Elena Vannuccini

1
In-flight performances of the PAMELA magnetic
spectrometer

• Elena Vannuccini
• (INFN Florence)
• on behalf of
• the PAMELA collaboration

2
The PAMELA experiment
PAMELA flight model

• MAIN TOPICS
• CR antiproton and positron spectra
• 104 antiprotons ? 80 MeV/c - 190 GeV/c
• 105 positrons ? 50 MeV/c - 270 GeV/c
• search for light antinuclei
• SECONDARY TOPICS
• Modulation of GCRs in the Heliosphere
• Solar Energetic Particles (SEP)
• Earth Magnetosphere

• PAMELA on board of Russian satellite Resurs DK1
• Orbital parameters
• inclination 70o (? low energy)
• altitude 360-600 km (elliptical)
• active life gt3 years (? high statistics)

More about PAMELA E.Mocchiutti H01 14 July
1100 M.Pearce E19 18 July 0955

• ? Launched on 15th June 2006
• ? First switch-on on 21st June 2006
• Detectors in nominal conditions (no problems due
  to the launch)
• Tested different trigger and hardware
  configurations
• Commissioning phase successfully ended on
  September 15th 2006
• ? PAMELA in continuous data-taking mode since
  then!

Launch from Baykonur
3
PAMELA detectors
Main requirements ? high-sensitivity antiparticle
identification and precise momentum measure
-

• Time-Of-Flight
• plastic scintillators PMT
• Trigger
• Albedo rejection
• Mass identification up to 1 GeV
• - Charge identification from dE/dX.
• Electromagnetic calorimeter
• W/Si sampling (16.3 X0, 0.6 ?I)
• Discrimination e / p, anti-p / e-
• (shower topology)
• Direct E measurement for e-
• Neutron detector
• plastic scintillators PMT
• High-energy e/h discrimination

GF 21.5 cm2 sr Mass 470 kg Size
130x70x70 cm3 Power Budget 360W

• Spectrometer
• microstrip silicon tracking system
  permanent magnet
• It provides
• - Magnetic rigidity ? R pc/Ze
• Charge sign
• Charge value from dE/dx

4
Antiprotons
Unexplored Region

• Secondary component
• CR propagation
• Primary source ((?))
• Dark matter
• Extragalactic primordial p-bar

Spectrometer required performances 4 mm
resolution on the bending view (x) ? MDR 740 GV
? spillover limit 190 GeV ( MDR Maximum
Detectable Rigidity ? DR/R1 _at_ RMDR where
Rpc/Ze )
5
The magnet

• 5 magnetic modules
• Permanent magnet (Nd-Fe-B alloy) assembled in an
  aluminum mechanics
• Magnetic cavity sizes (132 x 162) mm2 x 445 mm
• Geometric Factor 21.5 cm2sr
• Black IR absorbing painting
• Magnetic shields

Magnetic tower
Base plate prototype
6
The magnetic field

• MAGNETIC FIELD MEASUREMENTS
• Gaussmeter (F.W. Bell) equipped with 3-axis
  probe mounted on a motorized positioning device
  (0.1mm precision)
• Measurement of the three components in 67367
  points 5mm apart from each other
• Field inside the cavity
• 0.48 T _at_ center
• Average field along the axis 0.43 T
• Good uniformity
• External magnetic field magnetic momentum lt 90
  Am2

7
The tracking system

• 6 detector planes, each composed by 3 ladders
• Mechanical assembly
• aluminum frames
• carbon fibers stiffeners glued laterally to the
  ladders
• no material above/below the plane
• 1 plane 0.3 X0 ? reduced multiple
  scattering
• elastic rigid gluing

Carbon fibers
LADDER
Test of plane lodging inside the magnet
First assembled plane
8
Silicon detector ladders

• 2 microstrip silicon sensors
• 1 hybrid with front-end electronics
• Silicon sensors (Hamamatsu)
• 300 mm, double sided - x y view
• AC coupled (no external chips)
• double metal (no kapton fanout)
• 1024 read-out channels per view
• - strip/electrode coupling 20 pF/cm
• channel capacitance to ground
• - junction lt 10 pF
• - ohmic lt 20 pF
• Bias
• VY -VX 80 V fed through guard ring
  surrounding the strips
• Bias resistor
• - junction punch-through, gt 50 MO
• - ohmic polysilicon, gt 10 MO.
• Leakage current lt 1 µA/sensor.

