Resistive Plate Chambers for Time-of-Flight P. Fonte LIP/ISEC - - PowerPoint PPT Presentation

CBM2002@GSI 1

Resistive Plate Chambers for Time-of-Flight

• P. Fonte

LIP/ISEC Coimbra, Portugal.

Compressed Baryonic Matter

May 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

Electronics rise time Detector-related (not yet fully understood)

Experimental

CBM2002@GSI 4

Basic physical mechanisms 2 main questions: How can we reach 50 ps σ 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 level independent from the cluster position ⇒ exact timing

N= average number of visible primary clusters

• Av. cluster density

>> 1.4/0.3 = 4.6

ln( ) 1.4

N

e N ε ε

−

= = − =

CBM2002@GSI 5

Time resolution-principles

cathode

i i

anode

vt

i t i eα =

= ( ) p t N e ( ) − − e ( ) −τ τ ( ) BesselI , 1 2 e ( ) −τ N ( ) − eN 1 e ( ) −τ N

= τ α v t

[Mangiarotti and Gobbi, 2001]

n0eff=i0d/(ev)

= ( ) p i0 e v N e ( ) −i0 ( ) BesselI , 1 2 i0 N d ( ) − eN 1 i0 N

τ

Monte Carlo

i0

CBM2002@GSI 6

Time resolution-principles

[Mangiarotti and Gobbi, 2001]

= σt ( ) K N α v

τ=αvt

0.2 0.4 0.6 0.8 1 1.2 1.4 2 4 6 8 10 12

Average number of primary clusters (N) K(N) (time jitter in units of α v)

Standard deviation Sigma (gaussian fit)

1 gap 4 gaps

α=first Townsend coefficient v = electron’s drift velocity

CBM2002@GSI 7

y = 0.1967x - 9.6104 5 6 7 8 9 10 70 80 90 100

Applied field (kV/cm)

α v (GHz)

Signal properties 1-Signal shape cannot be changed by any linear system

Th1 Th2

TDC

t1 t2

2- αv can be easily measured

y = 20.69e8.7043x

10 100

• 0.1

0.1 0.2 Time difference (ns)

Threshold 1 (mV)

( )

1 2 1 2

ln( / ) Th Th s t t = −

Experimental Experimental

vt

i t i eα =

Linear system Z(s)

vt

v t i Z s v eα α =

• ≈

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 3.4

Applied Voltage (kV) Time resolution (ps σ) Time-charge corrected Uncorrected 0.75/(αv)

= σt ( ) K N α v = σt ( ) K N α v

Good agreement

CBM2002@GSI 9

The efficiency problem

There 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

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.1 0.2 0.3 0.4

Gas gap (mm) Efficiency/gap

Isobutane (IB) C2H2F4 (TFE) TFE+IB+SF6 Methane

* ln(1 ) efficiency clusters/mm L Lmin=ln(1 ) d L L d ε ε ε = − = = > −

Experimental

⇒Lmin=5/mm ⇒ Lmin=1.6/mm

CBM2002@GSI 10

F.Sauli CERN 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 /d min

* ln(1 ) efficiency clusters/mm G = G d L L ε ε = − = =

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 0.1 0.2 0.3 0.4

Gas gap (mm) d*/d

Isobutane (IB): L=9.5/mm C2H2F4 (TFE): L=9/mm TFE+IB+SF6: L=9/mm Methane: L=3/mm

G0=4x1014

CBM2002@GSI 12

Some hardware

CBM2002@GSI 13

Timing RPCs – Small single counters (9 cm2)

3σ

2 2

68 49 47

t

ps σ = − =

Resolution of the reference counter

[ Fonte 2000]

ε= 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 σ = 88 ± 9 ps ε = 97 ± 0.5 %

[ Spegel et al, 2000 ]

Crosstalk < 1% (not tested for multiple hits)

CBM2002@GSI 15

Very high frequency front-end electronics based on commercial chips

Amplifier-discriminator-delay module Pre-amplifier based on the

INA-51063 chip (HP/Agilent)

