The Silicon Strip Tracker of the Fermi Large Area Telescope Luca Baldini INFN–Pisa luca.baldini@pi.infn.it on behalf of the Fermi LAT collaboration HSTD-8, Taipei December 8, 2011
The Fermi observatory Large Area Telescope (LAT) ◮ Pair conversion telescope. ◮ Energy range: 20 MeV– > 300 GeV ◮ Large field of view ( ≈ 2 . 4 sr): 20% of the sky at any time, all parts of the sky for 30 minutes every 3 hours. ◮ Long observation time: 5 years minimum lifetime, 10 years planned, 85% duty cycle. Gamma-ray Burst Monitor (GBM) ◮ 12 NaI and 2 BGO detectors. ◮ Energy range: 8 keV–40 MeV. Luca Baldini (INFN) HSTD-8 2 / 18
The Large Area Telescope Large Area telescope ◮ Basic figures: ∼ 1 . 6 × 1 . 6 × 1 . 2 m 3 ; 3,000 kg weight; 650 W power consumption. ◮ Modular: 4 × 4 array of identical towers (each one including a tracker and a calorimeter module). ◮ Tracker surrounded by an Anti-Coincidence Detector (ACD) Anti-Coincidence Detector ◮ Segmented (89 tiles) to minimize self-veto at high energy. ◮ 0.9997 average efficiency Tracker (8 fiber ribbons covering gaps between tiles). ◮ Silicon strip detectors, W conversion foils; 1.5 X 0 on-axis. Calorimeter ◮ 9,216 sensors; 73 m 2 of ◮ 1536 CsI(Tl) crystal; 8.6 radiation silicon active area; 884,736 lengths on-axis. readout channels. ◮ Hodoscopic, 3D shower profile ◮ High-precision tracking, short reconstruction for leakage correction. instrumental dead time. Luca Baldini (INFN) HSTD-8 3 / 18
Fermi @ 3.5 years Launch ◮ Launched on June 11, 2008 from the Kennedy Space Center. ◮ Launch vehicle: Delta 7290H-10. ◮ Circular orbit, 565 km altitude, 25 . 6 ◦ inclination. ◮ Some milestones worth mentioning: 200 billion triggers as of October 2011, > 35 billion events downlinked to ground, > 600 million photon candidates released to the community, > 99% uptime. Luca Baldini (INFN) HSTD-8 4 / 18
Basic tracker layout ◮ 19 tray structures 3% X 0 W ◮ Basic mechanical framework ◮ 18 x - y detection planes ◮ Single sided SSDs, below the W foils Front ◮ Front: 12 planes with 0.03 X 0 converter ◮ Best angular resolution ◮ Back: 4 planes with 0.18 X 0 converters ◮ Increase the conversion efficiency ◮ Bottom: 2 planes with no converter ◮ Tracker trigger needs at least 3 x - y layers 18% X 0 W Back ◮ Total depth: 1.5 X 0 on axis Luca Baldini (INFN) HSTD-8 5 / 18
The Silicon Strip Detectors 8 . 95 × 8 . 95 cm 2 Outer size Strip pitch 228 µ m Thickness 400 µ m Depletion voltage < 120 V 1–2 nA/cm 2 @ 150 V Leakage current Breakdown voltage > 175 V ∼ 10 − 4 Bad channels # SSD tested 12500 # single strip tests ∼ 30M Rejected SSDs < 0 . 6% ◮ 18 flight towers integrated and tested in 9 months ◮ Flight Module A suffering from some processing issues during the set up of the assembly chain Luca Baldini (INFN) HSTD-8 6 / 18
Mechanical integration (1/2) ◮ Wafers glued and wired- bonded in 4 × 1 ladders. ◮ Four ladders integrated into a ∼ 36 × 36 cm 2 detection plane Luca Baldini (INFN) HSTD-8 7 / 18
Mechanical integration (2/2) ◮ < 2 mm spacing between layers ◮ Readout electronics on the tray sides: 90 ◦ pitch adapters, read out via flat cables ◮ 2 mm inter-tower separation ◮ Operate in vacuum, substain vibra- tional load at launch Luca Baldini (INFN) HSTD-8 8 / 18
The Tracker electronics system ◮ Basic design ◮ 24 front-end chips and 2 controllers handle one Si layer ◮ Data can shift left/right to either of the controllers (can bypass a dead chip) ◮ Zero suppression takes place in the controllers (hit strips + layer OR TOT in the data stream) ◮ Two flat cables complete the redundancy ◮ Key features ◮ Low power consumption ( ≈ 200 µ W/channel) ◮ Low noise occupancy ( ≈ 1 noise hit per event in the full LAT) ◮ Self-triggering (three x – y planes in a row, i.e. sixfold coincidence) ◮ Redundancy, Si planes may be read out from the right or from the left controller chip ◮ On board zero suppression Luca Baldini (INFN) HSTD-8 9 / 18
Hit efficiency Tower 15 Average hit efficiency 1 0.99 0.98 0.97 Average hit efficiency = (99.96 ± 0.01) % -1 Slope = (+0.0006 ± 0.0006) % year 0.96 Dec 31, 2008 Jul 02, 2009 Jan 01, 2010 Jul 02, 2010 Jan 01, 2011 Jul 03, 2011 Time (UTC) Tower 0 Average hit efficiency 1 0.99 0.98 0.97 Average hit efficiency = (98.46 ± 0.03) % -1 Slope = (-0.0449 ± 0.0030) % year 0.96 Dec 31, 2008 Jul 02, 2009 Jan 01, 2010 Jul 02, 2010 Jan 01, 2011 Jul 03, 2011 Time (UTC) Luca Baldini (INFN) HSTD-8 10 / 18