9
In-flight basic performaces
X view
Y view
N 4 ADC counts
N 9 ADC counts

• Tracking system calibrated _at_ every orbit (95
  min)
• Data acquisition in compressed mode (5)
• 12 x 250 B 3 kB/ev
• (5 kB/ev all detectors)
• system is stable
• good signal-to-noise performaces

Y view larger noise ? worse performances
X view lower noise ? better performances
10
Charge identification capabilities
Beam-test data (_at_GSI 2006)
flight data
12C projectiles on Al and polyethylene targets
(track average)
4He
B,C
3He
d
Be
p
Li

• Good charge discrimination of H and He
• Single-channel saturation at 10MIP affects B/C
  discrimination

11
Spatial resolution
Sensor instrinsic resolution
Spatial resolution studied by means of beam-test
of silicon detectors and simulation
Simulation
Simulation
COG
ETA4
ETA2
ETA3
ETA2
Non-linear algorythm with 2,3,4 strips
Center-Of-Gravity

• junction side (X) 3 mm _at_0o, lt 4 mm up to
  10o (? determines momentum resolution)
• ohmic side (Y) 813 mm

Sensor alignment (relative to mechanical
positions)

• Track-based alignment minimization of spatial
  residuals as a function of the roto-traslational
  parameters of each sensor
• _at_ground ? proton beam and atmospheric muons
  (cross-check) ? 1001 mm
• _at_flight ? observed displacements relative to
  ground alignment ? 10 mm

Necessary to align in flight !!
12
In-flight alignment

• ? Done with relativistic protons (high statistics)

Flight data Simulation
Spatial residuals (1st plane) protons 7-100 GV
X side
Y side

• After alignment
• residuals are centered
• width consistent with nominal resolution

13
Momentum resolution
Beam test - protons
Iterative c2 minimization as a function of track
state-vector components a
MDR 1TV

• ? 1/R ? magnetic deflection
• sR/R sh/h
• Maximum Detectable Rigidity (MDR)
• def _at_ RMDR ? sR/R1
• MDR 1/sh

Trajectory evaluated by stepwise integration of
motion equations by means of Runge-Kutta method
(not-homogeneous B field)

• Measured at beam test with protons of known
  momentum (CERN SPS, 2003)
• In-flight (possible) global distortions after
  alignment procedure ? deflection offset
• cross-check with electrons and positrons
• energy measured by the calorimeter ? DE/E lt 10
  above 5GeV

14
Spectrometer systematics

• ?z? lt 1 due to Bremstahlung effect in the
  material above the spectrometer
• the pdf of z depends only on the amount of
  traversed material

deflection offset
Calorimeter calibration uncertanty
electrons Positrons 520 GeV
P010-5
P00.403
Kolmogorov probability between ze- and ze (P0
0 1) with free parameter Dh Dh -10-3 GV -1
15
High-energy antiproton analysis

• Event selected from 590 days of data
• Basic requirements
• Clean pattern inside the apparatus
• single track inside TRK
• no multiple hits in S1S2
• no activity in CARDCAT
• Minimal track requirements
• energy-dependent cut on track c2 (95
  efficiency)
• consistency among TRK, TOF and CAL spatial
  information
• Galactic particle
• measured rigidity above geomagnetic cutoff
• down-ward going particle (no albedo)

