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full electromagnetic simulation of multi-strip detectors. Diego González-Díaz (GSI-Darmstadt) A. Berezutskiy (SPSPU-Saint Petersburg), G. Kornakov (USC-Santiago de Compostela), M. Ciobanu (GSI-Darmstadt), Y. Wang (Tsinghua U.-Beijing), J. Wang (Tsinghua U.-Beijing).

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slide1

full electromagnetic

simulation of multi-strip

detectors

Diego González-Díaz (GSI-Darmstadt)

A. Berezutskiy (SPSPU-Saint Petersburg), G. Kornakov (USC-Santiago de Compostela), M. Ciobanu (GSI-Darmstadt), Y. Wang (Tsinghua U.-Beijing), J. Wang (Tsinghua U.-Beijing)

Darmstadt, November 24th

some references used in this talk
Some references used in this talk

[1a] H. Alvarez Pol et al., 'A large area timing RPC prototype for ion collisions in the HADES spectrometer', NIM A, 535(2004)277.

[2a] A. Akindinov et al., 'RPC with low-resistive phosphate glass electrodes as a candidate for CBM TOF', NIM A, 572(2007)676.

[3a] J. Wang et al., paper in preparation.

[4a] L. Lopes et al., 'Ceramic high-rate RPCs', Nuclear Physics B (Proc. Suppl.), 158(2006)66.

[5a] D. Gonzalez-Diaz et al., 'The effect of temperature on the rate capability of glass timing RPCs', NIM A, 555(2005)72.

[6a] A. Ammosov et al., talk at XIII CBM collaboration meeting, Darmstadt, Germany.

[7a] L. Nauman et al., talk at XIV CBM collaboration meeting, Split, Croatia.

[1] A. Mangiarotti et al., 'On the deterministic and stochastic solution of Space-Charge models and their impact in high resolution timing' talk at RPC Workshop Seoul, 2005.

[2] G. Chiodini et al., 'Characterization with a Nitrogen laser of a small size RPC', NIM A 572(2007)173.

[3] A. Colucci et al., 'Measurement of drift velocity and amplification coefficient in C2H2F4-isobutane mixtures for avalanche-operated resistive-plate counters', NIM A, 425(1999)84.

[4] W. Riegler et al., 'Detector physics and simulations of resistive plate chambers', 500(2003)144 .

[5] E. Basurto et al., 'Time-resolved measurement of electron swarm coefficients in tetrafluoretane (R134a)', Proc. to 28th ICPIG, Prague, 2007.

[6] P. Fonte, V. Peskov, 'High resolution TOF with RPCs', NIM A, 477(2002)17.

[7] P. Fonte et al., 'High-resolution RPCs for large TOF systems', NIM A, 449(2000)295.

[8] A. Akindinov et al. 'Latest results on the performance of the multigap resistive plate chamber used for the ALICE TOF', NIM A 533(2004)74.

[9] G. Aielli et al., 'Performance of a large-size RPC equipped with the final front-end electronics at X5-GIF irradiation facility', NIM A 456(2000)77.

[10] S. An et al., 'A 20 ps timing device—A Multigap Resistive Plate Chamber with 24 gas gaps', NIM A 594(2008)39.

[11] A. Blanco et al., 'In-beam measurements of the HADES-TOF RPC wall', NIM A 602(2009)691.

[12] W. Riegler, D. Burgarth, 'Signal propagation, termination, crosstalk and losses in resistive plate chambers', NIM A 481(2002)130.

