Spark-protected high-rate...P.Fonte CERN 1998 a1 Spark-protected high-rate parallel geometry gas chambers P.Fonte Laboratório de Instrumentação e Física

Spark-protected high-rate... P.Fonte CERN 1998a1

Spark-protected high-rate parallel geometry gas chambers

P.Fonte

Laboratório de Instrumentação e Física Experimental de Partículas (LIP)

e

Instituto Superior de Engenharia de Coimbra (ISEC)

Coimbra, Portugal

CERN 1998


Summary

• Brief survey of the basic parallel plate detector types, physics and operation modes.

• Hybrid wire mesh/resistive plate detector, made with a medium resistivity plate for improved rate capability and spark protection.

• Rate-gain limitations on parallel geometry chambers and other gaseous detectors.

• Thin gap parallel mesh chambers.


Basic parallel plate detector types

metal plate

amplification gap

metal plateParallel Plate Chamber (PPC)

(J.S.Townsend, 1900)

resistive plate

amplification gap

resistive plateResistive Plate Chamber (RPC)

(Pestov, 1978)

wire mesh

amplification gap

wire meshParallel Plate Avalanche Chamber

(PPAC)(G.Charpak and F.Sauli, 1978)

Hybrid types

amplification gap

wire mesh cathode

resistive plate anode

A subject of this talk

amplification gap

wire mesh cathode

metal plate anode

Another subject of this talk

metal plate

amplification gap

resistive plate

Not very popular

etc.....


PPC operation modes

anode

cathode

ionizing particle

high-voltagepulsespark Spark Chamber

(optical or electric read out)

anode

cathode

ionizing particle

narrow high-voltage

pulse

streamersStreamer Chamber(optical read out)

anode

cathode

ionizing particle

constantvoltage

Proportional mode(electric readout)

avalanche*

* eventually there will be also a few sparks


RPC operation modes

ionizing particle

streamer

Streamer mode

(development of a sparkis avoided by the current

limitation provided by theresistive electrodes)

higher constant voltage

resistive anode

resistive cathode

resistive anode

resistive cathode

ionizing particle

Limited proportional mode(sub-exponential gain due

space-charge effect in single avalanches)

avalanche constantvoltage

PPAC operation modes

Proportional mode only*, but a large number of configurations canbe formed.

A drift gap providesgood energy resolutionand efficiency for MIPs

drift gap

pre-amplification gap

ionizing particlePhoton

transfer gap

amplification gap

transfer gap

read out electrode

gate electrode

Example of a multistep PPAC

* plus a few occasional sparks


Basic physics

Electron (fast) signal Ion (slow) signal

Charge in slow signalNi = Nt (1-1/ln(G))

in practice about 90% of Nt

Charge in slow signalNi = Nt (1-1/ln(G))

in practice about 90% of Nt

Charge in fast signalNe = Nt/ln(G)

in practice 7% to 14% of Nt

50ns 10 to 30 s

Gain: G(x)=exp(x); = First Townsend coefficient

Total collected charge: Nt = N0·G(x)

cathode

anode

Primary electron createdat distance x from the anode

Avalanche multiplication

xamplifying

gap

Primary electron cloud

x

Ionizationtrack

Ionizing particle

N0 = number of primary electrons

Photon

Total collected charge: Nt = N0 /d·G(d)

d

The electrons closer to the cathode generate most of the final charge localization of the entry point

If N0=50, then the equivalent numberof concentrated primary electrons will be only 500.07 = 3.5


From avalanche to streamer

Slow breakdown mode Fast breakdown mode

Slow breakdown depends on photon feedback to the cathode: “generations” mechanism.

Fast breakdown is a local process:“streamer” mechanism.

When the amount of hydrocarbons (quencher) is increased thereis always a transition from the slow to the fast breakdown mode.

No matter the gain or the gas composition there is a hard limit on the total avalanchecharge of a few times 108 electrons.