16
Antiproton identification

• dE/dx vs R (S1,S2,TRK) and b vs R
  proton-concistency cuts
• electron-rejection cuts based on
  calorimeter-pattern topology

-1 ? Z ? 1
p ( e)
p
e- ( p-bar)
spillover p
p-bar
17
Proton spillover background
p
p-bar
spillover p
MDR 1/sh evaluated event-by-event by the track
fitting routine

• MDR account for
• number and distribution of fitted points along
  the trajectory
• spatial resolution of the single position
  measurements
• magnetic field intensity along the trajectory

18
Proton spillover background
Minimal track requirements
MDR gt 850 GV

• Strong track requirements
• strict constraints on c2 (75 efficiency)
• rejected tracks with low-resolution clusters
  along the trajectory
• - faulty strips (high noise)
• - d-rays (high signal and multiplicity)

19
High-energy antiproton selection
p
p-bar
20
High-energy antiproton selection
p-bar
p
21
High-energy antiproton selection
p-bar
p
R lt MDR/10
22
Antiproton/proton ratio
Preliminary!
300 p-bar
23
Antiproton/proton ratio
Preliminary!
Secondary production CRISM? p-bar
24
Conclusions

• PAMELA is in space, continuously taking data
  since July 2006
• Detectors have been calibrated and in-flight
  performances has been studied
• ? PAMELA now ready for science!!
• Magnetic spectrometer
• - basic performances (noise, cluster signal,
  spatial resolution...) are nominal
• - tracking system alignment completed
  (incoherentcoherent)
• Spectrometer performances (momentum resolution)
  fulfill the requirements of the experiment
• Preliminary results about high-energy antiproton
  abundance could be obtained!!
• Work in progress to extend antiproton measurement
  further in energy
• ? thanks! ?

25
Spares
26
Spectrometer design

• PAMELA scientific objectives require extremely
  good momentum resolution
• silicon detectors ? low noise electronics ?
  spatial resolution mm
• reduced amount of traversed material ? minimize
  multiple scattering effect

Spectrometer flight model

• Satellite constraints
• mechanical stresses during launch phase (7.4 g
  rms, 50 g shocks)
• thermal variations (5-35 oC)
• small power consumption
• redundancy and safety
• protection against highly ionizing cosmic rays
• limited telemetry

27
Readout electronics

• Front-end ? VA1 chips
• 16 chip/ladder ? 288 chips
• 1.2 µm CMOS ASIC (CERN - Ideas, Norway)
• 6.2 mm 4.5 mm chip area 47 µm input pad pitch
• 128 low-noise charge preamplifiers
• shaping time 1 µs
• operating point set for optimal compromise
• Po wer consumption 1.0 mW/channel ? total
  dissipation 37 W for 36864 channels
• - voltage gain 7.0 mV/Fc ? dynamic range up to 10
  MIP
• ADC
• 1 ADC/ladder ? 36 ADCs
• event acquisition time 2.1 ms.
• DSP
• 1 DSP/view (ADSP2187L) ? 12 DSPs
• control logics on FPGA chips (A54SX)
• on-line calibration (PED,SIG,BAD)
• data compression
• - compression factor gt 95
• - compression time 1.1 ms

SADC-PED-CN 1 MIP150 ADC counts (_at_peak)
28
Orbital environment and in-flight operation

• Particle rate
• maximum at the poles (cutoff lt100 MV)
• minimum at the equator (cutoff 15 GV)
• Instrument operates also inside radiation belts

• Tracker operations
• Calibration performed every 95 min, soon after
  ascending node
• Data acquisition
• Special run after calibration ? full mode
• Physics run ? compressed mode (5)
• 12 x 250 B 3 kB/ev ? 5 kB/ev (all
  detectors)
• Slow controls , eg temperature sensors
• 21o _at_ power up, 28o _at_ regime
• ? lt 1º C variations along orbit
• Remote controls
• ? DSP configuration