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

10 20 30 40 50 60 70 10 100 1000

Signal fast charge per channel (fC) Time resolution per channel (σ/√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 σ resolution above 100 fC

[ Blanco 2000]

CBM2002@GSI 16

Double readout 4-gaps glass-metal counter

[Finck, Gobbi et al, RPC2001]

Edge effects

CBM2002@GSI 17

Timing RPCs – Large counter

Active area = 10 cm×160 cm = 0.16 m2 (400 cm2/electronic channel) 5 cm

4 timing channels

1,6 m

Top view Cross section

Ordinary 3 mm “window glass” Copper strips

HV

[Blanco 2001]

CBM2002@GSI 18

Timing RPCs – Large counter

Charge distribution Time distribution

2 4 6 100 200 300

2 4 6 8 10 10

1

10

2

10

3

Fast charge (pC) Events / 20 fC 0.5 pC

ε ≈ 98 % Time resolution essentially independent from electrode size

• 1000
• 500-300

300 500 1000 10 10

1

10

2

10

3

Time difference (ps) Events/20 ps

σ= 63.4 ps

“300 ps tails” ±1.5 σ fit

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

CBM2002@GSI 19

Timing RPCs – Large counter

40 50 60 70 80 90 100

• 80
• 70
• 60
• 50
• 40
• 30
• 20
• 10

10 20 30 40 50 60 70 80

Time resolution (ps σ)

σ = 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

10 20 30 40 50 60 70 80

Time efficiency Strip A Strip B Strips A+B

ε = 95 to 98 %

Center of the trigger region along the strips (cm)

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

[Blanco 2001]

CBM2002@GSI 20

Timing RPCs – Large counter

[Blanco 2001]

Position resolution along the strips

• 8
• 6
• 4
• 2

2 4 6 8

• 80 -70 -60 -50 -40 -30 -20 -10

10 20 30 40 50 60 70 80

∆ t/2 (ns) Strip A Strip B

y=0.0709 x + 0.0001 ⇔ v=14 .1 cm/ns

• 0.2
• 0.1

0.1 0.2 0.3

• 80 -70 -60 -50 -40 -30 -20 -10

10 20 30 40 50 60 70 80

Center of the trigger region along the strips (cm) Fit residuals (ns)

Very good linearity (± 1.4 cm) σX = 1.2 cm (0.75% of detector length)

• 350
• 300
• 250

50 100 150 200 250 300 350 400

TDC bins Events/bin

• 350
• 300
• 250

50 100 150 200 250 300 350 400

TDC bins Events/bin

• 350
• 300
• 250

50 100 150 200 250 300 350 400

TDC bins Events/bin

5.0 cm

CBM2002@GSI 21

Timing RPCs – single gap 2D-position sensitive readout

X left X right Time signal HV XY readout plane Y-strips (on PCB)

RC passive network RC passive network

X-strips (deposited on glass)

• ut left
• ut right

10 strips for each coordinate at 4 mm pitch 4 cm 2 mm thick black glass lapped to ~1µm flatness Well carved into the glass (avoid dark currents from the spacer) 300 µm thick high ρ glass disk (corners) metal box (no crosstalk)

Precise construction (for small and accurate TOF systems)

[Blanco 2002]

CBM2002@GSI 22

Charge distribution Time distribution

Time resolution of single-gap and 4-gap counters may be similar!

• 1000
• 500 -300

300 500 1000 10 10

1

10

2

10

3

Time difference (ps) Events/10ps σ = 66.6 ps (54 ps) 3σ Tails=1.9 % 300ps Tails=0.36 %

Events=26186

3σ

±1.5 σ fit

Timing RPCs – single gap

200 400 600 800 1000 500 1000 1500 2000 2500 3000

Fast charge (a.u.) Events/bin

500 1000 10 0 10 2 10 4

ε = 75 % ineficiency

2 - 2 = 54 ps

=

2 - 2 = 54 ps

σ

2 - 2 = 54 ps

= 672 - 402 = 54 ps σ

CBM2002@GSI 23

Timing RPCs – Single gap Efficiency and time resolution

σ = 50 to 60 ps ε = 75 to 80 % No influence from XY readout

40 50 60 70 80 90 100 2.4 2.6 2.8 3 3.2 3.4

Applied Voltage (kV) Time resolution (ps σ)