[13] T. Heubrandtner et al., NIM A 489(2002)439.

slide3

The Compressed Baryonic Matter Experiment

Transition

Radiation

Detectors

Tracking

Detector

Electro-

magnetic

Calorimeter

Muon

detection

System

Ring Imaging

Cherenkov

Detector

Silicon

Tracking

Stations

Projectile

Spectator

Detector

(Calorimeter)

Vertex

Detector

Dipole

magnet

Resistive

Plate

Chambers

(TOF)

slide4

The CBM-TOF wall. Design requirements

  • Overall time resolution (including start time) σT = 80 ps.
  • Occupancy < 5 % for Au-Au central collisions at E=25 GeV/A.
  • Space resolution ≤ 5 mm x 5 mm.
  • Efficiency > 95 %.
  • Pile-up < 5%.
  • Rate capability = 20 kHz/cm2.
  • Multi-hit capability (low cross-talk).
  • Compact and low consuming electronics (~65.000 electronic channels).
  • Multi-strip design in the outer region, due to the very low occupancies. Why? -> Why not?. If electrically possible it is mechanically much more easy.
the cbm tof wall simulation based on occupancies
The CBM-TOF wall. Simulation based on occupancies

In order to accommodate the different granularities as a function of the polar angle, five different regions were defined:

  • Padregion (1): 2.0 x 2.0 cm2 ( 27072 channels, ~10 m2)
  • Strip region (2): 2.0 x 12.5 cm2 ( 3840 x 2 channels, ~10 m2)
  • Strip region (3): 2.0 x 25.0 cm2 ( 5568 x 2 channels, ~30 m2)
  • Strip region (4): 2.0 x 50.0 cm2 ( 6150 x 2 channels, ~60 m2 )
  • Strip region (5): 2.0 x 100.0 cm2 ( 2900 x 2 channels, ~60 m2 )

TOTAL ( ~65000 channels, ~170m2)

slide6

A multi-gap RPC in general. Here a differential RPC ('a la' STAR), just for the sake of 'electrical elegance'

*parameters not from STAR

differential pre-amplifier

particle

HV insulator

with Vbreak>10-15 kV

standard PCB

with read-out

strips on one

side

Rin

+V

at least 4 gas gaps

(~0.3 mm thick)

-V

float glass

HV coating with

R~100 MΩ/□

slide7

More electrical schemes are (un)fortunately possible

HADES-SIS

FOPI-SIS

ALICE-LHC

-V

-V

V

V

-V

-V

STAR-RHIC

-V

-V

V

V

!

V

-V

V

-V

V

S. An et al., NIM A 594(2008)39 [10]

all these schemes are equivalent regarding the underlying avalanche dynamics... but the RPC is also a strip-line, and this is manifested after the avalanche current has been induced. And all these strip-lines have a completely different electrical behavior.

HV filtering scheme is omitted

slide8

Generation + induction + transmission + FEE. Sketch

transmission

1

2

generation + induction

4

3

multi-strip

FEE response

avalanche generation a simple avalanche model
Avalanche generation. A simple avalanche model

8.7

Raether limit

log10 nelectrons

space-charge

regime

~7.5

~7

exponential-growth

regime

threshold

~2

exponential-fluctuation

regime

0

to

tmeas

t

avalanche Furry-type

fluctuations

simplifying assumptions

  • The stochastic solution of the avalanche equation is given by a simple Furry law(non-equilibrium effects are not included).
  • Avalanche evolution under strong space-charge regime is characterized by no effective multiplication. The growth stops when the avalanche reaches a certain number of carriers called here ne,sat.
  • The amplifier is assumed to be slow enough to be sensitive to the signal charge and not to its amplitude. We work, for convenience, with a threshold in charge units Qth.
slide10

Parameters of the gas used for input: α* (effective Townsend coefficient), vd (drift velocity), no (ionization density)

little dependence

with mixture!

no [mm-1]

HEED

(from Lippmann[4])

αextrapolated to mixture by using

Freon's partial pressure:

αmixture = αFreon(E/fFreon) fFreon

*purely phenomenological!

continuous line: data from Basurto et al.