Mixtures with TEA

Mixtures with CH4 and C2H6


Streamer theory

( M e e k , L o e b , R a e t h e r , L o z a n s k i i )

P h o t o n - m e d i a t e d l o c a l f e e d b a c k i n a s t r o n g s p a c e - c h a r g e f i e l d

C o m p l e x p h y s i c a l p r o c e s s , i n v o l v i n g : e l e c t r o n t r a n s p o r t i n v a r i a b l e f i e l d s e l e c t r o n m u l t i p l i c a t i o n i n h i g h f i e l d s s p a c e - c h a r g e d i s t o r t e d e l e c t r i c f i e l d e m i s s i o n o f p h o t o n s a b l e t o p h o t o i o n i s e t h e g a s a t a

c e r t a i n d i s t a n c e ( g a s s e l f - p h o t o i o n i z a t i o n )

O n l y q u a l i t a t i v e a n a l y t i c a l s o l u t i o n s a r e p o s s i b l e , b u tn u m e r i c a l s o l u t i o n s h a v e b e e n s u c c e s s f u l l y a p p l i e d t o s e v e r a ld i s c h a r g e s i t u a t i o n s .

The streamer process can be separated in three stages- proportional avalanche stage- avalanche-streamer transition stage- streamer development stage- spark

No SQS mode


Proportional avalanche stage

Avalanche-streamer transition stage


Streamer development stage

The "precursor": Corresponds to the initial avalanche The current reduction between the precursor and the

discharge is due to the absorption of the majority of theelectrons by the anode

The electrons absorption causes important electric fielddistortions, so the precursor is a necessary feature ofthe breakdown process at low gains.


From streamer to sparkA

vala

nche

/str

eam

er

Glo

w f

orm

atio

n

Dif

fuse

glo

w

Fila

men

tary

glo

w

Spa

rk

(S.C.Haydon, 8th Int. Conf. on Phen. in Ion. Gases, Vienna, 1967)


Motivation for an hybrid PPAC/RPC detector

PPAC

•High counting rate (up to 105/mm2)•Violent sparks•Versatile (drift, multistep, etc..)•No dark noise•Proportional mode only•Almost not used

PPAC +RPC

•Fast signal (tens of ns)•Large areas•Breakdown via streamers at a few times 108 electrons/avalanche•Good position resolution (100 m) in 2 dimensions (prop.mode). •Good timing (less than 1 ns ).

RPC

•Low counting rate (up to a few times 10/mm2)•Mild sparks / “indestructible”•Less versatile (MIPs only)•Dark noise•Streamer mode or proportional mode•Widely used

Hi-rateRPC

•High counting rate•Mild sparks / “indestructible”•Versatile (MIPs, X-rays)•No dark noise•Proportional mode•Good timing•Widely used

(wish list)

PPAC RPC

18 orders of magnitude in electrode resistivity

Explore the electrode resistivity parameter

High-rate PPAC/RPC


How fast can a PPAC count and at what gain?

Pulse-height doesn’t depend on rate

Maximum counting rate is determined by the

appearance of sparks

For low gains the maximum counting rate Rmax and the gain G=Q/(eN0) are related by RmaxQ/D=C were C is

a constant with a value around 100 nA/mm or 1012 electrons/(s mm).

There is a linear dependence on the beam diameter and

not on the beam cross-section!

High-rate PPAC/RPC

Principle of measurement

1,0E+02

1,0E+03

1,0E+04

1,0E+05

1,0E+06

1,0E+02 1,0E+03 1,0E+04 1,0E+05 1,0E+06 1,0E+07

Rate/mm2 (from scaler and beam diameter)

Ga

in (

fro

m P

H)

Allowed region

Forbidden region


A new material is needed

if = 1011 cm 10 Hz/mm2

then = 107 cm 105 Hz/mm2

Our solution: epoxy+ink• black soft rubber (not staining)• = 1012 to 2107 cm• dielectric strength > 16 kV/mm

1,0E+06

1,0E+07

1,0E+08

1,0E+09

1,0E+10

1,0E+11

1,0E+12

1,0E+13

0% 10% 20% 30% 40% 50% 60% 70%

Ink concentration (per weight)

cm

97-06-30

97-07-15

97-07-31

• Controllable resistivity.

