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

[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.

Dipolemagnet

The Compressed Baryonic Matter Experiment

Ring ImagingCherenkovDetector

Transition Radiation Detectors

ResistivePlate Chambers(TOF)

Electro-magneticCalorimeter

SiliconTrackingStations

Tracking Detector

Muondetection System

Projectile SpectatorDetector(Calorimeter)

VertexDetector

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.

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

➔Pad region (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, ~170 m2)

The CBM-TOF wall. Simulation based on occupancies

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

Rin

standard PCBwith read-outstrips on oneside

HV insulatorwith Vbreak>10-15 kV

HV coating withR~100 MΩ/□

+V

-V

differential pre-amplifier

at least 4 gas gaps (~0.3 mm thick)

float glass

particle

*parameters not from STAR

More electrical schemes are (un)fortunately possible

ALICE-LHC

V

-V

-V

STAR-RHIC

V

-V

V

HADES-SIS

-V

-V

FOPI-SIS

-V

V

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.

-V

V

V

-V

V

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

!

HV filtering scheme is omitted

Generation + induction + transmission + FEE. Sketch

generation + induction

1transmission

2

FEE response

3

multi-strip

4

Avalanche generation. A simple avalanche model

• 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.

log 1

0 n e

lect

rons

~7

to t

space-charge regime

exponential-growthregime

~7.5

tmeas

avalanche Furry-typefluctuations

~2

Raether limit 8.7

exponential-fluctuationregime

threshold

0

simplifying assumptions

continuous line: data from Basurto et al.

in pure Freon [5]

α extrapolated to mixture by using Freon's partial pressure:

αmixture = αFreon(E/fFreon) fFreon

vd directly taken from Freon (inspired on microscopic codes)

vd,mixture = vd,Freon

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

density)

HEED(from Lippmann[4])

n o [m

m-1]

little dependencewith mixture!

*purely phenomenological!

results for wide-pad detectors

MC results. Efficiency and resolution for 'wide-pad' detectors

qinduced, prompt [pC]

qinduced, total [pC]

1-gap 0.3 mm RPC standard mixture

simulated

measured

Eff = 74%

Eff = 60%

Eff = 38%

measured

simulated

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

qinduced, prompt [pC]

assuming space-charge saturation at

4-gap 0.3 mm RPC standard mixture

data from Fonte, [6,7]

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

multi-strip detectors

Generation + induction + transmission + FEE

generation + induction

1transmission

2

FEE response

3

multi-strip

4

Induction and weighting field Ez

(t)nedzvEti )( t=2.5 mm

w=22 mm HV

read-out

wide-pad limit t << w

gap

totz C

C

gE

1

additionally when g<<t (typicalsituation) Ez does not depend on the position –z- along the gap

g=0.3 mm

ws-s ~0 mm

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

We adapted to multi-gap the formulas from: problem: under-estimation of Ez

for large inter-strip separations

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

om

mo

CC

CCC

om

mo

LL

LLL

om

moc ZZ

ZZZ

1ˆ TvTLZc

112 ˆ)ˆ(ˆ TLCTv

momo LLCC

20

02

vv

vv

v

cc

o

m

co

mc

c

ZZC

C

ZC

CZ

Z

roo

cCLv

1

)(3moom LCLCvv

cCC

LZ r

oo

oc

1

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

o

o

m

m

L

C

L

C true for

homogeneous transmission lines!

a 4-gap RPC seen as a transmission-line

om

mo

RR

RRR

dominated by skin-effect:small for typical dimensionsand rise-times

om

mo

GG

GGG

very small, due to the presence of gas and glass

)(ˆ CC

for typical materials (glass)

loss-less line!

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

if

1)

2) 0, mm LC

)(]9ln22)(

[)(2 v

ztI

tv

z

v

v

RZ

RZtI o

rise

o

inc

inmtc

for exponential signals

)(4

)(2,1v

ztItI o

tc

low-frequency /small distance/ non-dispersivelimit

high-frequency /large distance/ dispersivelimit

1

rise

o

tv

z

v

v

1

rise

o

tv

z

v

v

small dispersion

very large dispersion

zo = position along the strip where the signal is induced

see also [12]

the 2 modes are fullydecoupled

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

in

mg

mm

c

m

mgc

mg

mm

mg

RZ

Z

CC

C

L

L

Z

Z

CC

LZ

CC

C

L

L

v

v

CCLv

det

det

0

0

0

1

0

2

2

1

)(

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

log[i(t)]

t

ith

variations in base-line due to cross-talk

variations in time at threshold due to cross-talk 2talkcrossrms

space-charge

exponential regime

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.

Tctth

ctriseq

th

Fq

qF

t

q

rms

q

qrms

~

9lntalkcross

q

qF thct

cross-talk is expected to affect timing when

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

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

width = 5cm

strip separation = 1mm

glass = 3mm

gap = 0.3mm

length= 1.6m

Cross-talk in Fonte multi-strip RPC

Cg=521 pF/m

Cm=88 pF/m

Fct=50% !

BW=1.5 GHzRin=50 Ω

very dispersive!