in pure Freon [5]

vddirectly taken from Freon (inspired on microscopic codes)

vd,mixture = vd,Freon

slide13

MC results. Prompt charge distributions for 'wide-pad' detectors

1-gap 0.3 mm RPC standard mixture

4-gap 0.3 mm RPC standard mixture

Eff = 74%

Eff = 60%

Eff = 38%

simulated

simulated

measured

qinduced, prompt [pC]

qinduced, prompt [pC]

measured

assuming space-charge saturation at

ne,sat= 4.0 107 (for E=100 kV/cm)

data from Fonte, [6,7]

qinduced, total [pC]

slide15

Generation + induction + transmission + FEE

transmission

1

2

generation + induction

4

3

multi-strip

FEE response

induction and weighting field e z
Induction and weighting field Ez

read-out

ws-s ~0 mm

g=0.3 mm

t=2.5 mm

HV

w=22 mm

wide-pad limit t << w

additionally when g<<t (typical

situation) Ez does not depend on the position –z- along the gap

We adapted to multi-gap the formulas from:

problem: under-estimation of Ez for large inter-strip separations

T. Heubrandtner et al. NIM A 489(2002)439

slide17

Cross-talk in a 2-strip RPC modeled as a loss-less transmission-line (I)

W. Riegler, D. Burgarth, NIMA 481(2002)130 [12]

see

if

a 4-gap RPC seen as a transmission-line

dominated by skin-effect:

small for typical dimensions

and rise-times

very small, due to the presence of gas and glass

two different modes in the transmission line!. This causes 'modal dispersion' unless:

true for homogeneous transmission lines!

1)

for typical materials (glass)

loss-less line!

2)

slide18

Cross-talk in a 2-strip RPC modeled as a loss-less transmission-line (II). Limits.

zo = position along the strip where the signal is induced

for exponential signals

small dispersion

high-frequency /large distance

/ dispersive

limit

low-frequency /small distance

/ non-dispersive

limit

very large dispersion

the 2 modes are fully

decoupled

see also [12]

cross talk influence in the timing of a coincident double hit a simple derivation i
Cross-talk influence in the timing of a coincident (double) hit. A simple derivation (I).

space-charge

log[i(t)]

ith

exponential regime

t

variations in base-line due to cross-talk

variations in time at threshold due to cross-talk

slide20

Cross-talk influence in the timing of a coincident (double) hit. A simple derivation (II).

Assumptions:

  • Within the same primary collision cross-talk extends up-to infinite time.
  • It does not depend on position.
  • Fluctuations in time of cross-talk signal are smaller than fluctuations coming from the avalanche charge distribution.
  • Pick-up strips are separated by a typical distance bigger than the area of influence of the avalanche. Charge sharing during induction can be neglected!.
  • Cross-talk is small, given by Fct.

cross-talk is expected to affect timing when

slide21

History revisited: 1.6m-long 2-strip RPC (P. Fonte et al., 2002)

strip separation = 1mm

gap = 0.3mm

glass = 3mm

width = 5cm

length= 1.6m

slide23

Zc~13 Ω

Cross-talk in Fonte multi-strip RPC

Cm=88 pF/m

BW=1.5 GHz

Rin=50 Ω

experimental conditions:

Π, E=3.5 GeV, low rates, trigger width = 2 cm

Cg=521 pF/m

Fct=50%!

very dispersive!

transverse scan

'fine-tunning'

Fct=40%

HV=5.7 kV

80%-90% measured cross-talk levels reproduced

slide24

Minimizing cross-talk (I)

->increase strip

separation

Cg

Cm

Δv/v

trise

x10

slide25

Minimizing cross-talk (II)

->increase strip/width

separation

->reduce glass thickness

Cg

Cm

Δv/v

trise

/2.5

x6

/6

slide26

Minimizing cross-talk (III)

->increase strip/width

separation

->reduce glass thickness

->reduce band-width

Cg

Cm

Δv/v

low coupling

low dispersion

trise

BW/10

/2.5

x6

/6

slide27

Minimizing cross-talk (IV)

->put guard strip

Cg

Cm

Δv/v

trise

guard strip

slide28

Minimizing cross-talk (V)

->use only two electrodes

Cg

Cm

(it flips!)

