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Page 1
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Ion charge measurementwith the AMS-02 silicon tracker
1rst Int. Workshop on High Energy cosmic-Radiation DetectionOctober 17-18, 2012IHEP CAS, Beijing
Martin Pohl, Pierre SaouterCenter for Astroparticle Physics
University of Geneva
Alberto OlivaCIEMAT Madrid
Page 2
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Si Tracker Charge Measurement
Strip crosstalk
Gain (at VA level, using H, He and C)
Charge loss (position/angle dependence)
MIP scale conversion (saturation, non-linearities)
From ADC to energy deposition
Detector related corrections
From energy deposition to floating point charge estimators (Q)
From floating point chargeestimator to integer charge (Z)
Pathlength correction
Beta/Rigidity correction (layer dependent)
PDFZ(Edep)
Likelihood
Page 3
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Si Tracker Charge Measurement
• Physics:
• From physics to ADC:• Si material properties• Nuclear charge: z2
• β and βγ: eV/μm• Path length in Si: dx• Ionisation yield: eV fC• Charge collection efficiency on strips• ASIC response function• Channel cross talk: ADC
Page 4
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
The AMS Silicon Tracker
9 planes: 18 to 26 ladders Ladder : 7 to 15 double-sided silicon sensors. Implantation pitch p(n) side 27.5 (104) μm Readout pitch p(n) side 110 (208) μm (1/4 and 1/2 strips read out)
Ionization Energy Loss
• Signal usually collected by several adjacent strips (cluster)• Double threshold to eliminate insignificant strips
Cluster Amplitude
Page 5
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
VA64hdr
Front-end electronics
10 VAs on the p-side (Y direction) 6 VAs on the n-side (X direction)
Each VA reads 64 channels
• Each VA produces a signal with different characteristics • In particular differences in the gain are observed• FEE response curve is deliberately non-linear, different for p and n
p-side
n-side
Page 6
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Example of Gain Differences for He for p-side VAs of Ladder +307
Raw
AD
C
Typical ~10%, max ~35%
x 10
Helium Sample
Page 7
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
• Landau function convoluted with a Gaussian• MPV to characterize the gain of a given VA
Single VA Distribution forProton.
Amplitude distribution (protons, single VA)
Page 8
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Cluster pulse integral (single ladder) as function of ion charge
Alpat B. & al., 2004 (2003 Cern and GSI Test Beam)
Si
B
1. Two sides behave differently:• Maximum dynamic range• Good resolution at low charge
2. Two ~ linear response regimes
3. Same behavior expected for all VA
n side
p side
Page 9
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Charge Calibration Sample Selection
• Uncalibrated charge response with rather good resolution• Define charge samples using truncated mean of hits on n side, corrected for impact angle • 1σ selection ranges around MPV
Avoid any bias in selection: • separate ranges for each layer • truncated mean excluding layer under study
• (see later)
HHe
LiBe
BC
NO
Page 10
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
X-s
ide
Clu
ster
s
VA Number
• Proton• Helium• Carbon
Charge Calibration Sample Selection
Page 11
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Reference MPV values for each charge
• Proton• Helium• Carbon
Readout Region
Individual VA gains equalized on reference value
Page 12
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Good linearity of VA64 response
• Gain factor inde-pendent of particle impact location
• Small offset due to thresholds on seed and adjacent strips
Gain Corr. Fact
Page 13
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Offset must be taken into accountin gain correction!
Gain Correction Factors and Offsets
At most 10% correctionneeded.
Page 14
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Deviation of VA MPV values from Linear Fit
Systematic error ~ 3%
Page 15
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Gain Correction Effect on H, He and C Samples
• No Correction• Gain Correction• Including Offsets
RMS improvesby factor of 3.5
Page 16
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Gain Systematics
• Each point is mean of VA response per layer, with RMS as error• RMS is larger for layer 1• Systematics less than 0.5% << statistical error on gain factor
Layer 1 Layer 2 Layer 3 Layer 4 Layer 5 Layer 6 Layer 7 Layer 8 Layer 9
Page 17
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Nu
mb
er
Nu
clei
C
BeB N
O
F
Ne
Na
MgSi
Li
He
HBefore CorrectionAfter Gain Corrections
Track Truncated Mean n Side
Page 18
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
C
Be
BN
O
F
Ne
Na
Mg
Si
log
(N
um
ber
Nu
clei
) Before CorrectionAfter Gain Corrections
Zoom on High Charges n Side
Page 19
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Resolution of Charge Estimator After Gain Correction
A. Oliva
• n side before correction• n side after gain correction
Page 20
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Nu
mb
er
Nu
clei
C
BeB O
Ne
Li
He
H
Before CorrectionAfter Gain Corrections
Track Truncated Mean p Side
Page 21
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Charge Collection Efficiency
Particle very near a readout strip.
