1. Sergio Ricciarini ~ INFN Firenze on behalf of PAMELA collaboration Vertex 2006 15 th International Workshop on Vertex Detectors Perugia, 28 September 2006 PAMELA Silicon Tracker experience and operation

2. Summary  Introduction. The PAMELA experiment.  The magnetic spectrometer and silicon tracker. Detectors and read-out electronics.  Tracker performances. Preliminary analysis of a sample of data taken in flight.  Examples of events collected in flight by PAMELA. S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006

3. ITALY: INFN Florence and Physics Department of Florence University Istitute of Applied Physics “Nello Carrara”, Florence INFN Bari and Physics Department of Bari University INFN and Physics Department of Rome "Tor Vergata" INFN Naples and Physics Department of Naples University INFN Trieste and Physics Department of Trieste University INFN National Laboratories, Frascati GERMANY: Physics Department of Siegen University SWEDEN: Royal Institute of Technology, Stockholm RUSSIA: Ioffe Physico-Technical Institute, St Petersburg Cosmic Rays Laboratory, Moscow Engineering and Physics Institute, Moscow Lab. of Solar and Cosmic Ray Physics, P.N. Lebedev Physical Institute, Moscow S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006

4. PAMELA experiment  Main scientific objectives: antiparticles in cosmic rays; search for antimatter and dark matter; cosmic-ray propagation; solar modulation, solar physics.  Mission overview: on-board Resurs-DK1 Russian satellite, launched from Bajkonur (Kazakhstan) 15 June 2006; quasi-polar orbit 70° inclination, 350-600 km altitude; long expected duration (> 3 years);  efficient rejection of atmospheric background (albedo);  high statistics, also at lower energies (geomagnetic effect).  Design goals for PAMELA performance: sensitivity in anti-He/He ratio~ 10 -7 particlekin. energy range antiproton80 MeV - 190 GeV (expected ~ 10 4 /year) positron50 MeV - 270 GeV (expected ~ 10 5 /year) electron50 MeV - 400 GeV proton80 MeV - 700 GeV e - + e + up to 2 TeV nuclei Z ≤ 6100 MeV/n - 200 GeV/n S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006 operational launch (rest) Al container filled with N 2 at 1 atm 2 mm thick window Primary production  annihilation m(  ) = 964 GeV (Ullio 1999)

5.  Main requirements: high-sensitivity antiparticle identification, precise momentum measure.  Time-Of-Flight plastic scintillator strips + PMT:  trigger, albedo rejection;  mass identification up to E ~ 1 GeV;  charge identification from dE/dX.  Magnetic spectrometer with microstrip Si tracker:  charge sign and momentum from the curvature;  charge identification from dE/dX.  Electromagnetic calorimeter W/Si sampling; 16.3 X 0, 0.6 λ I :  discrimination e + / p, e - / from shower topology;  direct E measurement for e -. PAMELA apparatus max diameter: 102 cm height: 130 cm weight: 470 kg power: 355 W S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006

6. Magnetic spectrometer  Permanent magnet (5 modules): Nd-Fe-B alloy elements, residual magnetization 1.3 T; Al frames, tower height 44.5 cm; geometric factor 22.5 cm 2 · sr; B x ~ B z < 0.1 B y ; 3-axis map: 70000 points, 5 mm pitch.  Tracking system (6 planes, 8.9 cm apart): 3 independent ladders per plane: 2 Si microstrip sensors per ladder: double sided, with double metallization on ohmic view; integrated capacitive coupling; FE electronics (VA1 chips) integrated on hybrid boards. Si sensors hybrids l a d d e r VA1 chips B mean = 0.43 T B max = 0.48 T ADC boards S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006

7. Silicon detector ladder  Sensor dimensions: 70 mm x 53.3 mm x 300 μm.  Read-out: 1024 read-out channels per ladder view; strip/electrode coupling ~ 20 pF/cm; channel capacitance to ground: < 10 pF junction view, < 20 pF ohmic view.  Bias: V Y -V X = + 80 V fed through guard ring surrounding the strips. Bias resistor: junction: punch-through, > 50 MΩ; ohmic: polysilicon, > 10 MΩ. Leakage current < 1 μA/sensor. S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006

8. VA1 chip  Main features: 1.2 µm CMOS ASIC (CERN - Ideas, Norway); 6.2 mm · 4.5 mm chip area; 47 μm input pad pitch; ± 2 V power rails; 128 low-noise charge preamplifiers; shaping time set to 1 μs; ± 300 mV differential output range.  Operating point: chosen for optimal compromise; power consumption 1.0 mW/channel  total dissipation 37 W for 288 VA1 (36864 channels); voltage gain 7.0 mV/fC  output saturation at ~ 10 MIP. S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006