Δv/v

trise

not mirrored

slide29

Minimizing cross-talk (VI)

->use only two electrodes

->couple locally to ground

Cg

Cm

Δv/v

low coupling

NO dispersion

trise

not mirrored

coupling to PCB

35 cm long wide strip mirrored and shielded

Zc~18 Ω

35-cm long wide-strip, mirrored and shielded

Cm

experimental conditions:

~mips from p-Pb reactions at 3.1 GeV, low rates,

trigger width = 2 cm

BW=260 MHz

Rin=100 Ω

...

...

Cg

little dispersive

Fct=11%

transverse scan

'fine-tunning'

inter-strip region

dominated by trigger width

Fct=19%

probability of pure cross-talk:

1-3%

Analysis with high resolution tracking on-going.

1 m long counter 6 strip rpc 12 gap mirrored and shielded
1-mlong counter, 6-strip RPC, 12-gap, mirrored and shielded

experimental conditions:

~mips from p-Pb reactions at 3.1 GeV, low rates,

trigger width = 2 cm (< strip width)

long run. Very high statistics.

...

...

No simulations available yet

1 m long counter 12 gap mirrored and shielded
1-m long counter, 12-gap, mirrored and shielded

double-hit in any of 3rd neighbors

double-hit in any of 2nd neighbors

double-hit in any of 1st neighbors

no double hit

No simulations available yet

conclusions and outlook
Multi-strip design of timing RPCs at 1-m scale with acceptable cross-talk, small cluster size and small deterioration of time resolution seems doable.

Further optimized structures based on simulations are on the way (Fct~1%).

For making a multi-strip fully robust against streamer-crosstalk there is still a long way to go (maybe impossible).

-> Detailed optimization based on physics performance soon to follow. Then we will know if cross-talk is 'high' or not.

conclusions and outlook
the fopi counter
The FOPI counter

Multi-strip-MRPC (MMRPC)

Glass: ε=7.5, strip width = 1.64 mm, strip gap = 0.9 mm, strip length = 900 mm

copper (20 μm)

8 gaps

1.1 mm

0.22 mm

0.5 mm

1.1 mm

slide45

Cross-talk in an un-terminated line

50

50

anode 1

cathode 1

50

50

50

50

anode 2

cathode 2

50

50

signal from BC420

scintillator (used as current generator)

50

50

anode 3

cathode 3

50

50

50

anode 4

cathode 4

50

50

50

anode 5

cathode 5

50

50

slide46

Cross-talk in a terminated line

50

50

anode 1

cathode 1

50

50

50

50

anode 2

cathode 2

50

50

50

50

anode 3

cathode 3

50

50

50

50

anode 4

cathode 4

50

50

50

50

anode 5

cathode 5

50

50

slide47

Cross-talk and signal shape

low dispersion counter, typical working conditions, BW=260 MHz

cross-talk

constant, very independent from the signal shape

Take as a typical shape the one of an avalanche produced at the cathode

Even for dispersive counters it is reasonable since most of the charge is coming from that region

the fopi counter 11 th strip
The FOPI counter (11th strip)

cathode

50

50

anode 0

50

50

anode 1

50

50

..........

50

50

anode 11

50

50

anode 12

50

50

anode 13

50

50

anode 14

50

50

anode 15

the fopi counter 9 th strip
The FOPI counter (9th strip)

cathode

50

50

anode 0

50

50

anode 1

50

50

..........

50

50

anode 9

50

50

anode 10

50

50

anode 11

50

50

anode 12

50

50

..........

slide52

several electrons (I)

  • An ionizing particle at fixed energy creates an average number of ionizations no randomly distributed along the gap, with each cluster having a (1/ne in cluster)2 probability to produce more than 1 electron. This is very easy to generate. Then each cluster can be made to fluctuate according to Furry law.

