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

2. Summary Introduction. The magnetic spectrometer and silicon tracker.
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. • INFN Florence and Physics Department of Florence University
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: Mission overview:
• 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, 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:
Primary
production
 annihilation
m() = 964 GeV
(Ullio 1999)
operational
launch (rest)
Al container
filled with N2 at 1 atm
2 mm thick window
particle
kin. energy range
antiproton
80 MeV GeV
(expected ~ 104/year)
positron
50 MeV GeV
(expected ~ 105/year)
electron
50 MeV GeV
proton
80 MeV GeV
e- + e+
up to 2 TeV
nuclei Z ≤ 6
100 MeV/n GeV/n
sensitivity in anti-He/He ratio
~ 10-7
S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006

5. PAMELA apparatus
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;
Electromagnetic calorimeter
W/Si sampling; 16.3 X0, 0.6 λI:
 discrimination e+ / p, e- /
from shower topology;
 direct E measurement for e-.
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 cm2 · sr;
• Bx ~ Bz < 0.1 By ;
• 3-axis map: points, 5 mm pitch.
Bmean = 0.43 T
Bmax = 0.48 T
l a d d e r
ADC boards
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.
VA1 chips
Si sensors
hybrids
S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006

7. Silicon detector ladder
Sensor dimensions: 70 mm x mm x 300 μm.
Read-out:
• 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:
• VY -VX = + 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: Operating point:
• 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. 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).
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.
flight data
calibration
physics run
S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006

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).
At power-up: 21º C
(5000 s ~ 0.9 orbits)
8 days after power-up: 28º C
(10000 s ~ 1.8 orbits)
S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006

12. Noise in flight Pedestal Noise X view (DSP 6) 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.
Pedestal
X view (DSP 6)
Y view (DSP 3)
flight data
flight data
Noise
X view (DSP 6)
Y view (DSP 3)
flight data
flight data
N ~ 4 ADC counts
N ~ 9 ADC counts
yellow line = ground data average
S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006

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

14. Signal/noise ratio
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)
flight data - preliminary
plane 1
S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006

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

16. Spatial resolution Critically depends on the signal/noise ratio.
Resolution for junction (X, bending) view determines the momentum measurement.
Beam-test data - orthogonally incident MIP
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).
sx = (2.77 ± 0.04) mm
sy = (13.1 ± 0.2) mm
S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006

17. Momentum resolution MDR ~ 1 TV
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.
MDR ~ 1 TV
mult. scatt.
spat. resol. X
magnetic rigidity R = |pc/Z|
magnetic deflection η = 1/R = |Z/pc|
S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006

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

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

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

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

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

23. Conclusions PAMELA is taking data since 11 July 2006.
• Up to now >900 Gbyte of data downlinked to ground.
• Acquired ~ 90 · 106 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.
trigger rate
S. Ricciarini – PAMELA Silicon Tracker – Vertex 2006, Perugia 28 September 2006