9.  ADC stage: 36 ADC sections, 1 ADC / ladder; ADC are operated in parallel at 0.5 Msps; event acquisition time 2.1 ms.  DSP stage: 12 DSP on 2 boards, 1 DSP/view (ADSP2187L); control logics on FPGA chips (A54SX); typical data compression factor 15  ~ 4 kbyte/event; typical compression time 1.1 ms. Tracking system electronics  General characteristics: segmentation/redundancy of power and functional sections; devices qualified for radiation hardness (TID, SEE); compact mechanical assembly; limited total power consumption (63 W); limited data bandwidth occupation (~ 10 Gbyte/day available). S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006 flight data calibration physics run

10. Tracker performances  Preliminary analysis of a sample of data taken in ~12 hours of flight. Data show that the tracking system is working nominally as expected.  Thermal environment.  Noise performances.  Cluster multiplicity and total signal.  Signal correlation between X and Y views.  Signal/noise.  Charge discrimination capabilities.  Spatial resolution.  Momentum resolution. S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006

11. Temperatures in flight  After power-up temperature remains stable: < 1º C variations along orbit; < 10º C difference between PAMELA off and on.  Heat from VA1 on hybrids radiated to the magnetic tower: black IR absorbing painting on the walls; heat released from magnetic tower to cooling loop (liquid iso-octane). S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006 At power-up: 21º C (5000 s ~ 0.9 orbits) 8 days after power-up: 28º C (10000 s ~ 1.8 orbits)

12. Noise in flight S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006 Noise Pedestal Y view (DSP 3)  Noise performance is nominal, as can be seen for a typical calibration taken in flight.  Y (ohmic) view has worse performance because of double metallization. X view (DSP 6) Y view (DSP 3) X view (DSP 6) N ~ 4 ADC countsN ~ 9 ADC counts flight data yellow line = ground data average

13. all planes flight data - preliminary all planes flight data - preliminary one plane flight data - preliminary one plane Cluster characteristics S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006  Cluster inclusion cuts: S > 7 N (seed), S > 4 N (neighbours).

14.  Signal/noise ratio, calculated as Σ(S/N) over the cluster channels.  This sample contains also non-MIP cosmic rays (He etc.).  Typical average signal/noise measured at beam-test for orthogonally incident MIP: 56 (X view) 26 (Y view) Signal/noise ratio S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006 flight data - preliminary plane 1

15.  Full charge discrimination capabilities studied with beam-test data (GSI Darmstadt, 2006). Fragmentation of 12 C projectiles on different targets (Al, polyethylene).  Single-channel saturation at ~ 10 MIP affects B- C discrimination. Charge discrimination S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006 beam–test data average cluster signal events Good H-He charge discrimination capability. flight data - preliminary He H magnetic rigidity R = |pc/Z| all planes

16. Spatial resolution  Critically depends on the signal/noise ratio.  Resolution for junction (X, bending) view determines the momentum measurement. S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006  x = (2.77 ± 0.04)  m  y = (13.1 ± 0.2)  m  Best spatial resolution obtained with non-linear η algorithm for normally incident MIP.  Resolution measurement and sensor alignment done at the last beam test of the flight model with protons of known energies (CERN SPS, 2003).  Whole-tracker alignment checked with cosmic-rays collected at ground level during final qualification tests (INFN Rome “Tor Vergata” laboratories, 2005).  In flight: alignment parameters will be checked with high-energy electrons after collecting a sufficient statistical sample (at least 3 months of data taking). Beam-test data - orthogonally incident MIP

17. Momentum resolution  Measured at beam test with protons of known momentum (CERN SPS, 2003).  In flight cross-check with E measured by calorimeter for high-energy electrons. S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006 MDR ~ 1 TV mult. scatt. spat. resol. X magnetic rigidity R = |pc/Z| magnetic deflection η = 1/R = |Z/pc|

18. S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006 Flight data: 10 GV electron with electr. shower

19. S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006 Flight data: 1.56 GV positron with electr. shower

20. S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006 Flight data: 36 GV proton with hadronic shower

21. S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006 Flight data: 18 GV antiproton without shower

22. S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006 Flight data: 9.7 GV He nucleus without shower

23. Conclusions  PAMELA is taking data since 11 July 2006. Up to now >900 Gbyte of data downlinked to ground. Acquired ~ 90 · 10 6 events. Apparatus operating also within radiation belts (SAA).  Magnetic spectrometer on-flight performances are nominal.  Data processing and analysis tools have been developed and used; they are now being finalized.  Next step: systematic data analysis. Precise determination of detector characteristics. Application to physics research items. S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006 trigger rate