HEED

calculation

slide53

A parentheses: rate capability of various CBM prototypes

for small fluxes and in a simple DC-model

see for instance: D. Gonzalez-Diaz et al.

Nucl. Phys. B (Proc. Suppl.) 158(2006)111

slide55

How (we believe) is the avalanche produced?

g/ve~ 1 ns

g/vi~1 μs

ith

τg~1 s(glass relaxation time)

prompt (e-) component

Slow (ion) component

avalanche growth

decreases!

space-charge

limitation

Eav~E

E=ΔV/g

e--I+

particle

see [4],

for instance

slide56

More electrical schemes are (un)fortunately possible

HADES-SIS

FOPI-SIS

ALICE-LHC

-V

-V

V

V

-V

-V

STAR-RHIC

-V

-V

V

V

!

V

-V

V

-V

V

S. An et al., NIM A 594(2008)39 [10]

all these schemes are equivalent regarding the underlying avalanche dynamics... but the RPC is also a strip-line, and this is manifested after the avalanche current has been induced. And all these strip-lines have a completely different electrical behavior.

HV filtering scheme is omitted

slide57

First of all... what is a strip?

In standard language:- strip: something read-out in two ends/something 'quite rectangular'- pad: something read-out in one end/something 'quite squared'

In this talk:

A strip is a read-out structure that must be described (due to the phenomena under study) like a transmission-line. In the simplest single-strip description, it is something characterized by 2 magnitudes: a transmission coefficient and a propagation velocity.This is a definition based on the electrical properties of the structure.

slide58

Induction + transmission + FEE. Sketch (II)

  • Five stages in order to get a predictive result
  • Avalanche generation with the previous code.
  • [->Comparison with eff vs V and fine-tune, if needed, of threshold value. This approach seems to be flexible enough.]
  • Induction, based on analytical formulas from [13], extrapolated to multiple-gaps by using the effective series permittivity of the corresponding group of layers.
  • Propagation based on HF simulator APLAC (http://web.awrcorp.com/Usa/Products/APLAC/).
  • [-> Validation of APLAC for the structure of interest with a pulse generator (nowadays we do not need this step anymore)]
  • Termination and other circuit elements are included, together with FEE, simulated also with APLAC.
slide59

A 2-strip RPC as a loss-less transmission-line. Example (III)

2-strip geometry and signal taken from [12]

non-dispersive limit

(zo=0)

dispersive limit (zo->∞)

injected signal

cross-talk signal

->Continuous line is the exact analytical solution from [12].

->Dashed and dotted lines are the numerical solution from APLAC used later in this work.

slide61

Typical plots where to look at

  • Transverse profile of the efficiency, with and w/o valid charge.
  • Cross-talk probability. Integral and as a function of the charge in the main strip.
  • Resolution when a second hit is present in the module.
  • Cluster sizes (not shown here).
  • Dependence with HV of the above observables (not shown here).
50 cm long wide strip mirrored and not shielded

Zdet~20 Ω

50-cm long wide-strip, mirrored and not shielded

Cm=18 pF/m

BW=260 MHz

Rin=100 Ω

experimental conditions:

~mips from p-Pb reactions at 3.1 GeV, low rates,

trigger width = 2 cm (< strip width)

...

...

Cg=276 pF/m

Cm/Cg =6.5%

Fct=11.5%

dispersive

similar cross-talk levels than in previous case

probability of pure cross-talk: 1-3%

30 cm long narrow strip differential

Zdiff=80 Ω

30-cm long narrow strip, differential

Cm=20 pF/m

experimental conditions:

~mips from p-Pb reactions at 3.1 GeV, low rates,

high resolution (~0.1 mm) tracking

transverse scan

...

...

Cdiff=23 pF/m

Fct=9%

dispersive

intrinsic strip profile is accessible!

probability of pure cross-talk:

1-3%