GLAST Large Area Telescope: Status of System-level Performance Simulations

1. Gamma-ray Large Area Space Telescope GLAST Large Area Telescope: Status of System-level Performance Simulations UPDATE (to 26 July presentation) S. Ritz Goddard LAT Instrument Scientist ritz@milkyway.gsfc.nasa.gov Representing the PDR/Baseline simulation group For status see: http://www-glast.slac.stanford.edu/software/PDR/

2. System-level Performance Simulations Outline • Brief review of July 26 presentation, actions • Work done since July • background fluxes settled • large generation runs • trigger rate studies • CAL bug fix, ACD segmentation final update • tools for peeling off events, readback of events • Status of actions • The LIST (what’s left to do now that the tools are in place) Result of much direct work by many people: Toby, Richard, Heather, Sasha, Ian, Karl, Leon, Tracy, Eduardo, Arache, Regis, … and based on the foundation provided by many others.

3. Summary of Requirements We use the simulation to evaluate the expected performance of the instrument design: • Effective Area as a function of energy • FOV (Effective Area as a function of angle) • Energy resolution • PSF (68%, 95%) as a function of energy and angle. Make parameterization for physics studies. • Background rejection (and all of the above after background rejection selections) • Trigger rates and data volume after L1T and L3T • Failure mode effects on performance We use the beam tests and other measurements to verify the simulation.

4. Work done for proposal We did this work for the proposal: Why do it again for PDR/Baseline? quantitative assessment of performance impact of incremental design changes better modeling of backgrounds check results and make improvements. move analysis forward. side benefits: opportunity to use and improve tools. simulation and reconstruction have undergone major architectural changes -- lost some functionality but gained much more solid foundation. this is an opportunity to pull everything back together.

5. PDR Simulation Work Requirements (2/2001) For PDR, we must present the basic performance parameters of the instrument and show they meet the relevant LAT Performance Specifications. The following elements must be in place to begin this evaluation process: • updated, validated, and documented geometry to match the current design, along with configuration control of the design parameters; review noise and threshold parameters; • updated source fluxes, incorporating the improved understanding; • a documented release of the simulation and reconstruction in the new framework, including the new event storage format; • a version of the event display that is compatible with the new simulation, reconstruction, and event storage format; • machinery in place to generate the necessary statistics of signal and background events. We specify a requirement of >10 million background events, with a goal of 100 million background events generated and analyzed. The machinery must include a simple mechanism to identify each event uniquely, and the capability to produce from an event list files with a subset of the full event sample; • an updated version of merit, or an equivalent tool, to run in the new environment. Ideally, this tool could run both on the stored events and on a new standard tuple. Depending on the size of the full events, a standard tuple may be a practical necessity. • a basic analysis framework to operate on the new event stores.

6. CAL Geometry Update • new carbon cell design implemented • detailed description of top and bottom supporting frames • detailed description of cell closeout and electronics compartment at the sides of towers. • all calorimeter dimensions are up-to-date. Support frame CsI cylindrical end supports Work done by Sasha (CAL group)

7. CAL Geometry Update (II) 41.5 mm

8. ACD and GRID Geometry Updates • ACD support structure consists of an approximated core material and two face sheets. • The gap between the tiles and the towers reflects the current design. • Thermal blanket is modeled as it was for the AO, using one average density material. • Back-most side rows now single-tiles NEW • GRID flange between TKR and CAL (treated as a separate volume) is undergoing modification to reflect the current design. • Also adding ACD base frame. Work done by Heather (ACD group) with help from Eduardo

9. MCM Boards tray face, glue Bias board, glue • New Features: • Dimensions correspond to latest design • Better treatment of top and bottom trays • More accurate composite materials • MCM boards included • Better segmentation of tray faces Tungsten Silicon Bias board, tray face, glue TKR Geometry Update closeouts carbon-fiber walls + screws Work done by Leon (TKR group)

10. Level 1 Trigger • TKR 3-in-a-row • CAL-LO = any single log with recorded energy > 100 MeV • CAL-HI = any tower with 3-layers-in-a-row each with >0 logs with recorded energy > 1 GeV (NEW!see LAT-TD-00245-01, 16 July) • L1 Trigger word: ___ ___ ___ ___ ___ <- (LSB) • Will continue to study details of performance of new CAL-HI proposal in this round of simulations. • ACD throttle of L1T now included in tuple. TKR CAL-HI ACD LO CAL-LO ACD (unused) NEW

11. Implementation by source, comparisons and justifications Sanity checking – EGRET A-Dome rates L1T rates Background Fluxes NEW • see • 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

12. New Orbit-max Fluxes (the new tools are great!!) total Integrates to ~10 kHz/m2

13. New Orbit-avg Fluxes Integrates to ~4.2 kHz/m2

14. Sanity Check • The measurements of the flux components may have unsubtracted backgrounds – adding them up could result in double-counting. Also, fluxes below 100 MeV are mostly blind guesses. Need a unitarity check – look at EGRET A-dome rates. • The proposed orbit-max flux amounts to ~10 kHz/m2 • Also sent fluxes to Eric Grove, Ormes and Kamae for comments, and discussed fluxes with experts at Goddard.

