Page 1 Martin Pohl DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE Ion charge measurement with the AMS-02 silicon tracker 1rst Int. Workshop on High

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

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Martin Pohl


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

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Martin Pohl


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

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Martin Pohl


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

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Martin Pohl


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

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Martin Pohl


Example of Gain Differences for He for p-side VAs of Ladder +307

Raw

AD

C

Typical ~10%, max ~35%

x 10

Helium Sample

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Martin Pohl


• Landau function convoluted with a Gaussian• MPV to characterize the gain of a given VA

Single VA Distribution forProton.

Amplitude distribution (protons, single VA)

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Martin Pohl


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

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Martin Pohl


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

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Martin Pohl


X-s

ide

Clu

ster

s

VA Number

• Proton• Helium• Carbon

Charge Calibration Sample Selection

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Martin Pohl


Reference MPV values for each charge

• Proton• Helium• Carbon

Readout Region

Individual VA gains equalized on reference value

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Martin Pohl


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

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Martin Pohl


Offset must be taken into accountin gain correction!

Gain Correction Factors and Offsets

At most 10% correctionneeded.

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Martin Pohl


Deviation of VA MPV values from Linear Fit

Systematic error ~ 3%

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Martin Pohl


Gain Correction Effect on H, He and C Samples

• No Correction• Gain Correction• Including Offsets

RMS improvesby factor of 3.5

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Martin Pohl


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

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Martin Pohl


Nu

mb

er

Nu

clei

C

BeB N

O

F

Ne

Na

MgSi

Li

He

HBefore CorrectionAfter Gain Corrections

Track Truncated Mean n Side

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Martin Pohl


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

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Martin Pohl


Resolution of Charge Estimator After Gain Correction

A. Oliva

• n side before correction• n side after gain correction

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Martin Pohl


Nu

mb

er

Nu

clei

C

BeB O

Ne

Li

He

H

Before CorrectionAfter Gain Corrections

Track Truncated Mean p Side

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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

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Martin Pohl


Charge Collection: Impact Point and Angle

ZXZ Projected Track

θXZ

X

X

Y

Z

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Martin Pohl


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

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Martin Pohl


Ne

O

C

B

Be N

F

• No Corr• Gain Corr• Gain + Charge Loss

Track Truncated Mean n Side

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Martin Pohl


Resolution of Charge Estimator After Correction

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Martin Pohl


OC

Mg

FeSi

BBe

Li

Track Truncated Mean p Side

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Path Length Correction

Normalization to 300 μm of Silicon traversed.

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Martin Pohl


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

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Martin Pohl


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

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Beta CorrectionProtons

Helium

TOF measures

β inside AMS

β > βTOF

βTOF

β < βTOF

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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.)

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Martin Pohl


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

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Martin Pohl


• 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

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Martin Pohl


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

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Martin Pohl


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

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Martin Pohl


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

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Martin Pohl


Tracker and ToF

HHe

LiBe B

CN O

FNe

NaMg

AlSi

Cl Ar K Ca Sc Ti

V Cr

P SFe

Ni

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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

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Page 1 Martin Pohl DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE Ion charge measurement with the AMS-02 silicon tracker 1rst Int. Workshop on High