiently & quickly with loose cuts, • reduce computing load • remove any noise triggers • tracker hits ~line up x x x • track does not point to hit ACD tile L 2 was motivated by earlier DAQ design that had one processor per tower. Usingle-tower info only, background rate reduction was typically a factor 5. On-board filtering hierarchy being redesigned. L 3 T: full instrument Function: reduce data to fit within downlink • complete event reconstruction • signal/bkgd tunable, depending on analysis cuts: : cosmic-rays ~ 1: 1 Total L 3 T Rate: <30 Hz> (average event size: ~7 kbits) On-board science analysis: transient detection (AGN flares, bursts) Spacecraft 17

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Pre-Proposal Formal Peer Reviews Subsystem designs and critical technology development decisions are periodically reviewed by independent panels that include members outside of the collaboration. S. Ritz 18

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Flow into Tracker Subsystem Effective Area -ray conversion efficiency Effective Area Knowledge Other material Converter configuration Aspect ratio Charged particle detection efficiency Single photon angular resolution requirements Geometric area Spatial measurement resolution Dead area Ionization measurement Field of view Self-trigger Data noise occupancy Trigger efficiency Bkgd rejection Dead time Trigger noise rate Dead time Total mass Trigger saturation recovery time MSS&IRD requirements Total power Environmental Tel. bndwdth S. Ritz 19

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Flow into Tracker Subsystem Effective Area -ray conversion efficiency Effective Area Knowledge Other material Converter configuration Aspect ratio Charged particle detection efficiency Single photon angular resolution requirements Geometric area Spatial measurement resolution Dead area Ionization measurement Field of view Self-trigger Data noise occupancy Trigger efficiency Bkgd rejection Dead time Trigger noise rate Dead time Total mass Trigger saturation recovery time MSS&IRD requirements Total power Environmental Tel. bndwdth S. Ritz 20

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Flow into Calorimeter Subsystem Energy range Energy resolution Depth Geometric area Passive material Background rejection Spatial resolution of energy depositions Trigger signals Effective area knowledge Data noise occupancy Dynamic range Measurement dead time Dead time Total mass MSS&IRD requirements Total power Environmental Tel. bndwdth S. Ritz 21

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Flow into ACD Subsystem Background rejection Energy range Energy resolution Charged particle detection efficiency Coverage Maximum false veto due to backsplash Noise CNO signals Effective area knowledge Trigger signals Deadtime Dead time Total mass MSS&IRD requirements Total power Environmental Tel. bndwdth S. Ritz 22

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Flow into Electronics Subsystem Energy range Trigger Dead/live time measurement Time accuracy Transient detection and notification Effective area knowledge Data flow, deadtime Command & configuration Transient analysis and notification Environmental & Housekeeping data flow Event filtering Dead time Total mass MSS&IRD requirements Total power Environmental Tel. bndwdth S. Ritz 23

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Flow into Flight Software Energy range Commanding Data flow Transient detection and notification Effective area knowledge Monitoring, reporting Calibration Event filtering Transient detection & onboard science analysis Reprogrammability Dead time Integration and Test Tel. bndwdth S. Ritz 24

GLAST LAT Project DOE/NASA Review of the GLAST/LAT Project, Feb. 13 -15, 2001 Looking forward to… S. Ritz 25