15. EGRET A-dome Rates (courtesy of Dave Bertsch) A-dome has an area of ~6 m2, so orbit max rate (outside SAA and no solar flares) corresponds to ~16 kHz/m2 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

16. Orbit Max Rates – Preliminary! • Notes: • as we expected, the unthrottled L1T rate is now > 10 kHz • with the ACD throttle on the TKR trigger, the total max rate is 6 kHz. If the max A-dome rate is due entirely to particles that trigger us (it isn’t), naïve scaling results in an orbit-max throttled rate of 9 kHz. Appears we have some margin. • we have a ceiling, finally.

17. Data set generation planning • Main challenge is generating backgrounds ~ 50M events (~half-hour equiv.) • Would take months to generate on a PC • Use SLAC batch (Linux/Solaris) system, requires 2-3 days • Linux build of pdrApp now running (7/24/01) • Disk space (800 GB fileserver) is in place • DataManager ready to generate and book-keep events • Since early June, we have been generating increasingly larger photon and background data sets: • iterate to find the bugs and missing pieces (inspect sets of events, study distributions, effects of cuts, use tools, …) • complete and exercise the infrastructure • Generated 5M event data set ~1 week ago. Demonstrated. 50M event run gearing up. Using avg flux. [one minor glitch last night] • Exercised tuple preselections and chaining; subset selectivity (remaining backgrounds at any stage of filtering); readback/re-analysis/event display of event subsets last week. Need to finish event history display. NEW NEW

18. Failure Modes Modeling • Now part of our regular planning (see, e.g., May study on using ACD as L1T throttle). Loss of functionality will be handled in the simulation as an “after-burner” analysis on the large data set. Tools to do this not yet tested. Difficult part is putting awareness into the reconstruction (as we would if failure really happens) – results for PDR will be rudimentary. • ACD: loss of a single tile in three locations (front corner, front middle, side). • TKR: impact of loss of a layer on trigger efficiency. • TKR+CAL loss of a tower. • Not possible to explore the infinite combinatorics. Highest priority prior to PDR is to do one tile and 1 TKR layer failure: L1T, L3T, effective area impacts.

19. Actions from 26 July Presentation • “Please make sure low energy positrons (100-300 MeV) are included in a worst-case background analysis.” Done, extended below 100 MeV. • “Please simulate the loss of ½ the light from 18 PMTs on one of the 12 electronics boards.” Withdrawn. • “Please run the simulation with isotropic electrons of 10 GeV and show that no more than 3 in 104 are accepted as gammas, along with the resulting effective area.” On the list. • “Will the trade study to identify potential ACD extension impact the grid design, and will it be complete by September? If not, when? It may impact S/C.” The essential gaps and geometries are now in the new simulation, along with the potentially problematic fluxes. Already see not a likely problem for on-board operations. Answer on ultimate ground-based background rejection analysis will come with the full analysis automatically. First hand-scan of residual events not worrisome, but new cuts are necessary and must be carefully evaluated. The analysis is only part of the issue. The issue should be systematically addressed by the IDT. • Check that the physics of the simulation code to simulate low energy proton interactions. On the list. If this is a problem, it will show up in the analysis of the 50M event run shortly. Tools exist to evaluate.

20. Much Left To Do Re-do overall ground-based background rejection selections validation. Deal with new fluxes. Recalculate Aeff and FOV. Specifically evaluate combined effect of ACD gap and low energy particle fluxes. Demonstrate 10 GeV electron rejection. PSF evaluation vs. energy, angle, front/back. Energy resolution. Finish baselining L3 algorithms Tools now ready. Help is needed. Join the fun.

21. Backups

22. L3 Filtering (orbit max case) Open L1T rate: 13,025 Hz Throttled L1T rate: 5,968 Hz cut at 25 || CAL_Hi chime albeo_p albdeo_g electron albedo e+e- Not checked yet! ACD_DOCA caveat: the current version of ROOT interactive cuts is FLAKY! Beware.

23. L3 Filtering (Orbit Max Case) 72 Hz at orbit max => ~30 Hz avg DOCA cut, (#tracks>0||CAL>5 GeV visible): 167 Hz #xtals note! >30 Hz even at orbit max due to <100 MeV e+e-. Is that real? No matter, we can deal with it. Track-CAL match Cal_Fit_errNrm<15. Not checked yet!

