CBM2002@GSI 1

Resistive Plate Chambersfor

Time-of-Flight

P. FonteLIP/ISEC

Coimbra, Portugal.

Compressed Baryonic MatterMay 13 – 16, 2002

GSI Darmstadt/Germany

CBM2002@GSI 2

• Main physical mechanisms

• Description of several. interesting devices

• High frequency front-end electronics

• Small segmented counters

• Bidimensional position-sensitive single-gap counters

• Large area

• Practical difficulties to be addressed for future applications

• Crosstalk

• Ageing

• Tails

• Background

• Rate capability

Plan of the Presentation

CBM2002@GSI 3

Timing RPCs – General features

• Several (~4) thin (~ 0.3 mm) gas gaps

• Atmospheric pressure operation

• Both charge and time readout

• Offline correction for time-charge correlation.

Time-charge correlation

1-2 ns

100 ps

Electronicsrise time

Detector-related(not yet fully understood)

Experimental

CBM2002@GSI 4

Basic physical mechanisms

2 main questions: How can we reach 50 ps resolution in a gas counter? How is it possible to reach efficiencies of 75% in a 0.3 mm gas gap?

cathode

anode

i

i

Detection levelindependent fromthe cluster position exact timing

N= average number of visible primary clusters

Av. cluster density>> 1.4/0.3 = 4.6

ln( ) 1.4

Ne

N

CBM2002@GSI 5

Time resolution-principles

cathode

i

i

anode0( ) vti t i e

( )p tN e

( ) e( )

( )BesselI ,1 2 e

( ) N

( ) eN

1 e( )

N

v t

[Mangiarotti and Gobbi, 2001]

n0eff=i0d/(ev)

( )p i0e v N e

( ) i0( )BesselI ,1 2 i0 N

d ( ) eN

1 i0 N

Monte Carlo

i0

CBM2002@GSI 6

Time resolution-principles

[Mangiarotti and Gobbi, 2001]

t

( )K N

v

=vt

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 2 4 6 8 10 12Average number of primary clusters (N)

K(N

) (t

ime

jitte

r in

un

its

of v) Standard deviation

Sigma (gaussian fit)

1 gap 4 gaps

=first Townsend coefficientv = electron’s drift velocity

CBM2002@GSI 7

y = 0.1967x - 9.6104

5

6

7

8

9

10

70 80 90 100Applied field (kV/cm)

v

(G

Hz)

Signal properties

1-Signal shape cannot be changed by any linear system

Th1

Th2

TDC

t1

t2

2- v can be easily measured

y = 20.69e8.7043x

10

100

-0.1 0 0.1 0.2Time difference (ns)

Th

resh

old

1 (

mV

)

1 2 1 2ln( / )Th Th s t t

ExperimentalExperimental

0( ) vti t i e Linear system Z(s)

0( ) ( ) vtv t i Z s v e

CBM2002@GSI 8

Time resolution-theory vs. measurements

Single 0.3mm gap

40

50

60

70

80

90

100

110

120

2.4 2.6 2.8 3 3.2 3.4Applied Voltage (kV)

Tim

e re

solu

tio

n (

ps

)

Time-charge corrected

Uncorrected

0.75/(v)

t

( )K N

vt

( )K N

v

Good agreement

CBM2002@GSI 9

The efficiency problemThere should be at least one cluster in the efficient region of the gap

d

cathode

anode

i

i

d*

Inefficient region(too small avalanches)

Efficient region

Gmin~106

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4

Gas gap (mm)

Eff

icie

ncy

/gap

Isobutane (IB)

C2H2F4 (TFE)

TFE+IB+SF6

Methane

* ln(1 )

efficiency

clusters/mm

L Lmin= ln(1 )

d L

L

d

Experimental

Lmin=5/mm

Lmin=1.6/mm

CBM2002@GSI 10

F.SauliCERN 77-09

C4H10

C2F4H2

50 60 70 80

CH4

[Fisher, NIMA238]

L(cm-1)

The efficiency problem-cluster density data

[Finck, RPC2001]

0.1 mm gap in C4H10

Experimental

Lmin

Lmin

No contribution from cathode(would also provide an effect to explain the t-q correlation)Calculation

(HEED)

Use this data

[Riegler, RPC2001]

CBM2002@GSI 11

The efficiency problem (maybe solved)

Large values of G0 must be assumed (much above the Raether limit of 108).Possible by an extremely strong avalanche saturation effect (space charge effect).