Strip masks trending Number of masked strips 6 1 4 4 1 9 8 3 400 2 3 6 6 0 3 6 4 9 1 4 5 3 3 3 6 2 2 3 3 3 1 3 3 3 1 8 300 Full LAT 2 0 2 3 6 2 0 0 2 2 200 8 1 1 0 1 1 6 9 100 8 7 Tower 0 5 3 6 Tower 3 5 4 7 3 8 2 4 0 2 1 1 1 6 8 1 5 0 Jul 02, 2008 Dec 31, 2008 Jul 02, 2009 Jan 01, 2010 Jul 02, 2010 Jan 01, 2011 Jul 03, 2011 Time (UTC) ◮ Some 200 noisy strip masked prior to launch (0.02%) ◮ 213 additional noisy strips masked over the first three years of mission, for a total of 416 (0.05%) ◮ Two major contributors ◮ Tower 0 (Fligth Module A): the first one being assembled, suffering from some processing issues—showed some evolution throughout the first year ◮ Tower 3 (Flight Module 15): noise issue in one ladder—more on that later Luca Baldini (INFN) HSTD-8 11 / 18
Trigger efficiency SIU reboot Tower 15 1.1 1.05 1 0.95 0.9 ± 0.85 Average trigger efficiency = (99.90 0.54) % ± -1 Slope = (-0.018 0.062) % year 0.8 Oct 01, 2008 Dec 31, 2008 Apr 02, 2009 Jul 02, 2009 Oct 01, 2009 Jan 01, 2010 Time (UTC) SIU reboot Tower 0 1.1 1.05 1 0.95 0.9 ± Average trigger efficiency = (99.83 0.57) % 0.85 ± -1 Slope = (+0.043 0.066) % year 0.8 Oct 01, 2008 Dec 31, 2008 Apr 02, 2009 Jul 02, 2009 Oct 01, 2009 Jan 01, 2010 Time (UTC) Luca Baldini (INFN) HSTD-8 12 / 18
Time over threshold LAT average 6 Average TOT peak position (fC) New TOT charge scale (SSC-181) Timing change trigger window set to 14 ticks 5.5 5 4.5 Average TOT peak = (4.831 0.000) fC ± -1 Slope = (+8.69e-03 4.95e-05) fC year ± 4 Dec 31, 2008 Jul 02, 2009 Jan 01, 2010 Jul 02, 2010 Jan 01, 2011 Jul 03, 2011 Time (UTC) ◮ Long term trending of the position of the MIP peak in the Tracker Time Over Threshold (averaged over the LAT) ◮ The two noticeable discontinuities are due to hardware/software changes ◮ Analog signal remarkably stable (within much less than 1%) since the last of the two changes. Luca Baldini (INFN) HSTD-8 13 / 18
Noise occupancy Tower 15, plane 10 Layer noise occupancy -1 10 -2 10 Average layer occupancy = (4.31e-03 1.47e-05) ± -1 Slope = (-9.13e-05 ± 1.60e-06) year -3 10 Dec 31, 2008 Jul 02, 2009 Jan 01, 2010 Jul 02, 2010 Jan 01, 2011 Jul 03, 2011 Time (UTC) ◮ Long term trending of the noise occupancy for a typical silicon layer ◮ Measured accumulating counts on the silicon layers far from triggering towers (and cross-checked with dedicated periodic triggers) ◮ Noise occupancy at the level of 4 × 10 − 3 for a layer (1536 strips) ◮ Translating into 2–3 × 10 − 6 at the single strip level (dominated by accidental coincidences). . . ◮ . . . or 2–3 noise hits per event in the full LAT Luca Baldini (INFN) HSTD-8 14 / 18
A minor hardware issue January 1, 2010 July 1, 2010 10 -1 10 -1 Strip occupancy Strip occupancy Noisy ladder Noisy ladder (strips 0--384) (strips 0--384) -2 -2 10 10 10 -3 10 -3 10 -4 10 -4 -5 -5 10 10 10 -6 10 -6 0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 Strip number Strip number January 1, 2011 July 1, 2011 10 -1 10 -1 Strip occupancy Noisy ladder Strip occupancy Noisy ladder (strips 0--384) (strips 0--384) -2 -2 10 10 -3 -3 10 10 10 -4 10 -4 -5 -5 10 10 -6 -6 10 10 0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 Strip number Strip number ◮ Noise in one silicon ladder steadily increasing since January 2010 ◮ Really only one of the 2304 silicon ladders in the LAT Luca Baldini (INFN) HSTD-8 15 / 18
A minor hardware issue To be debugged in space Tower 3 bias current Detector bias current Time UTC ◮ One power supply per tower ◮ We only monitor the currents at the tower level (i.e. each HV line is biasing 36 × 4 = 144 silicon ladders) ◮ Not trivial to measure a relative increase in the leakage current at the level of a single ladder ◮ Test runs with reduced bias HV (40, 60, 80 V vs. nominal 105 V) ◮ Normal data taking, charge injection calibration ◮ No obvious root cause identified ◮ Even if we lose the entire ladder it’s less than 0.05% of the tracker ◮ No evidence of similar phenomena in any other part of the LAT Luca Baldini (INFN) HSTD-8 16 / 18
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