24. L3 Filtering (Orbit Max Case) CAL Hi Rate: 40 Hz “L3” inspection of CAL_Hi not implemented yet Visible CAL energy Not checked yet! Require fraction in 1st 3 CAL layers >10% OR Cal_Energy > 10 GeV: 34 Hz at orbit max. Fraction of energy in front 3 CAL layers Just first looks with new fluxes and geometry. Gamma distributions on the way.

25. Results: status • First peek at PSF looks ~not terrible (much to do!) results are pre-pre-pre- preliminary! Note: normal incidence PSF is not particularly relevant for physics – just a well-defined comparison point for performance changes and code checking. “Normal” in Table means 0-8 deg bin.

26. Aside: some definitions Effective area(total geometric acceptance) • (conversion probability) • (all detector and reconstruction efficiencies). Real rate of detecting a signal is (flux) • Aeff 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-dimensional 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%

27. Energy loss mechanisms: Pair-Conversion Telescope  anticoincidence shield conversion foil particle tracking detectors e– • calorimeter • (energy measurement) e+ Experimental Technique Instrument must measure the direction, energy, and arrivaltime 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: determinephoton direction clear signature for background rejection - limitations on angular resolution (PSF) low E: multiple scattering => many thin layers high E: hit precision & lever arm • instrument 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.

28. Primary Design Impacts of Science Requirements  e– e+ Background rejection requirements drive the ACD design (and influence the calorimeter and tracker layouts). Effective area and PSF requirements drive the converter thicknesses and layout. PSF requirements also drive the design of the mechanical support. Field of view sets the aspect ratio (height/width) Energy range and energy resolution requirements set thickness of calorimeter Electronics Time accuracy provided by electronics and intrinsic resolution of the sensors. On-board transient detection requirements, and on-board background rejection to meet telemetry requirements, drive 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) drive the overall system performance.

29. Design and Simulations gaps, dead areas included Zoom in on a corner of the instrument The GLAST baseline instrument design is based on detailed Monte Carlo simulations. Two years of work was put into this beforeany significant investment was made in hardware development. • Cosmic-ray rejection of >105:1 with 80% 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 are possible. • Practical scheme for triggering. • Design optimization. Simulations and analyses are all OO (C++), based on GISMO toolkit. 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

30. Simulations validated in detailed beam tests Experimental setup in ESA for tagged photons: X Projected Angle 3-cm spacing, 4% foils, 100-200 MeV Data Monte Carlo GLAST Data (errors are 2) Monte Carlo

31. x x x x x x • CAL: LO – independent check on TKR trigger. HI – indicates high energy event ground Instrument Triggering and Onboard Data Flow Level 3 Processing Level 1 Trigger Level 2 Processing Function: • reject background efficiently & quickly with loose cuts, • reduce computing load • remove any noise triggers Hardware trigger based on special signals from each tower; initiates readout Function: • “did anything happen?” • keep as simple as possible L3T: full instrument Function: reduce data to fit within downlink • complete event reconstruction • signal/bkgd tunable, depending on analysis cuts: :cosmic-rays~ 1:~few • TKR 3 x•y pair planes in a row** workhorse gtrigger • tracker hits ~line up • track does not point to hit ACD tile OR L2 was motivated by earlier DAQ design that had one processor per tower. On-board filtering hierarchy being redesigned. Total L3T Rate: <30 Hz> (average event size: ~7 kbits) Upon a L1T, all towers are read out within 20ms On-board science analysis: transient detection (AGN flares, bursts) Instrument Total L1T Rate: <5 kHz> Spacecraft 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.

32. LAT Instrument Basics • 4x4 array of identical towersAdvantages of modular design. • Precision Si-strip Tracker (TKR) Detectors and converters arranged in 18 XY tracking planes. Measure the photon direction. • Hodoscopic CsI Calorimeter(CAL)Segmented array of CsI(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. • Central 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 MeV - >300 GeV.

33. Background Rejection Overview • First developer of background rejection analysis: Bill Atwood (lots of work also by Jay, Toby, Heather, Cathie, Sawyer, Jose, Paul, Taro, SR, … • Analysis done thus far for two main reasons: • (1) A reasonable way to quote our effective area. • (2) A proof of principle and a demonstration of the power of the instrument design. • Not the final background analysis! Other techniques are available to reduce the backgrounds further with good efficiency (particularly using TKR pattern recognition). The analysis for the AO response was done in triage mode, and there is much to do now. • Some science topics may require less stringent background rejection than others. Issues of duration, visible energy range, etc. Same points also hold for the event reconstruction we have thus far.