29
PAMELA nominal capabilities

• energy range particles in 3 years
• Antiproton flux 80 MeV - 190 GeV 104
• Positron flux 50 MeV 270 GeV 105
• Electron flux up to 400 GeV 106
• Proton flux up to 700 GeV 108
• Electron/positron flux up to 2 TeV (from
  calorimeter)
• Light Nuclei up to 200 GeV/n He/Be/C
  107/4/5
• AntiNuclei search sensitivity of 3x10-8 in He/He

• Simultaneous measurement of many cosmic-ray
  species
• New energy range
• Unprecedented statistics

Taking into account live time and geometrical
factor 1 HEAT-PBAR flight 22.4 days PAMELA
data 1 CAPRICE98 flight 3.9 days PAMELA data
30
Dead/Live time
31
Temperatures

• After power-up temperature remains stable
• lt 1º C variations along orbit
• lt 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)
32
Calibration
Pedestal
Y view
X view
Noise
X view
Y view
N 4 ADC counts
N 9 ADC counts

• Calibration performed _at_ every orbit (95 min),
  soon after the equator
• Calibration parameters are
• used on-line to compress data (95 compresion
  factor)
• transmitted to ground for off-line data reduction

33
Cluster signal
The energy deposited by charged particles is
collected by more than 1 strip
(low energy protons and He nuclei)
Muliplicity
Signal
Signal correlation (Y vs X )
Y view larger noise ? worse performances
Signal-to-noise
X view lower noise ? better performances
34
11.6 GV interacting anti-proton
35
32.3 GV positron
36
Track recognition
X view
Y view
Top view
Y-view ambiguity

• R 23.2 GV
• h -0.043 GV-1
• c2 0.36
• b 1.08

• Hough transform circle sector (X) straight
  line (Y)
• ? Track recognition efficiency close to nominal
  value (sensor geometry)
• Ambiguity on Y view solved with help from ToF
  and calorimeter information

37
Track fitting

• Iterative c2 minimization as a function of track
  state-vector components a
• h 1/R ? magnetic deflection
• Trajectory evaluated by stepwise integration of
  motion equations by means of Runge-Kutta method
  (not-homogeneous B field)

Flight data - protons

• Intrinsic sensor resolution
• It accounts for
• track-angle dependence
• cluster-noise

Energy dependence due to multiple scattering
effects
38
MDR

• MDR naturally account for
• angular effects on the spatial resolution
• measurements positions along the trajectory

?MDR? 720 GV
39
(No Transcript)
40
Number of selected antiprotons
Preliminary!
2 0.438034 0.5814 10 0.5814 0.753128 10
0.753128 0.956853 27 0.956853 1.19688 32
1.19688 1.47837
23 30.7925 47.6105 6 47.6105 99.0664
39 1.47837 1.8075 51 1.8075 2.19179 54
2.19179 2.64048 59 2.64048 3.16508 68
3.16508 3.7802 78 3.7802 4.50482 93
4.50482 5.36419 83 5.36419 6.39306 60
6.39306 7.64097 73 7.64097 9.18169 81
9.18169 11.1308 59 11.1308 13.6811 80
13.6811 17.1812 65 17.1812 22.3332 51
22.3332 30.7925
41
Tracker alignment

• Evaluation of the position of silicon sensors
  relative to the nominal
• mechanical positions
• System aligned _at_ground with proton beam
  (CERN-SPS ) and atmospheric muons (cross-check)
• sensor displacements relative to mechanical
  positions ? 100 mm
• In-flight correction are necessary
• observed displacements after launch, relative to
  ground alignment ? 10 mm
• Track-based alignment
• minimization of spatial residuals as a function
  of the roto-traslational parameters
  (w,b,g,Dx,Dy,Dz) of 34 sensors (6 x 6 - 2 fixed),
  relative to mechanical position
• Tracks bent by the magnetic field
• ? the shape of the particle trajectory has to be
  known a priori

((? solutions!!))

