ion charge measurement with the ams-02 silicon tracker

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

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

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

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

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

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

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)

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

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

Martin Pohl


X-s

ide

Clu

ster

s

VA Number

• Proton• Helium• Carbon

Charge Calibration Sample Selection

Martin Pohl


Reference MPV values for each charge

• Proton• Helium• Carbon

Readout Region

Individual VA gains equalized on reference value

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

Martin Pohl


Offset must be taken into accountin gain correction!

Gain Correction Factors and Offsets

At most 10% correctionneeded.

Martin Pohl


Deviation of VA MPV values from Linear Fit

Systematic error ~ 3%

Martin Pohl


Gain Correction Effect on H, He and C Samples

• No Correction• Gain Correction• Including Offsets

RMS improvesby factor of 3.5

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

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

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

Martin Pohl


Resolution of Charge Estimator After Gain Correction

A. Oliva

• n side before correction• n side after gain correction

Martin Pohl


Nu

mb

er

Nu

clei

C

BeB O

Ne

Li

He

H

Before CorrectionAfter Gain Corrections

Track Truncated Mean p Side

Martin Pohl


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

Martin Pohl


Charge Collection: Impact Point and Angle

ZXZ Projected Track

θXZ

X

X

Y

Z

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

Martin Pohl


Ne

O

C

B

Be N

F

• No Corr• Gain Corr• Gain + Charge Loss

Track Truncated Mean n Side

Martin Pohl


Resolution of Charge Estimator After Correction

Martin Pohl


OC

Mg

FeSi

BBe

Li

Track Truncated Mean p Side

Martin Pohl


Path Length Correction

Normalization to 300 μm of Silicon traversed.

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

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

Martin Pohl


Beta CorrectionProtons

Helium

TOF measures

β inside AMS

β > βTOF

βTOF

β < βTOF

Martin Pohl


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.)

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

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

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

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

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

Martin Pohl


Tracker and ToF

HHe

LiBe B

CN O

FNe

NaMg

AlSi

Cl Ar K Ca Sc Ti

V Cr

P SFe

Ni

Martin Pohl


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

ion charge measurement with the ams-02 silicon tracker

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