34. Background Analysis • Ideally (and usually) cut variable distributions are examined several ways, first to check the distribution is sensible and then for implementing the selections: • 1) raw (after triggers, depending on tuple) • 2) cumulative- the distribution of the next cut variable after all previous cuts. Note, this order is arbitrary (mostly) and the distributions can be misleading, so…. • 3) “all but”: look at each variable distribution with every cut but this one applied. • 4) niche areas: check for effects of each cut in different energy ranges and different angles of incidence. (usually done with merit first) • 5) interplay with track quality cuts: the effects of the track quality cuts and the background rejection cuts are not orthogonal: track quality cuts usually help in background rejection somewhat, and background cuts sometimes help clean up PSF. In one case, an “all but” background distribution was empty! Optimize these together. • 6) n-dimensionally (usually 2 at a time) : look for correlations and domains of well-clustered S/B for like variables. Note that a neural net very well addresses (3), (5) and (6). These cuts are not orthogonal, and there is a better space in which to make them.

35. Background Analysis (cont) • try to keep the cuts away from steep areas, or right next to individual events (avoid fine-tuning). • process is iterative: • With each variable, look at distributions for gammas and background and choose a preliminary cut value. • Scan remaining background events and lost gamma events for adjusting cut and to determine potentially new cut variables. • Check for cut redundancy and correlation. Check impact on instrument performance. Merit is particularly useful here. • As a practical matter, some days are spent mostly improving the rejection and other days are spent mostly improving the gamma efficiency.

36. AO Overview: Visible (CAL) Energy Distributions at Various Stages Important: at the time of the large data set generation for the AO, the albedo proton flux was not implemented. It was implemented for the rate studies, but the energy spectrum was wrong. We thought this was pessimistic. (38, actually, but who’s counting?) Although the cosmic ray spectrum peaks around 4-20 GeV, the deposited energy is typically much lower. The region below 1 GeV is the most difficult for background rejection for several reasons. Note, after all selections, no background events remain with visible energy greater than 200 MeV. This wasn’t easy.

37. Some references (beyond meeting presentations): 1) DoE proposal (1998) 2) Note of 9 August 1999 (describes cuts and problem areas fairly well, needs distributions included) 3) AO response however, better documentation is needed and will be done in this round of studies. STEPS (note: description uses AO tuple variable names) • The famous VETO_DOCA (only for CsI_Energy_Sum<20) - getting better, but still somewhat broca. Needs improvement. • “Hit pattern” - Surplus_Hit_Ratio, with an energy-dependent application. Surplus_Hit_Ratio>2.25 || (CsI_Energy_Sum>1&&fst_X_Lyr>13) || CsI_Energy_Sum>5.

38. Background Analysis Steps (II) CsI_Xtal_Ratio>0.25||CsI_No_Xtals<1 (CsI_Energy_Sum<1.&&CsI_Fit_errNrm<10.)||CsI_Fit_errNrm<4.||CsI_No_Xtals<1 (Will work on making this one continuous in energy with a normalized moment) CsI_eLayer8/CsI_Energy_Sum<0.08 || CsI_eLayer1/CsI_Energy_Sum>0.25 || CsI_Energy_Sum>0.35||CsI_No_Xtals<1 CsI_moment1<15. || CsI_moment1<80.&&CsI_Energy_Sum>0.35||CsI_Energy_Sum>1.|| CsI_No_Xtals<1 CsI_Z>-30.||CsI_No_Xtals<1 Surprisingly efficient even at high energy CsI_No_Xtals_Trunc<20.||CsI_Energy_Sum>75.||fst_X_Lyr<12 Only needed in BACK STEPS (continued) • “CAL info” - CsI_Fit_errNrm, CsI_Xtal_Ratio -- keep events w/no CAL info whenever possible. • “Track quality” (most recent selections developed by Jose) • “S/C induced event cuts” - designed to remove cosmics whose primary interaction is in the S/C. This is our single largest residual background! No_Vetos_Hit<1.5 || (CsI_Energy_Sum>1. && No_Vetos_Hit<2.5) || CsI_Energy_Sum>50. Quality_Parm>10 (composite track quality parameter, cut effective against low-energy stubs from splash-up)

39. Background Analysis To Do Next steps: • Use better background flux model. Low energy p and e albedo must be dealt with. Demonstrate high-energy electron rejection. • Improve low energy Aeff, work on inefficiencies (Surplus_Hit_Ratio, CsI_Fit_errNrm) • VETO_DOCA: needs work. Mainly a tracking issue. Seed tracks with hit tiles, track quality selections for loop. • Document, put correct implementation into merit. • Simplify analysis (make prettier, simpler). Bring in neural net. More sophisticated tracking (downward “ ”) & CAL pattern recognition. (post-PDR) • Further improvements in rejection (at time of AO, integrated residual background rate was ~ 6% of extragalactic diffuse rate). Also, study background rate differentially (by visible energy bin). More work on upward-going energy events. • Evaluate impact of limited set of instrument failure modes (see end of talk) V