• INPUT
• particle energy
• magnetic field

• OUTPUT
• sensor rototraslation parameters

real
measured
aligned
42
Alignment method

• Track-based alignment
• Minimization of c2 of track spatial residuals
• Several strategy can be applied (iterative
  procedures or global fit)
• Intrinsic problem ? possible insensitivity to
  certain global distortions
• Resisting high statistics
• Depends on data set

43
In-flight alignment STEP 1

• Step 1 ? Correction for random displacements of
  the sensors (incoherent alignment)
• Done with relativistic protons
• Input trajectory evaluated from (misaligned)
  spectrometer fit

measured
step 1
Flight data Simulation

• After incoherent alignment
• residuals are centered
• width consistent with nominal resolution
  alignment uncertainty (1mm)

X side
Y side
protons 7-100 GV (6x6y, all plane included in
the fit)
44
In-flight alignment STEP 2

• After Step1
• ? (possible) uncorrected global distortions might
  mimic a residual deflection
• spectrometer systematic effect
• Step 2 ? Correction for global distortions of the
  system (coherent alignment)
• Done with electrons and positrons
• Energy determined with the calorimeter
• ? DE/E lt 10 above 5GeV

step 2
step 1

• Energy-rigidity match
• HOWEVER, the energy measured by the calorimeter
  can not be used directly as input of the
  alignment procedure, for two reasons
• Calorimeter calibration systematic uncertainty
• Electron/positron Bremstrahlung above the
  spectrometer

deflection offset
calorimeter calibration uncertanty
45
Bremsstrahlung effect

• From Bethe-Heitler model
• The probability distribution of z
• depends on the amount of traversed material
• does not depend on the initial momentum
• it should be the same for electrons and
  positrons!!
• With real data
• Spectrometer systematic gives a charge-sign
  dependent effect
• Calorimeter systematic has the same effect for
  both electrons and positrons

e
P0
? t0.1X0
PSpe
PCalP0
g e
46
Spectrometer systematic
Clean sample of electrons and positrons with
ECal520 GeV
electrons positrons
P010-5
P00.403
Kolmogorov probability between ze- and ze (P0
0 1) with free parameter Dh Dh -10-3 GV -1
47
Charge collection
h2 ? 2-strip center-of-gravity

• Non-linear charge collection ? best estimate of
  impact coordinate given by h-algorithm

(standard implementation)
Experimental h-distribution

• for small angles (lt10o) 2-strip algorithm gives
  best resolution ? xs(h2)

48
Angular systematic
Standard implementation of the h-algorithm relies
on the assumption of symmetric signal
distribution ? condition not satisfied in case of
inclined tracks
49
Angular correction
Angular effect studied in ref. Landi G., NIMA,
554 (2005) 226 - study of discretization
effects on position reconstruction with silicon
microstrip detectors, by means of analytical
model (signal theory) and Monte Carlo simulation
(both tuned on PAMELA tracker sensors)
X side ( 51 mm pitch )
Y side ( 66.5 mm pitch )
1mm
1mm
-1mm
-1mm

• Results
• Standard h-algorithm has a systematic error up
  to 2mm ? significant on the x view!
• Correction can be derived from data itself
• ( Center of gravity with 4 strips (or more) has
  no systematic (but worse resolution) )

50
Angular systematic (beam test)
On 2006 test of detector prototypes _at_SPS (protons
50-100-150 GV/c)
Angular systematic has opposite sign on the x
view of the middle plane
h4
Angular correction
?? used to align the system ? ? used to check
alignment

• Theoretical results confirmed
• Correction account also for intrinsic asymmetries

51
Angular systematic (flight data)
Spatial x-residual on the bottom plane
Angular systematic has opposite sign on the x
view of the last plane
X side (51 mm pitch)
Angular correction
52
Angular correction in flight
Y side (66.5 mm pitch)
X side (51 mm pitch)
protons 7100 GV