*

1

1-d /d0 min

* ln(1 )

efficiency

clusters/mm

G = G

d L

L

d

cathode

anode

i

i

d*

Gmin~106

G0

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4

Gas gap (mm)

d*/

d

Isobutane (IB): L=9.5/mmC2H2F4 (TFE): L=9/mmTFE+IB+SF6: L=9/mmMethane: L=3/mm

G0=4x1014

CBM2002@GSI 12

Some hardware

CBM2002@GSI 13

Timing RPCs – Small single counters (9 cm2)

3

2 268 49 47t ps

Resolution of thereference counter

[ F

onte

200

0]

= 99.5 % for MIPs(75%/gap)

-HV

4x0.3 mm gaps

Aluminum Glass

(optimum operating point 1% of discharges)

CBM2002@GSI 14

Timing RPCs – Array of 32 small counters

= 88 ± 9 ps = 97 ± 0.5 %

[ S

pege

l et a

l, 20

00 ]

Crosstalk < 1%

(not tested for multiple hits)

CBM2002@GSI 15

Very high frequency front-end electronics based on commercial chips

Amplifier-discriminator-delaymodule

Pre-amplifier based on the INA-51063 chip (HP/Agilent)

• 2.5 GHz bandwidth• 20 db power gain• 3 dB noise figure

0

10

20

30

40

50

60

70

10 100 1000Signal fast charge per channel (fC)

Tim

e re

solu

tio

n p

er

chan

nel

(

2)

(ps)

RPC at Threshold=12.5 fC

RPC at threshold=25 fC

RPC at threshold=50 fC

Pulser at threshold=25 fC

Realistic tests using chamber-generated signals

i=i0 est

s=8.7 GHz

Input signal(experimental)

10 ps resolutionabove 100 fC

[ B

lanc

o 20

00]

CBM2002@GSI 16

Double readout 4-gaps glass-metal counter

[Fin

ck, G

obbi

et a

l, R

PC

2001

]

Edge effects

CBM2002@GSI 17

Timing RPCs – Large counterActive area = 10 cm160 cm = 0.16

m2

(400 cm2/electronic channel)

5 cm 4 timingchannels

1,6 m

Top view Cross section

Ordinary 3 mm “window glass”

Copper strips

HV

[Bla

nco

2001

]

CBM2002@GSI 18

Timing RPCs – Large counter

Charge distribution Time distribution

0 2 4 60

100

200

300

0 2 4 6 810

0

101

102

103

Fast charge (pC)

Eve

nts

/ 20

fC 0.5 pC

98 %

Time resolution essentially independent from electrode

size

-1000 -500-300 0 300 500 100010

0

101

102

103

Time difference (ps)

Eve

nts

/20

ps = 63.4 ps

“300 ps tails”

1.5 fit

642 - 352 = 54 ps= 642 - 352 = 54 ps 642 - 352 = 54 ps= 642 - 352 = 54 ps

CBM2002@GSI 19

Timing RPCs – Large counter

40

50

60

70

80

90

100

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80

Tim

e re

solu

tio

n (

ps

) = 50 to 75 ps

Center of the trigger region along the strips (cm)

93%

94%

95%

96%

97%

98%

99%

100%

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80

Tim

e e

ffic

ien

cy

Strip A

Strip B

Strips A+B

= 95 to 98 %

Center of the trigger region along the strips (cm)

No degradation when the area/channel was doubled to 800 cm2/channel

[Bla

nco

2001

]

CBM2002@GSI 20

Timing RPCs – Large counter

[Bla

nco

2001

]

Position resolution along the strips

-8

-6

-4

-2

0

2

4

6

8

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80

t/

2 (n

s)

Strip A

Strip B

y=0.0709 x + 0.0001 v=14 .1 cm/ns

-0.2

-0.1

0

0.1

0.2

0.3

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80

Center of the trigger region along the strips (cm)

Fit

res

idu

als

(ns

) Very good linearity ( 1.4 cm)

X = 1.2 cm(0.75% of detector

length)

