Luca Baldini (INFN–Pisa) luca.baldini@pifn.it for the Fermi LAT collaboration

1. 8th International Conference on Position Sensitive Detectors Glasgow, September 1—5, 2008 Luca Baldini (INFN–Pisa) luca.baldini@pi.infn.it for the Fermi LAT collaboration

2. The launch Launch from Cape Canaveral Air Station 11 June 2008 at 12:05PM EDT.

3. Fermi in orbit Circular orbit, 565 km altitude (96 min period), 25.6 degrees inclination http://observatory.tamu.edu:8080/Trakker(track the satellite) http://www.nasa.gov/mission_pages/GLAST/news/glast_online.html(look at Fermi in the sky from your place)

4. Start Year 1 Science Ops Start Year 2 Science Ops “first light” whole sky LAT, GBM turn-on check out spacecraft turn-on checkout pointed + sky survey tuning sky survey week week week week month 12 months LAUNCH L+60 days initial tuning/calibrations in-depth instrument studies Outline • Fermi timeline in brief: • First 60 days for commissioning (Launch and early orbit): • Spacecraft turn on and check out; • Instruments turn on, check out and initial calibration; • “First light” data, pointing and sky-survey mode tuning. • Just finished the commissioning phase and entered the first year sky-survey mode. • Talk outline: • Description of the instrument. • Highlights from the first two months of operations of the instrument (on orbit rates, calibration, SAA studies). • First light.

5. The observatory • Large Area Telescope (LAT): • Pair-conversion telescope. • Tracker/converter + calorimeter, surrounded by an anticoincidence shield. • Energy range: 20 MeV – 300 GeV. • Gamma-ray Burst Monitor (GBM): • Set of12 NaI + 2 BGO scintillators. • Monitoring the entire unocculted sky, optimized to detect Gamma-Ray Bursts. • Energy range: 10 keV – 25 MeV. • Key features: • Large field of view (>2 sr for the LAT, 2p sr for the GBM) • Huge energy range (including the essentially unexplored 10--100 GeV band). • Small dead time (26 ms minimum). • Enormous increase in sensitivity with respect to the previous missions.

6. The LAT: experimental technique The goal is to measure direction, energy and arrival time of the incoming photons, being able at the same time to reject the charged particle background. • Tracker/converter: photon conversion and direction reconstruction. • Calorimeter: energy measurements. • Anti-coincidence shield: background rejection. All the three subsystem participate to the trigger logic.

7. g e+ e- Overview of the Large Area Telescope • Overall modular design: • 4x4 array of identical towers - each one including a Tracker, a Calorimeter and an Electronics Module. • Surrounded by an Anti-Coincidence shield (not shown in the picture). • Tracker/Converter (TKR): • Silicon strip detectors (single sided, each layer is rotated by 90 degrees with respect to the previous one). • W conversion foils. • ~80 m2 of silicon (total). • ~106 electronics chans. • High precision tracking, small dead time. • Anti-Coincidence (ACD): • Segmented (89 tiles). • Self-veto @ high energy limited. • 0.9997 detection efficiency (overall). • Calorimeter (CAL): • 1536 CsI crystals. • 8.5 radiation lengths. • Hodoscopic. • Shower profile reconstruction (leakage correction)

8. Trigger/Onboard processing • Level 1 trigger: • Hardware trigger, single-tower level. All subsystem contributes to the trigger: • TKR: three consecutive tracker x-y planes in a row fired. • CAL_LO: single log with E > 100 MeV (adjustable). • CAL_HI: single log with E > 1 GeV (adjustable). Disengage the use of the ACD. • ROI: MIP signal in one of the ACD tiles close to the triggering Tracker Tower. • CNO: heavy ion signal in the ACD. • Upon L1 trigger the entire detector is readout---the particular combination of trigger primitives determines the readout settings (prescale, zero suppression). • Onboard filter(s): • Each event is presented to (up to) four different onboard filters: • GAMMA: rough photon selection (full instrument information available to the onboard processor). • HIP: heavy ions (collected continuously for CAL calibration). • DGN: prescaled unbiased sample for diagnostics purposes. • MIP: only running in calibration runs. • Final gamma selection is performed on the ground (see following slides).

9. Large Area Telescope & GBM m • sec GPS • - • Telemetry 1 kbps GLAST Spacecraft • TDRSS SN S & Ku DELTA 7920H • • S - - • GN • LAT Instrument Science Ops Center White Sands Schedules Mission Operations Center (MOC) GLAST Science Support Center HEASARC • Test: • test Schedules GBM Instrument Ops Center GRB Coordinates Network Alerts Data, Command Loads The data flow

10. On orbit rates in nominal configuration • Overall trigger rate: ~few KHz • Huge variations due to orbital effects. • Downlink rate: ~400—500 Hz • ~90% from GAMMA filter • ~20—30 Hz from DGN filter • ~5 Hz from HIP filter • Rate of photons after the standard background rejection cuts for source study: ~1 Hz • Most of the downlinked events are in fact background, final 100:1 rejection is done in ground processing.

11. Calorimeter calibration C O Protons B N Ne Mg Be Si Fe • Heavy ions continuously collected (through the HIP fillter) for the energy scale calibration (about 100 hours of data shown in the plots). • MIPs collected continuously and in dedicated calibration runs. • Peak positions stable to better than 1% during the commissioning phase.

12. Tracker performance and calibration • No evidence of a reduction in hit efficiency (well above 99% on average) with respect to the ground calibrations. • No significant change in the alignment constants (intra and inter-tower) after the launch (the LAT underwent up to 4 g acceleration). • No evidence of any increase in the overall noise level (~1 noise hit per event for the full LAT).

13. Mapping of the SAA • The South Atlantic Anomaly (SAA) is a region with a high density of trapped particles (mostly low energy protons). • We do not take physics data when in the SAA (ACD HV turned off), but we do count the TKR and CAL trigger to map the radiation intensity. • Started with a conservative boundary definition, new polygon already uploaded on the spacecraft (time spent in the SAA reduced from ~18% to ~15%).

14. First light skymap (4 days)

15. First results from the vela Off-pulse On-pulse • One of the primary observation targets during the commissioning phase, for several different reasons: • Timing; • Background rejection (on pulse vs. off pulse); • PSF verification (on pulse photons).

16. Conclusions • The GLAST Launch and Early Orbit phase has been extremely successful. • We didn’t have any real surprise, as far as the instrument performance is concerned: • The post-launch calibration is complete; • The on-orbit rates are reasonably close to the predictions; • The LAT is performing extremely well. • The first light images have just been released. • We’re just beginning the first year sky survey. Stay tuned for the upcoming science results!

17. Spares

18. Tracker design • Aggressive mechanical design: • Less than 2 mm spacing between x and y layers, with front-end electronics lying on the four sides of the trays. • 90° pitch adapters from the front end chips to the silicon sensors. • 2 mm inter-tower separation in order to minimize the inactive area.

19. LAT performance • Energy Resolution: ~10% (~5% off-axis) • PSF (68%) at 100 MeV ~ 3.5o (thin section) • PSF (68%) at 10 GeV ~ 0.1o • Field Of View: 2.4 sr • Point Source sens. (>100 MeV): 4x10-9 cm-2 s-1