GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz 1 GLAST Large Area Telescope: Instrument Design Steven M. Ritz Goddard Space Flight Center LAT Instrument Scientist Gamma-ray Large Area Space Telescope

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

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

GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz 4 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.

GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz 5 Experimental Technique Instrument must measure the direction, energy, and arrival time of high energy photons (from approximately 20 MeV to greater than 300 GeV): - photon interactions with matter in GLAST energy range dominated by pair conversion: determine photon direction clear signature for background rejection  e+e+ e–e– calorimeter (energy measurement) particle tracking detectors conversion foil anticoincidence shield Pair-Conversion Telescope Energy loss mechanisms: - limitations on angular resolution (PSF) low E: multiple scattering => many thin layers high E: hit precision & lever arm high E: hit precision & lever arm must detect  -rays with high efficiency and reject the much higher flux ( x ~10 4 ) 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.

GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz 6 Some Constraints (MSS & IRD) on Instrument Lateral dimension < 1.8m Restricts the geometric area. Mass < 3000 kg Primarily restricts the total depth of the CAL. Power < 650W 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.

GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz 7 Overview of LAT 4x4 array of identical towers4x4 array of identical towers Advantages of modular design. Precision Si-strip Tracker (TKR)Precision Si-strip Tracker (TKR) Detectors and converters arranged in 18 XY tracking planes. Measure the photon direction. Hodoscopic CsI Calorimeter(CAL)Hodoscopic CsI Calorimeter(CAL) Segmented array of CsI(Tl) crystals. Measure the photon energy. Segmented Anticoincidence Detector (ACD)Segmented Anticoincidence Detector (ACD) First step in reducing the large background of charged cosmic rays. Segmentation removes self-veto effects at high energy. Electronics SystemElectronics System Includes flexible, highly-efficient, multi-level trigger. Systems work together to identify and measure the flux of cosmic gamma rays with energy 20 MeV - >300 GeV.

GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz 8 Choice of Detectors TRACKER single-sided silicon strip detectors for hit efficiency, low noise occupancy, resolution, reliability, readout simplicity. CALORIMETER hodoscopic array of CsI(Tl) crystals with photodiode readout ANTICOINCIDENCE DETECTOR segmented plastic scintillator tiles with wavelength shifting fiber/phototube readout for high efficiency and avoidance of ‘backsplash’ self-veto. 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.

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

GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz 10 Benefits of Modularity 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.

GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz 11 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.  Cosmic-ray rejection of >10 5 :1 with high gamma ray efficiency.  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.  Practical scheme for triggering.  Design optimization. Simulations and analyses are all OO (C++), based on GISMO toolkit. Zoom in on a corner of the instrument First TKR module plane module walls scintillators front scintillators gaps, dead areas included The instrument naturally distinguishes most cosmics from gammas, but the details are essential. A full analysis is important. proton gamma ray

GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz 12 X Projected Angle 3-cm spacing, 4% foils, MeV Data Monte Carlo Experimental setup in ESA for tagged photons: (errors are 2  ) GLAST Data Monte Carlo Monte Carlo Modeling Verified in Detailed Beam Tests See NIM A446(2000), 444; and SLAC-Pub 8682 (submitted to NIM)

GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz Beam Test at SLAC 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 Using beams of positrons, tagged photons and hadrons, with a ~flight-size tower, studies of

GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz 14 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.

GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz 15 Evolving understanding of the fluxes, new sources of backgrounds included. Right now: Work is ongoing: update background fluxes and selections. L1T rate estimate being revised. Background Rejection Source% rateAvg L1T Rate [Hz] Chime  albedo 4196 Electron130 Albedo p TOTAL5470 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.

GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz 16 Performance Plots Performance Plots Derived performance parameter: high-latitude point source sensitivity (E>100 MeV), 2 year all-sky survey: 1.6x10 -9 cm -2 s -1, a factor > 50 better than EGRET’s (~1x10 -7 cm -2 s -1 ). (after all background rejection cuts, being updated ) FOV w/ energy measurement due to favorable aspect ratio Effects of longitudinal shower profiling

GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz 17 Instrument Triggering and Onboard Data Flow Hardware trigger based on special signals from each tower; initiates readout Function: “did anything happen?” keep as simple as possible TKR 3 x y pair planes in a row ** workhorse  trigger x x x Instrument Total L1T Rate: Function: reject background efficiently & quickly with loose cuts, reduce computing load remove any noise triggers x x x tracker hits ~line up track does not point to hit ACD tile L3T: full instrument Function: reduce data to fit within downlink Total L3T Rate: complete event reconstruction signal/bkgd tunable, depending on analysis cuts:  cosmic-rays  ~ 1:1 Spacecraft OR (average event size: ~7 kbits) rates are orbit averaged; peak L1T rate is approximately 10 kHz. L1T rate estimate being revised. ** ACD may be used to throttle this rate, if req. Upon a L1T, all towers are read out within 20  s Level 1 Trigger CAL: LO – independent check on TKR trigger. HI – indicates high energy event ground Level 2 Processing Level 3 Processing L2 was motivated by earlier DAQ design that had one processor per tower. Using single-tower info only, background rate reduction was typically a factor 5. On-board filtering hierarchy being redesigned. On-board science analysis: transient detection (AGN flares, bursts)

GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz 18 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.

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

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

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

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

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

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

GLAST LAT ProjectDOE/NASA Review of the GLAST/LAT Project, Feb , 2001 S. Ritz 25 Looking forward to…