-350 -300 -2500

50

100

150

200

250

300

350

400

TDC bins

Eve

nts

/bin

-350 -300 -2500

50

100

150

200

250

300

350

400

TDC bins

Eve

nts

/bin

-350 -300 -2500

50

100

150

200

250

300

350

400

TDC bins

Eve

nts

/bin

5.0 cm

CBM2002@GSI 21

Timing RPCs – single gap 2D-position sensitive readout

X leftX right

Time signalHV

XY readout plane

Y-strips(on PCB)

RC

passive netw

ork

RC passive network

X-strips(deposited on glass)

out left

out right

10 strips for eachcoordinate

at 4 mm pitch

4 cm

2 mm thick black glass lapped to ~1m flatness

Well carved into the glass (avoid dark currents from the spacer)

300 m thick high glass disk (corners)

metal box (no crosstalk)

Precise construction

(for small and accurate TOF systems)

[Bla

nco

2002

]

CBM2002@GSI 22

Charge distribution Time distribution

Time resolution of single-gap and

4-gap counters may be similar!

-1000 -500 -300 0 300 500 100010

0

101

102

103

Time difference (ps)

Eve

nts/

10p

s

= 66.6 ps (54 ps)

3 Tails=1.9 %

300ps Tails=0.36 %

Events=26186

3

1.5 fit

Timing RPCs – single gap

0 200 400 600 800 10000

500

1000

1500

2000

2500

3000

Fast charge (a.u.)

Eve

nts

/bin

0 500 100010 0

102

104

= 75 %

ine

ficie

ncy

2 - 2 = 54 ps= 2 - 2 = 54 ps 2 - 2 = 54 ps= 672 - 402 = 54 ps

CBM2002@GSI 23

Timing RPCs – Single gap

Efficiency and time resolution

= 50 to 60 ps = 75 to 80 %

No influence from XY readout

40

50

60

70

80

90

100

2.4 2.6 2.8 3 3.2 3.4Applied Voltage (kV)

Tim

e re

solu

tio

n (

ps

)

55

60

65

70

75

80

85

Eff

icie

ncy

(%

)

#2 (time only)

#3 (time + XY)

Efficiency

Resolution

CBM2002@GSI 24

Eve

nts

/mm

2

Y(m

m)

X (mm)

Timing RPCs – single gap

Bidimensional position resolution

edges 3 mm

resolution 3 mm FWHM(strips=4mm)

-25 -20 -15 -10 -5 0 5 10 15 20 250

500

1000

1500

Position along X (mm)

Eve

nts/

0.5

mm

-25 -20 -15 -10 -5 0 5 10 15 20 250

500

1000

1500

Position along Y (mm)

Eve

nts/

0.5

mm

4 mm strip pitch

Trigger edge (3 mm)

Chamberedge

CBM2002@GSI 25

Timing RPCs – single gap

Edge effects?

Eve

nts

/mm

2

Y (

mm

)

X (mm)

-1000 -500 0 500 100010

0

101

102

Time difference (ps)

Eve

nts/

4ps

Sigma=74.0 ps (62 ps)3-Sigma Tails=3.0 %300ps Tails=1.5 %

Center

Essentially no

edge effects

Sigma=76.9 ps (66 ps)3-Sigma Tails=2.9 %300ps Tails=1.8 %

Edge & Corner

-1000 -500 0 500 100010

0

101

102

Time difference (ps)

Eve

nts/

4ps

CBM2002@GSI 26

Timing RPCs – multilayer

4 layers of single-gap chambers

= 50 ps @ = 99 %3 Tails=1.5 %300ps Tails=0.20%

Single layer of 4-gap chambers

Layer = 75 %, = 60 ps +

+ 1% K @ 300 ps

3 Tails=1.4 %300ps Tails=0.0%

-1000 -500 0 500 100010

0

101

102

103

Time difference (ps)E

ven

ts/1

0ps

-1000 -500-300 0 300 500 100010

0

101

102

103

10

Time difference (ps)

Eve

nts

/10p

s

Events=51215

4

= 32 ps = 94.9 %

-300 0 300 5000

50

100

Time difference (ps)

Eve

nts

/10p

s

-300 0 300 5000

20

40

Eve

nts/

10p

s

Time difference (ps)

tail-dominated

M.C.