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

Fct=40%

'fine-tunning'

80%-90% measured cross-talk levels reproduced

Zc~13 Ω

transverse scan

Cross-talk in Fonte multi-strip RPC

HV=5.7 kV

x10

->increase stripseparation

Cg

Cm

Δv/v

trise

Minimizing cross-talk (I)

x6 /2.5

/6

->increase strip/widthseparation->reduce glass thicknessCg

Cm

Δv/v

trise

Minimizing cross-talk (II)

x6 /2.5

/6

BW/10

->increase strip/widthseparation->reduce glass thickness->reduce band-width

Cg

Cm

Δv/v

trise

low couplinglow dispersion

Minimizing cross-talk (III)

guard strip

->put guard strip

Cg

Cm

Δv/v

trise

Minimizing cross-talk (IV)

not mirrored

->use only two electrodes

Cg

Cm

Δv/v

trise

(it flips!)

Minimizing cross-talk (V)

not mirrored

coupling to PCB

->use only two electrodes->couple locally to groundCg

Cm

Δv/v

trise

low couplingNO dispersion

Minimizing cross-talk (VI)

Minimizing cross-talk + detector response (I)

x10

Minimizing cross-talk + detector response (II)

x6 /2.5

/6

Minimizing cross-talk + detector response (III)

x6 /2.5

/6

BW/10

Minimizing cross-talk + detector response (IV)

not mirrored

coupling to PCB

Minimizing cross-talk + detector response (V)

Ideal case: no cross-talk + perfect tracking

'some' of the new CBM prototypes(preliminary short compilation)

35-cm long wide-strip, mirrored and shielded

... ...

Zc~18 Ω

BW=260 MHzRin=100 Ω

Fct=11%little dispersive

experimental conditions:~mips from p-Pb reactions at 3.1 GeV, low rates, trigger width = 2 cm

Fct=19%

'fine-tunning'inter-strip regiondominated by trigger width

probability of pure cross-talk:1-3%

Analysis with high resolution tracking on-going.

transverse scan

Cg

Cm

1-m long 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

no double hitdouble-hit in any of 1st neighborsdouble-hit in any of 2nd neighborsdouble-hit in any of 3rd neighbors

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

No simulations available yet

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

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.

Appendix

Multi-strip-MRPC (MMRPC)

1.1 mm

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

1.1 mm

0.5 mm

0.22 mm

copper (20 μm)

8 gaps

The FOPI counter

Induction. Example FOPI case.

n(t)dwvEtI )(

cathode 150 anode 1

50

50

50

cathode 250 anode 2

50

50

50

cathode 350 anode 3

50

50

50

cathode 4anode 4 50

50

cathode 550 anode 5

50

50

50

Cross-talk in an un-terminated line

signal from BC420scintillator (used as current generator)

cathode 150 anode 1

50

50

50

cathode 250 anode 2

50

50

50

cathode 350 anode 3

50

50

50

cathode 450 anode 4

50

50

50

cathode 550 anode 5

50

50

50

Cross-talk in a terminated line

Cross-talk and signal shape

cross-talkconstant, very independent from the signal shape

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

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 (11th strip)

50 anode 0 50

50 anode 1 50

50 .......... 50

anode 11 50

50 anode 12 50

50

cathode

50 anode 13 50

50 anode 14 50

50 50anode 15

The FOPI counter (9th strip)

50 anode 0 50

50 anode 1 50

50 .......... 50

anode 9 50

50 anode 10 50

50

cathode

50 anode 11 50

50 anode 12 50

50 .......... 50

50-cm long, mirrored and not shielded

... ...

~1-m long, non-mirrored and shielded

... ...

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.

HEEDcalculation

A parentheses: rate capability of various CBM prototypes

for small fluxes and in a simple DC-model

gappergapper dqg

EE1

)(

see for instance: D. Gonzalez-Diaz et al. Nucl. Phys. B (Proc. Suppl.) 158(2006)111

dqAoTT ,)(

dqBo )(

A parentheses: rate capability and DC-model systematics

In first order, it fits! dqAoTT ,)(

dqBo )(

prompt (e-) component

Slow (ion) component

g/ve ~ 1 ns g/vi ~1 μs

E=ΔV/g

ev

D

/1

p

particle

e--I+

How (we believe) is the avalanche produced?

vtwo eEqti )(

ith

space-chargelimitation

Eav~E

avalanche growth

decreases!

τg ~ 1 s (glass relaxation time)

see [4],for instance

More electrical schemes are (un)fortunately possible

ALICE-LHC

V

-V

-V

STAR-RHIC

V

-V

V

HADES-SIS

-V

-V

FOPI-SIS

-V

V

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.

-V

V

V

-V

V

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

!

HV filtering scheme is omitted

First of all... what is a strip?

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.

In standard language:

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

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.

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

2-strip geometry and signal taken from [12]

injected signal cross-talk signal

non-dispersive limit(zo=0)

dispersive limit (zo->∞)

->Continuous line is the exact analytical solution from [12].->Dashed and dotted lines are the numerical solution from APLAC used later in this work.

Measurements of cross-talk with RPC mockup

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

... ...

probability of pure cross-talk: 1-3%

similar cross-talk levels than in previous case

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

trigger width = 2 cm (< strip width)

BW=260 MHzRin=100 Ω

Zdet~20 Ω

Cm=18 pF/m

Cg=276 pF/m

dispersiveCm/Cg =6.5%

Fct=11.5%

30-cm long narrow strip, differential

... ...

Cm=20 pF/m

Cdiff=23 pF/m

Fct=9%

experimental conditions:~mips from p-Pb reactions at 3.1 GeV, low rates, high resolution (~0.1 mm) tracking

probability of pure cross-talk:1-3%

intrinsic strip profile is accessible!

Zdiff=80 Ω

dispersive

transverse scan