55 60 65 70 75 80 85

Efficiency (%) #2 (time only) #3 (time + XY) Efficiency Resolution

CBM2002@GSI 24

Events/mm2 Y(mm) X (mm)

Timing RPCs – single gap Bidimensional position resolution

edges ≤ 3 mm ⇓ resolution ≤ 3 mm FWHM (strips=4mm)

• 25
• 20
• 15
• 10
• 5

5 10 15 20 25 500 1000 1500

Position along X (mm) Events/0.5mm

• 25
• 20
• 15
• 10
• 5

5 10 15 20 25 500 1000 1500

Position along Y (mm) Events/0.5mm

4 mm strip pitch Trigger edge (3 mm) Chamber edge

CBM2002@GSI 25

Timing RPCs – single gap Edge effects?

Events/mm2 Y (mm) X (mm)

• 1000
• 500

500 1000 10 10

1

10

2

Time difference (ps) Events/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

500 1000 10 10

1

10

2

Time difference (ps) Events/4ps

CBM2002@GSI 26

Timing RPCs – multilayer

4 layers of single-gap chambers

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

Single layer of 4-gap chambers

Layer ε = 75 %, σ = 60 ps +

π + 1% K @ 300 ps

3 σ Tails=1.4 % 300ps Tails=0.0%

• 1000
• 500

500 1000 10 10

1

10

2

10

3

Time difference (ps) Events/10ps

• 1000
• 500
• 300

300 500 1000 10 10

1

10

2

10

3

10

Time difference (ps) Events/10ps

Events=51215

4

σ = 32 ps ε = 94.9 %

• 300

300 500 50 100

Time difference (ps) Events/10ps

• 300

300 500 20 40

Events/10ps Time difference (ps)

tail-dominated M.C. Experimental

CBM2002@GSI 27

Open problems

CBM2002@GSI 28

Channel “A” Neighbouring channel “B”

The subtle crosstalk problem

1 1.2 1.4 1.6 1.8 2

• 10
• 8
• 6
• 4
• 2

2 4 6 8 10

Time (ns) Current (µA)

Threshold

Few 100 ps jitter Realistic (measured) current shape

Baseline shift due 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)

[Kubo et al, RPC2001]

Problem water+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 Dark current (nA) Days Temperature (ºC) Integrated charge (mC≈108 avalanches)

Test in progress

CBM2002@GSI 32

Tails

• 300

300 500 50 100

Time difference (ps) Events/10ps

• 300

300 500 20 40

Events/10ps 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

500 1000 1500 2000 2500 3000 3500 1.0E+09 1.0E+10 1.0E+11 1.0E+12 1.0E+13

Resistivity (Ω cm) Max Counting Rate (Hz/cm2)

LHCb CERN+Bologna Warsaw CERN+Rio ATLAS CMS-forward CMS-barrel ALICE-TOF Beijing ALICE-muon

"Multigap" RPC Streamer mode

uniform and continuous illumination

(survey of recently published results)

CBM2002@GSI 34

Rate capability – Special RPCs

Drift gap Amplification gap Resistive anode on a metal base

ρ=107 Ω cm

Wire meshes (50 µm wires at 0.5 mm pitch)

15 mm 3.5 mm

microRPC

[Fonte 1997] [Carlson et al, NSS2001]

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) Effective gain

N0 = 200 e-

=

Metallic (PPC) limit

Si plate ρ=104 Ω cm-HV

0.1-0.5 mm

PPAC

microRPC PPAC

107 Hz/cm2 Gain 5×104 Future GSI experiment

CBM2002@GSI 35

• Main physical mechanisms – mostly understood except t-q correlation.
• Several interesting devices have been sucessfully tested with σ <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