1. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 1 GLAST Large Area Telescope: Science Requirements and Instrument Design Concepts Steven Ritz Goddard Space Flight Center LAT Instrument Scientist steven.m.ritz@nasa.gov Gamma-ray Large Area Space Telescope

2. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 2 Outline  Context: flow of requirements, definitions, design drivers  Instrument overview  Simulations  Performance calculation results review and status  Design modifications since PDR  Failure modes modeling and science impact  Summary

3. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 3 Flow of instrument requirements from science goals and requirements was developed as part of the LAT proposal. LAT will meet or exceed requirements in GLAST Science Requirements Document (433-SRD-0001). From Science Requirements to Design

4. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 4 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)

5. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 5 Aside: some definitions Effective area (total geometric acceptance) (conversion probability) (all detector and reconstruction efficiencies) Point Spread Function (PSF) Angular resolution of instrument, after all detector and reconstruction algorithm effects. The 2-dimensional 68% containment is the equivalent of ~1.5  (1-dim error) if purely Gaussian response. The non-Gaussian tail is characterized by the 95% containment, which would be 1.6 times the 68% containment for a perfect Gaussian response. 68%95%

6. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 6 Science Performance Requirements Summary From the SRD: ParameterSRD Value Peak Effective Area (in range 1-10 GeV)>8000 cm 2 Energy Resolution 100 MeV on-axis<10% Energy Resolution 10 GeV on-axis<10% Energy Resolution 10-300 GeV on-axis<20% Energy Resolution 10-300 GeV off-axis (>60º)<6% PSF 68% 100 MeV on-axis<3.5° PSF 68% 10 GeV on-axis<0.15° PSF 95/68 ratio<3 PSF 55º/normal ratio<1.7 Field of View>2sr Background rejection (E>100 MeV)<10% diffuse Point Source Sensitivity(>100MeV)<6x10 -9 cm -2 s -1 Source Location Determination<0.5 arcmin GRB localization<10 arcmin

7. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 7 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 larger (~10 4 :1) flux 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.

8. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 8  e+e+ e–e– Science Drivers on Instrument Design Energy range and energy resolution requirements bound the thickness of calorimeter Effective area and PSF requirements drive the converter thicknesses and layout. PSF requirements also drive the sensor performance, layer spacings, and the design of the mechanical supports. Field of view sets the aspect ratio (height/width) Time accuracy provided by electronics and intrinsic resolution of the sensors. Electronics Background rejection requirements drive the ACD design (and influence the calorimeter and tracker layouts). On-board transient detection requirements, and on-board background rejection to meet telemetry requirements, are relevant to the electronics, processing, flight software, and trigger design. Instrument life has an impact on detector technology choices. Derived requirements (source location determination and point source sensitivity) are a result of the overall system performance.

9. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 9 IRD and MSS Constraints Relevant to LAT Science Performance 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.

10. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 10 e+e+ e–e–  Overview of LAT Precision Si-strip Tracker (TKR)Precision Si-strip Tracker (TKR) 18 XY tracking planes. Single-sided silicon strip detectors (228  m pitch) Measure the photon direction; gamma ID. Hodoscopic CsI Calorimeter(CAL)Hodoscopic CsI Calorimeter(CAL) Array of 1536 CsI(Tl) crystals in 8 layers. Measure the photon energy; image the shower. Segmented Anticoincidence Detector (ACD)Segmented Anticoincidence Detector (ACD) 89 plastic scintillator tiles. Reject background of charged cosmic rays; segmentation removes self-veto effects at high energy. Electronics SystemElectronics System Includes flexible, robust hardware trigger and software filters. Systems work together to identify and measure the flux of cosmic gamma rays with energy 20 MeV - >300 GeV. Calorimeter Tracker ACD [ surrounds 4x4 array of TKR towers]

11. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 11 Tracker Optimization Converter thickness profile iterated and selected. Resulting design: “FRONT”: 12 layers of 3% r.l. converter “BACK”: 4 layers of 18% r.l. converter followed by 2 “blank” layers Large A eff with good PSF and improved aspect ratio for BACK. Two sections provide measurements in a complementary manner: FRONT has better PSF, BACK enhances photon statistics. TKR has ~1.4 r.l. of material. Combined with ~8.4 r.l. CAL provides ~9.8 r.l. total.

12. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 12 Design Performance Validation: LAT Monte-Carlo Model LAT design based on detailed Monte Carlo simulations. Integral part of the project from the start.  Background rejection  Effective area and resolutions  Trigger design  Overall design optimization Simulations and analyses are all C++, based on standard HEP packages. proton gamma ray Detailed detector model includes gaps, support material, thermal blanket, simple spacecraft, noise, sensor responses… Instrument naturally distinguishes gammas from backgrounds, but details matter.

13. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 13 (errors are 2  ) GLAST Data Monte Carlo Monte Carlo Modeling Previously Verified in Detailed Beam Tests 1997 SLAC beam test (photons, positrons) Demonstrate silicon conversion telescope principle Published in NIM A446 High-level performance parameters Detailed detector characteristics (e.g., PSF) (e.g., hit multiplicities) 1999-2000 SLAC beam test (photons, positrons, protons) flight-scale tower Published in NIM A474

14. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 14 LAT Balloon Flight  Help validate the basic LAT design at the single tower level.  Demonstrate the ability to take data in the isotropic background flux of energetic particles in the balloon environment.  Record events for background flux analysis. Purpose of balloon test flight: expose prototype LAT tower module to a charged particle environment similar to space environment and accomplish the following objectives: All Objectives met by Balloon Flight on August 4, 2001 (3 hrs at 38 km float) background event candidate: gamma event candidate: All subsystems performed properly. Trigger rate <1.5 kHz, well below BFEM 6 kHz capability.

15. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 15 Calibration Strategy Every LAT science performance requirement has a defined test. LAT energy range and FOV are vast. Testing will consist of a combination of simulations, beam tests, and cosmic ray induced ground-level muon tests. –beam tests verify the simulation and sample performance space; analysis using the simulation is used to verify the full range of performance parameters. For science performance verification, beam tests can be done with a few towers together. Full-LAT tests are functional tests. 55º, 1 GeV  side view front view See LAT-TD-00440 and LAT-MD-00446

16. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 16 Implemented Maximum Background Fluxes total Integrates to ~10 kHz/m 2 LAT-TD-00250-01 Mizuno et al Note by Allan Tylka 12 May 2000, and presentations by Eric Grove AMS Alcaraz et al, Phys Lett B484(2000)p10 and Phys Lett B472(2000)p215 Comparison with EGRET A-Dome rates provides a conservative ceiling on the total rate. orbit-max fluxes used for trigger rate calculations

17. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 17 Implemented Average Background Fluxes Integrates to ~4.2 kHz/m 2 orbit-avg fluxes used for downlink and final background rejection calculations

18. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 18 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: ** subset of full background rejection analysis, with loose cuts only use quantities that  are simple and robust  do not require application of sensor calibration constants full instrument information available to processors. Function: reduce data to fit within downlink Hierarchical filter process: first make the simple selections that require little CPU and data unpacking. Total L3T Rate: complete event information signal/bkgd tunable, depending on analysis cuts:  cosmic-rays  ~ 1:~few Spacecraft OR (average event size: ~8-10 kbits) **4 kHz average without throttle (1.3 kHz with throttle); peak L1T rate is approximately 12 kHz without throttle and 3.8 kHz with throttle). 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 disengage use of ACD. On-board Processing On-board science analysis: transient detection (AGN flares, bursts)

19. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 19 On-board Filters select quantities that are simple to calculate. Intelligent use of ACD information to preserve acceptance of high-energy events. Filter scheme is tunable. Filters use –ACD info: match simple tracks to selected hit ACD tiles, count # hit tiles at low energy –CAL info: energy deposition pattern consistent with downward- going electromagnetic interactions. –TKR info: remove low-energy particles up the ACD-TKR gap by projecting track to CAL face and selecting on XY position; for very low CAL energy, require TKR hit pattern inconsistent with single prong. filter first designed using LAT simulation/reconstruction, then pieces implemented by FSW group. Now wrapping existing FSW code for use in LAT simulation/recon packages for verification and optimization for science.

20. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 20 On-board Filters Design Results After all selections, average background rate is 17 Hz. chime albedo albedo CRe albedo p  e+e- 5 Hz line 2 Hz line 1 Hz line composition: Additional margin available: much of the residual rate is due to high-energy proton and electron events with CAL E>5GeV -- if apply ACD selections onboard to higher energy, rate can be cut in half (to 8 Hz), with ~5% reduction in Aeff at 10 GeV. 16.5 Hz total rate

21. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 21 Ground-based Background Rejection Analysis Evaluation of the performance parameters (Aeff, PSF, etc.) requires the final background rejection analysis. Different science analyses can optimize final background rejection selections differently. –The most stringent requirement driver is the extragalactic diffuse flux. Exercises the Science Analysis Software infrastructure: –generate 50M background events, with a unique tag for each. Machinery to cull lists of events, single event display, analysis, etc. –Also generate adequate sample of extra-galactic diffuse photon flux and pass it through the analysis chain.

22. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 22 Summary of Ground-Based Background Rejection Analysis The background rejection analysis meets the requirements and is continuing to evolve, becoming more sophisticated, effective, and efficient. Current selections using subsystem info: –TKR: use information from track fits, hit pattern inconsistent with single track at low energy, PSF cleanup cuts, location cuts, loose consistency between track multiple scatter and CAL energy. –CAL: require xtal hit patterns (shower shapes) to be consistent with downward-going EM showers; more precise track-CAL energy centroid matching. –ACD: very low energy events require quiet ACD; hidden shower rejection.

23. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 23 Background Rejection Results Requirement: 100 MeV Residual background is 5% of the diffuse (6% in the interval between 100 MeV and 1 GeV). Important experimental handle: large variation of background fluxes over orbit – compare diffuse results over orbit. Below 100 MeV [no requirement], without any tuning of cuts for low energy, fraction rises to 14%. This will improve. Peak effective area: 10,000 cm 2 (at ~10 GeV). At 20 MeV, effective area after onboard selections is 630 cm 2. Different physics topics will require different (and generally less stringent) background rejection on the ground. Diffuse flux, after cuts, scaled to generated background log(E) corrected (MeV) 100 MeV 1 GeV 10 GeV 100 GeV

24. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 24 Energy Resolution Energy corrections to the raw visible CAL energy are particularly important for –depositions in the TKR at low photon energy (use TKR hits) –leakage at high photon energy (use longitudinal profile) Normal-incidence 100 MeV  (require <10%) uncorrectedcorrected E(MeV) 9% 18% >60° off-axis 300 GeV  (require <6%) corrected E(GeV) 4.3%

25. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 25 PSF 68% containment 95% containment 100 MeV, normal incidence FRONT 68% containment radius: 100 MeV Requirement: <3.5° 3.37° FRONT 4.64° Total 10 GeV Requirement: <0.15° 0.08° FRONT 0.115° Total 95/68 ratio Requirement: <3 FRONT: 2.1 (BACK: 2.6) NOTE: With older version of TKR reconstruction software, the 95/68 ratio is not <3 everywhere (particularly at high energy). This is a software issue, not an instrument design issue. TKR reconstruction software under upgrade to improve hit association and more precise vertexing.

26. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 26 Field of View CAL TKR CAL TKR Defined as the integral of effective area over solid angle divided by peak effective area: FOV LAT cos(  )  0 10 20 30 40 50 60 70 deg Requirement: >2 sr LAT current: 2.4 sr 10 GeV, uniform flux, all cuts 10 GeV, uniform flux, all cuts Favorable LAT aspect ratio provides large FOV

27. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 27 Science Performance Requirements Summary From the SRD: ParameterSRD ValuePresent Prediction Peak Effective Area (in range 1-10 GeV)>8000 cm 2 10,000 cm 2 at 10 GeV Energy Resolution 100 MeV on-axis<10%9% Energy Resolution 10 GeV on-axis<10%8% Energy Resolution 10-300 GeV on-axis<20%<15% Energy Resolution 10-300 GeV off-axis (>60º)<6%<4.5% PSF 68% 100 MeV on-axis<3.5°3.37° (front), 4.64° (total) PSF 68% 10 GeV on-axis<0.15°0.086° (front), 0.115° (total) PSF 95/68 ratio<32.1 front, 2.6 back (100 MeV) PSF 55º/normal ratio<1.71.6 Field of View>2sr2.4 sr Background rejection (E>100 MeV)<10% diffuse6% diffuse (adjustable) Point Source Sensitivity(>100MeV)<6x10 -9 cm -2 s -1 3x10 -9 cm -2 s -1 Source Location Determination<0.5 arcmin<0.4 arcmin (ignoring BACK info) GRB localization<10 arcmin5 arcmin (ignoring BACK info)

28. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 28 Updates to the Simulation/Reconstruction Simulation and reconstruction packages completely rewritten since PDR: –support flight, with more robust architecture –very large effort, validation proceeding –re-evaluating instrument performance using new reconstruction. Some major improvements already apparent: 100 MeV, normal incidence (<5º), front: 68% containment – 2.4º 98% containment – 7.1º VERY PRELIMINARY –support for misalignments –hooks into hardware databases (bad channels, etc.) and ability to introduce hardware failures easily Now wrapping onboard filter into recon to have high-fidelity assessment of performance (expected complete July) Support for science verification and calibration planning (see I&T talk) Support for Data Challenges (end-to-end testing, see SAS status talk)

29. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 29 Design Modifications Since PDR Most changes are in the mechanical/thermal system and are outside the instrument FOV. Negligible impact on particle propagation. Deleted geographic information in TKR trigger signal (used only for optional ACD hardware throttle). Now using much simpler throttle, with whole tower-tile associations. –Very small impact on instrument triggered FOV when throttle is turned on: –Even smaller after analysis cuts cos  65° 45°

30. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 30 Failure Modes Impacts on Science Performance Support for reliability calculations and design evaluation Can analyze identical events with/without failure turned on. Difficult part is putting awareness into the reconstruction (as we would if failure really happens). Highest priority has been ACD: loss of a single tile. Potential impact on trigger rates and downlink volume – could we operate? Needed primarily for micrometeoroid shield reliability requirements. TKR: impact on trigger efficiency of loss of a layer within a tower: thick converter layer: 0.6% loss of triggered Aeff thin converter layer: <0.1% loss of triggered Aeff Effects of loss of a tower

31. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 31 Impact of ACD Tile Failures YES Main issue here is the micrometeoroid shield reliability analysis. Can we tolerate loss of a single tile? YES DamageAfter L1T throttle After filters [Hz]After simple mitigation [Hz] none1.8 kHz17N/A center top tile1.9 kHz2719 corner top tile1.9 kHz2417 Simple mitigation when a top tile is non-functional: require found track NOT to start in silicon layer directly underneath. Loss of effective area is small. Additional actions available, if needed.

32. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 32 Impact of Loss of a Tower Three kinds of towers, by symmetry –corner, edge, core Requirement: <20% degradation in 5 years  peak Aeff>6,400 cm 2 ; FOV>1.6 sr Use simulation to study loss of a TKR tower (#6, core) on triggered Aeff: –1 GeV normal incidence: 6% loss –1 GeV 40° off axis: 4% loss Difficult to estimate all impacts accurately – an actual loss will trigger more intense study of mitigations. Rough estimates of impact: corner: peak Aeff ~9,000 cm 2, FOV ~ 2.4 sr edge: peak Aeff ~7,500 cm 2, FOV ~2.1 sr (asymmetric) core: peak Aeff ~7,500 cm 2, FOV ~1.9 sr (asymmetric)

33. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 33 LAT Has Three Tower Location Types corner coreedge Loss of a corner is not as bad as loss of an edge is not as bad as loss of a core. Simplifying assumption: “tower loss” means both CAL and TKR. ACD is still intact.

34. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 34 Summary LAT will meet or exceed science requirements from the SRD. Much work done over the past year on simulation and reconstruction. Moving forward: –first filter integration complete: July –new instrument response functions, using improved reconstruction algorithms: September (collaboration science meeting, data challenge start) In addition to design optimization and performance evaluation, detailed simulation is being used as a system engineering tool, e.g., supporting FMEA. See following talks for design details.

35. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 35 Backup Slides

36. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 36 EGRET A-dome Rates (from D. Bertsch, EGRET team) A-dome has an area of ~6 m 2, so orbit max rate (outside SAA and no solar flares) corresponds to ~16 kHz/m 2 This represents a conservative upper-limit for us, since the A-dome was sensitive down to 10’s of keV. Note peak rate is at (24.7,260) SAA

37. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 37 Onboard Filter Development Filter designs done with the full simulation and ground-based reconstruction, in consultation with FSW group. Demonstration of principles, included in science performance evaluations. FSW implemented most of the filter design for benchmarking on the flight processor. –filtering is hierarchical. Most important to implement the selections that are run first (highest rate, largest multiplier on CPU demand). More cycles/event available for remaining event sample after each step. FSW implementation is being wrapped for inclusion in the simulation/recon packages. –very early functional testing of the flight algorithms, with high fidelity. Examine details (e.g., existing track finding) using full set of SAS tools, event display, etc. –detailed evaluation of the filter effects on the science performance –opportunity for a tuning iteration and optimization of the final set of selections

38. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 38 Summary of Filter and Status Primary InfoDesign SelectionFSW Status ACDTile counts (energy dependent)DONE ACD-TKRTrack match with tileDONE CALSimple energy selectionsDONE CALLayer ratiosDONE CALSimple topologies TKR-CALTrack match with energy centroid TKRSkirt only cutDONE TKRSimple hit pattern inconsistent with single prong at low energy TKR-CALMinimal #tracks and CAL E, or make additional demands DONE TKREarth direction TKRTKR hits consistent with a track near CAL if E>0 DONE

39. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 39 Detector Choices TRACKER single-sided silicon strip detectors for high hit efficiency, low noise occupancy, resolution, reliability, readout simplicity. Noise occupancy requirement primarily driven by trigger. 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 (0.9997 flows from background rejection requirement) 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.

40. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 40 SRD LAT Requirements (I)

41. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 41 SRD LAT Requirements (II)

42. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 42 SRD LAT Requirements (III)

43. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 43 Testing Trigger Efficiencies Two kinds of inefficiencies Test this on the ground in LAT using cosmic-ray induced muons LOCAL at least 2 methods to measure: spatial distributions of L1T’s compare TKR hits with TKR trigger pattern using both TKR triggers CAL-LO triggers (independent sample!) GLOBAL example: global trigger drops every 10 th trigger. How would we know? Two types of global inefficiencies: Time-dependent monitor pulsar fluxes over time Constant At least 3 methods to measure: count prescales periodic triggers (both hard and soft) use a sensor with a counter independent of trigger system (e.g., ACD tile) to generate heavily prescaled triggers.

44. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 44 On-board Science Requirements Requirements on transient recognition and localization onboard are modest. Requirements specify the characteristics of the flux (#photons, energy, time period) to be detected: –10 arcmin(3 arcmin goal) GRB localization accuracy onboard for any burst that has >100 detected photons with E>1 GeV arriving within 20 seconds. –5 second (2 second goal) notification time to spacecraft relative to time of detection of GRB – NO requirements on AGN flare detection onboard in SRD. NO mission minimum.

45. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 45 Broad Implications for Onboard Reconstruction With 100 photons, the mean per-photon pointing error for E>1 GeV onboard is comfortably LARGE. Rudimentary gamma direction reconstruction onboard is anyway needed for earth albedo gamma rejection. The crude precision needed to meet the SRD requirements is not a driver. The expected (non-transient) gamma rate (E>20 MeV) is a few Hz; the residual rate for downlink (independent of science analysis onboard) is ~30 Hz. Recognizing that >100 photons with E>1 GeV have arrived within a 20 second interval from one direction should not require precision reconstruction or background rejection better than that needed to meet telemetry requirements. Simple (low resource) prototype algorithms exist, but not yet in FSW environment. Attempt first to reuse filter’s track finding once embedded into recon environment to study performance.

46. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 46 Additional Goals (no requirement) AGN flare localization 2 x 10 -6 ph cm -2 s -1 (E > 100 MeV) that last for more than 1000 secs. For these flare parameters, notify spacecraft <1 min after recognition of flare. This recognition and localization goal is not easy: Assuming 30 Hz event rate (residual photon candidate rate after standard onboard filter), on average there will be ~5 photons per square degree bin in the FOV after 1000s. Change in flux corresponds to ~5 detected photons. => Meeting the goal is not possible without additional onboard filtering (additional factor 10 needed --- not crazy to consider). This is a GOAL, and is not a driver. First approach should be to extend GRB onboard analysis to longer timescales and see how well we do. A simple energy cut will also help here. The science justifies making an effort! However, it is lower priority than other onboard software tasks.The science justifies making an effort! However, it is lower priority than other onboard software tasks.

47. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 47 On-board Filters select quantities that are simple to calculate and that do not require individual sensor calibration constants. Filter scheme is flexible – current set is basis for flight implementation. order of selections to be optimized. Grouped by category for presentation purposes: –ACD info: match track to hit tile, count # hit tiles at low energy Background 100 MeV  outside tile boundary no tile hit inside tile boundary Rate after ACD selections is 180 Hz orbit-avg (360 Hz orbit-max) [cm]

48. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 48 On-board Filters (II) –CAL info: most of the residual rate at this point is due to albedo events and other upward-going energy events. Require track-CAL energy centroid loose match, fractional energy deposit in front layer reasonably consistent with downward EM energy flow. If no CAL energy, require track pattern inconsistent with single-prong. –TKR info: low-energy particles up the ACD-TKR gap easily dealt with: project track to CAL face and require XY position outside this band; for low CAL energy, require TKR hit pattern inconsistent with single prong. X (cm) Y(cm) Rate after CAL selections is ~80 Hz orbit-avg (130 Hz orbit-max)

49. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 49 Effects of Rocking: Albedo Gammas cos (  ) full background flux L1T with Throttle frontback albdeo gamma As we rock, the spike spreads in  : At zenith, earth horizon is at 113 degrees. Study what happens when observatory rocks to 35 and 60 degrees off zenith. 35 degree rock60 degree rock Front Back cos(   

50. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 50 Albedo Gamma Rates L1T rate [Hz]L1T rate with Throttle [Hz] After filters [Hz] After fiducial cut [Hz] zenith25019022 (no cut) rock 35°26020033 (no cut) rock 60°30025081 (<45°) 3 (<53°) Notes: rates for other backgrounds will be reduced somewhat by the same angle cut, not taken into account here. small incremental L1T rate not a problem calculating the gamma direction only happens at a relatively low rate, if needed (after other filters), so incremental CPU load not a problem. can reduce the downlink contribution to whatever we need with a tighter fiducial cut.

51. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 51 Pointing Knowledge  Context: requirements and their origins  Overview of pointing knowledge elements  Details  Experimental technique  Intrinsic LAT measurement characteristics  LAT mechanical/thermal distortion effects  Observatory/instrument coordinates to sky coordinates  Resulting pointing knowledge and source localization  Calibration techniques and stability timescales  Precision of corrections and uncorrectable effects  Sequence of calibrations on orbit  Required supporting ground-based calibrations

52. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 52 Context: Requirements Summary Observatory pointing knowledge: 10 arcsec –The single photon direction measurement ability of the LAT is not very precise, on average, but the best photon LAT will ever measure will have an error of ~ 30 arcsec. –The pointing knowledge should be better than the best single photon error to avoid throwing away information. The single photon accuracy does not, by itself, set the scale of the pointing knowledge, however, since LAT will measure many photons from each source. (see following slide on localization) The mission assigned suballocations as follows: –LAT internal: 7 arcsec –Spacecraft (both GN&C and thermal/mechanical): 6 arcsec –Calibration residual: 4 arcsec –Jitter: 1 arcsec

53. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 53 LAT Source Localizations Systematics will dominate here figure is from LAT proposal. Will be updated. 10 arcsec, 1  radius SRD 0.5 arcmin 1  radius for 10 -7 source note! There is no critical value (original SRD requirement was a factor 2 more stringent).

54. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 54 Overview of Pointing Knowledge Elements Single photon direction measurement (observatory/instrument coordinates) Experimental technique Intrinsic low-level LAT measurement characteristics High-level LAT measurement characteristics Internal LAT mech/thermal effects Single photon direction measurement (sky coords) Source localization Correction algorithm Uncorrected effects Ancillary packet info LAT-SC calibration

55. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 55  e+e+ e–e– Experimental Technique and Science Drivers on Instrument Design Energy range and energy resolution requirements bound the thickness of calorimeter Effective area and PSF requirements drive the converter thicknesses and layout. PSF requirements also drive the sensor performance, layer spacings, and drive the design of the mechanical supports. Field of view sets the aspect ratio (height/width) Time accuracy provided by electronics and intrinsic resolution of the sensors. Electronics Background rejection requirements drive the ACD design (and influence the calorimeter and tracker layouts). On-board transient detection requirements, and on-board background rejection to meet telemetry requirements, are relevant to the electronics, processing, flight software, and trigger design. Instrument life has an impact on detector technology choices. Derived requirements (source location determination and point source sensitivity) are a result of the overall system performance.

56. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 56 Tracker/Converter Issues Expanded view of converter-tracker:  X Y X Y X Y At low energy, measurements at first two layers dominate due to multiple scattering At 100 MeV, opening angle ~ 20 mrad At higher energies, more planes contribute information: Energy# significant planes 100 MeV2 1 GeV ~5 10 GeV >10 Some lessons learned from simulations All detectors have some dead area: if isolated, can trim converter to cover only active area Low energy PSF completely dominated by multiple scattering effects:  0 ~ 2.9 mrad / E[GeV] (scales as (x 0 ) ½ ) High energy PSF set by hit resolution/lever arm ~1/E Roll-over and asymptote (  0 and  D ) depend on design E PSF At low energy, direction measurement is dominated by planes closest to conversion point.

57. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 57 Intrinsic LAT Measurements Characteristics Low-level –Strip pitch: 228  m –Plane spacing: ~3 cm –Tower size: ~60 cm tall, ~37 cm wide –Geometric characteristics: hit resolution/plane spacing: ~2 mrad (410 arcsec) hit resolution/tower height: 0.1 mrad (21 arcsec) –thus, on average, a tower rotation of 7 arcsec will have a very small impact on the strip hit distribution in a single event. stack diagonal angle: 32 degrees High-level –Photon direction determined by reconstruction software algorithms. Different categories of events and corresponding measurement quality. Kalman filter is used. Optimal algorithm to handle both multiple scattering and geometric effects. At low energy, first measurements dominate due to multiple scattering. Even for cross-tower events, it is the measurements in the tower containing the conversion that dominate the direction measurement. At high energy, it is the tower with the largest track path length that dominates the direction measurement. To good approximation, the individual tower misalignment is also the photon location misalignment. This also represents the pessimistic case.

58. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 58 Overview of Pointing Knowledge Elements Single photon direction measurement (observatory/instrument coordinates) Experimental technique Intrinsic low-level LAT measurement characteristics High-level LAT measurement characteristics Internal LAT mech/thermal effects Single photon direction measurement (sky coords) Source localization Correction algorithm Uncorrected effects Ancillary packet info LAT-SC calibration

59. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 59 Converting to Sky Coordinates Information from ancillary data packet –Attitude information is conveyed to LAT from spacecraft every 200ms. Includes both attitude and angular velocities. maximum angular velocity is 30 deg/minute, or 360 arcsec/update, so velocity info is necessary with a timestamp better than ~0.5ms. OK. angular acceleration terms are negligible (<0.25 arcsec) LAT-SC calibration residual –The error from the calibration is carried forward, but can be updated over time (see later slide on on-orbit calibrations).

60. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 60 Calibration Techniques and Stability Timescales Internal LAT alignment using cosmic rays (straight trajectories) –standard technique for particle trackers –no external measurements or references needed –ground: muons from cosmic ray airshowers –on-orbit: proton cosmic rays –statistics set by detail needed –individual sensors require higher statistics. Stable. –tower-to-tower lower statistics needed. Varies on thermal timescales. LAT to Observatory –done on orbit. Observe known sources. Precision/statistics source-dependent. Brighter sources provide ~10 arcsec localization knowledge in ~1 month. –after initial alignment observation, all-sky survey data can be used for continuous update/refinement => effects that are slow compared to ~2 months are automatically calibrated out.

61. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 61 Sequence of On-Orbit Calibrations LAT SVAC Plan (LAT-MD-00446). First, LAT internal alignment using CR’s. Then, initial two-week observations to know LAT-SC alignment to better than 15 arcsec (easily sufficient for most science topics). –optimization of initial observing strategy (source selection, optimal orientation, etc.) under investigation. Then, proceed with sky survey and use known sources to reduce the error over year 1.

62. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 62 Supporting Ground Measurements (I) Captured in LAT survey plan, SVAC plan, I&T survey procedures documents. Demonstration of technique: –introduce misalignments into the simulation and reconstruction –implement muon flux to simulate the measurements, with LAT pointed horizontally. Remaining systematic offsets under study, but are already sufficiently small. The ground-based tower-to-tower alignment information doesn’t carry forward significantly to flight. Launch loads will change the alignment, and it will anyway be re-established using cosmic rays on orbit.

63. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 63 Supporting Ground Measurements (II) The main purpose of the ground-level muon measurements is to test functionality and as part of the mechanical/thermal alignment requirements testing and analysis. LAT ground-level muon measurements will be made with LAT Z-axis parallel to the ground (“sideways”). Verified rate of tracks crossing tower pairs with the simulation: tower pairs Current estimate: sufficient statistics in one day to do tower-to-tower verification measurement. Good match to LAT thermal timescale. 

64. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 64 Alignment Summary Single photon direction measurement (observatory/instrument coordinates) Experimental technique Intrinsic low-level LAT measurement characteristics High-level LAT measurement characteristics Internal LAT mech/thermal effects Single photon direction measurement (sky coords) Source localization Correction algorithm Uncorrected effects Ancillary packet info LAT-SC calibration

65. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 65 Loss of corner tower easiest case to estimate Aeff loss ~10% FOV loss very small. corner coreedge good experimental handle on-orbit: can pretend any other working corner tower is off and check for incremental background leakage and other systematic effects.

66. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 66 Loss of single edge tower corner closest to dead edge tower also significantly impacted FOV. For purposes of conservative estimate, assume only 3x4 LAT left. Aeff loss ~25% FOV loss ~10%. corner coreedge good experimental handle on-orbit: can pretend any other working edge tower is off and check for incremental background leakage and other systematic effects.

67. GLAST LAT ProjectDOE/NASA CD3-Critical Design Review, May 12, 2003 S. Ritz Document: LAT-PR-01967-01Section 03 Science Requirements and Instrument Design Concepts 67 Loss of single core tower most difficult (and painful) case. For purposes of conservative estimate, assume whole quadrant compromised. Looks like 2 LATs, each 1x2, with overlap. Aeff loss ~25% FOV loss ~15%. (~35% loss in 1-dim in the bottom left and top right quadrants) corner coreedge good experimental handle on-orbit: can pretend any corresponding tower is off and check for incremental background leakage and other systematic effects.