Experimental

CBM2002@GSI 27

Open problems

CBM2002@GSI 28

Channel “A”

Neighbouringchannel “B”

The subtle crosstalk problem

1 1.2 1.4 1.6 1.8 2-10-8-6-4-202468

10

Time (ns)

Cu

rre

nt (

µA

)

Threshold

Few 100 psjitter

Realistic (measured) current shape

Baseline shiftdue to crosstalk

CBM2002@GSI 29

The crosstalk problem – main approaches

1 – Do nothing and hope the problem is small or correctable offline

2 – Accept crosstalk and use many more channels than strictly needed.

3 – Shield channels and try to totally avoid crosstalk

None of these approaches has been proven so far (as of Nov 2001)

+ Robust performance.+ Testable in the lab.- Segmented mechanics.

+ Simple and economic+ Being massively implemented

(ALICE, STAR)- Possible fragile performance

+ Good 2D position resolution- Expensive- Effective granularity must be

defined by testing- Possible fragile performance.

CBM2002@GSI 30

Ageing in streamer mode glass RPCs (BELLE)

[Kub

o et

al,

RP

C20

01]

Problemwater+freon+streamers

Fluoridric acid

Glass corrosion

Dark current

Ineficiency

Freonless gas

Freon gas

CBM2002@GSI 31

Ageing in avalanche mode glass RPCs?

• 3 glass cathode and 3 aluminium cathode counters• Gas: (85% C2H2F4+10% SF6 +5%C4H10 ) + 10% rel. humidity

Glass cathode

Aluminium cathode

Da

rk c

urr

ent (

nA)

Days

Te

mp

era

ture

(ºC

)

Integrated charge (mC108 avalanches)

Test in progress

CBM2002@GSI 32

Tails

-300 0 300 5000

50

100

Time difference (ps)

Eve

nts

/10p

s

-300 0 300 5000

20

40

Eve

nts/

10p

s

Time difference (ps)

tail-dominated

Background

Theoretical time distribution has a tail

Almost completely compensated

by t-q correction

Safe cure: redundancy (in a real application counters will be imperfect)

Highly ionising background more streamers lower rate capability lower resolution

Problem was not studied so far. Severity unknown.

CBM2002@GSI 33

Rate capability – Standard RPCs

Streamer mode < 300 Hz/cm2

Avalanche mode < 3000 Hz/cm2

Typically max. rate corresponds to an efficiency drop of a few percent

0

500

1000

1500

2000

2500

3000

3500

1.0E+09 1.0E+10 1.0E+11 1.0E+12 1.0E+13

Resistivity ( cm)

Max

Co

un

tin

g R

ate

(Hz/

cm2)

LHCbCERN+BolognaWarsawCERN+RioATLASCMS-forwardCMS-barrelALICE-TOFBeijingALICE-muon

"Multigap"RPC

Streamer mode

uniformand

continuous illumination

(survey of recently published results)

CBM2002@GSI 34

Rate capability – Special RPCs

Drift gap

Amplification gap

Resistive anode on a metal base=107 cm

Wire meshes (50 m wires at 0.5 mm pitch)

15 mm

3.5 mm

microRPC

[Fon

te 1

997]

[Car

lson

et a

l, N

SS

2001

]

1.0E+04

1.0E+05

1.0E+06

1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06

Counting rate (Hz/mm2)

Eff

ectiv

e ga

in

N0 = 200 e-

=

Metallic (PPC) limit

Si plate =104 cm-HV

0.1-0.5 mm

PPAC

microRPC

PPAC

107 Hz/cm2

Gain 5104

Future GSI experiment

CBM2002@GSI 35

• Main physical mechanisms – mostly understood except t-q correlation.

• Several interesting devices have been sucessfully tested with <100 ps and very small tails or edge effects

• Small segmented counters.

• 2D position-sensitive single-gap counters.

• Large area counters.

• Practical difficulties to be addressed for future applications

• Crosstalk – several approaches proposed, none proven.

• Ageing – tests in progress, no problem so far. If problem: avoid water, freon or glass cathodes. Use chemistry

to avoid formation of fluoridric acid.

• Tails – redundancy should solve any problem if really needed.

• Background – severity unknown.

• Rate capability – solutions should exist up to ~105/cm2.

Conclusions