Particle passes in between two readout strips.
Capacitive coupling between strips allows to estimate impact positionof the traversing particle (COG).
Charge loss ~30 % for Helium
0
Loss of collection efficiency in thenon-readout region
Page 22
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Charge Collection: Impact Point and Angle
ZXZ Projected Track
θXZ
X
X
Y
Z
Page 23
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Implant structure and n/p side differences
• n - side: 1 out of 2 strips read out + saturation• p - side: 1 out 4 strips readout + non linearity at low charges (B,C,O)
different charge collection behavior
Charge Loss For Carbon Sample
N-Side / Z=6 / ~28%P-Side / Z=6 / ~35%
ADC ADC
Page 24
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Ne
O
C
B
Be N
F
• No Corr• Gain Corr• Gain + Charge Loss
Track Truncated Mean n Side
Page 25
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Resolution of Charge Estimator After Correction
Page 26
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
OC
Mg
FeSi
BBe
Li
Track Truncated Mean p Side
Page 27
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Path Length Correction
Normalization to 300 μm of Silicon traversed.
Page 28
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Beta Correction: Layer-by-Layer (II)
Z = 1 Z = 2
Z = 2 Z = 1 Layer 4 Layer 4
Layer 1Layer 1
Effect of TRD + upper TOF
Effect of TRD + upper TOF
Page 29
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Beta Correction: Layer-by-Layer (III)
Z = 1 Z = 2
Z = 2 Z = 1 Layer 8Layer 8
Layer 9Layer 9
Effect of RICH + lower TOF
Effect of RICH + lower TOF
Page 30
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Beta CorrectionProtons
Helium
TOF measures
β inside AMS
β > βTOF
βTOF
β < βTOF
Page 31
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Tracker Charge Measurement
Z>10 should use p-side
n
Track Truncated Mean p–Side (c.u.)
Trac
k Tr
unca
ted
Mea
n n–
Sid
e (c
.u.)
Page 32
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
MIP Correction • Transforms corrected response into charge units.• Accounts for saturation and non-linearity• Directly provided as an outcome of the charge loss correction
• Gives almost linear charge estimator• Some residual deviation left in the non-linearity regions
n side
p side
Page 33
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
• Combine the n and p measurement with a weighted sum. • Weights depend on the number of hits used• Weights assumed to be independent of Z (approximately correct)
H x 10-3
He x 10-2
Be
C
O
Si
Fe
Joint Track Charge Estimator
Page 34
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Going to PDF
1 23
45
6
78
9
1012
14
26
• This shapes should be understood in detail• Tails from wrong hit associated to tracks, interactions…• Specific ladder behavior • Dependencies on external parameters: t, T …
Layer 2 charge distributions
Page 35
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
ZTRK_L1=6.1
ZTRD=5.9
ZTOF_UP=5.9
ZTOF_LOW=5.8
ZTRK_IN=5.8
ZRICH=6.1
Carbon: Rigidity=215 GV, P=1288 GeV, Ekin/A=106 GeV/n
Page 36
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
ZTRK_L1=4.9
ZTRD=4.5
ZTOF_UP=5.0
ZTOF_LOW=5.1
ZTRK_IN=4.9
ZRICH=5.2
Boron: Rigidity=187 GV, P=935 GeV, Ekin/A=93 GeV/n
Page 37
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Tracker and ToF
HHe
LiBe B
CN O
FNe
NaMg
AlSi
Cl Ar K Ca Sc Ti
V Cr
P SFe
Ni
Page 38
Martin Pohl
DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE
Conclusions• AMS Si tracker shows excellent nuclear charge
identification:– Excellent charge separation– Simple unfolding of species
• Complete calibration chain in place:– Floating point charge estimator– Probabilistic approach based on PDF
• Redundancy of subdetectors is key to systematic accuracy:– Tracker– ToF– RICH
• Chemical composition of cosmic rays GeV to TeV