• Ohmic behavior

• Mechanically inconvenient. Too soft and the surface resistivity is strongly affected by dryness.

• A much more convenient material, based on ABS plastic, is now being investigated in collaboration with a specialized group.

Bulk resistivity

1.1E+07

1.3E+07

1.5E+07

1.7E+07

1.9E+07

100 400 700 1000

Applied voltage (V)

cm

60% Ink

High-rate PPAC/RPC


Setup

15 mm

3.5 mm

Drift

Amplification

3

300 pF

To scopeResistive plate

over a metal base

Wire meshes

Sharp focusX-ray gen.

40 mm

Collimator

To current amp

Gas: Ar + 10 to 20% C2H6 + 30 % of methanol V.P.

Since Cgap « Cplate « Creadout

the voltage change across Cgap

is mainly determined by the signal charge stored in Cplate

3 Cgap

Cplate

Creadout

Signalcurrent

Rplate

Equivalent electrical circuit

Mechanical arrangement

High-rate PPAC/RPC


Low-rate behavior

0

50

100

150

200

1 8

15

22

29

36

43

50

57

64

71

Channel

Co

un

ts

55Fe = 3x108 cmFWHM = 20%

0

200

400

600

800

1000

1200

1.00E+04 1.00E+05 1.00E+06 1.00E+07

Gain

Cou

nt

rate

(H

z)

55Fe

Dark

Sparks

Dark sparks

= 3108 cmL = 1.5 mm

High-rate PPAC/RPC


Counting rate capabilities

Ohmic model

fittedmeasured V01 V02

4.0E+07 5.8E+07 3.8E+07 3.8E+08 3.5E+08 1.8E+08

4.1E+11 8.7E+11 6.1E+11

Reasonable agreement,considering that the model doesn’t take into account beam-edge effects, materialnon-linearities, etc...

High-rate PPAC/RPC

1,0E+04

1,0E+05

1,0E+06

1,0E-01 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)

Effe

ctiv

e ga

in

N0 = 200 e-

"metallic" limit (PPC)=4x107 cm

=3x108 cm=4x1011 cm

open symbols: 5 mm diam. beamsolid symbols: 2 mm diam. beam

It is known that RPCs can reach higher gains than

PPCs


Streamer charge

50 mV/div 250 ns/div

20 mV/div 10 s/div



50 mV/div 10 s/div

50 mV/div 10 s/div

Streamer current on 3 (not all pictures on same gas)

= 41011 cmL = 0.25 mm (melamine)

Qmeas. 33 nCQstored 40 nC/cm2

= 3108 cmL = 1 mm


= 4107 cmL = 1.5 mm


There is some contribution from conduction current across the plate, but it is comparable to the discharge of the plate-equivalent capacitor.

High-rate PPAC/RPC


Summary of high-rate PPAC/RPC

• The detector can operate in proportional mode up to the intrinsic counting rate limits of metallic PPACs (about 105 Hz/mm2 with gain above 104).

• Materials with suitable mechanical properties are needed with resistivity about 107 cm.

• The streamer has a charge typically of the order of 10 nC (relatively independent of the substrate resistivity) and poses no threat to the integrity of the detector.

• Beam area seems to have only a minor effect on rate capability.

High-rate PPAC/RPC


0.5 s

Detector current (on 1 M)

Slow currentincrease justbeforebreakdown

Cyclic breakdown, with frequency dependenton detector current.At higher gain there is continuous sparking.

This kind of continuous sparking is totally absent in low-rate sparking.

1 s

The breakdown pulse is precededby many individual spurious avalanchesat a growing rate and amplitude

We call this the “cathode excitation” effect (cannot be photon feedback). May be improved by choice of cathode materials, geometry or gases.

Why there is rate-induced breakdown in PPACs?

Rate-induced breakdown

Aftercurrent


What about other detectors?


Data presented by V.Peskov at the Wien WCC98

It seems that there is a general tendency for a reduction in themaximum gain as the counting rate increases.

Unknown physical origin.


How can we improve gain x rate (at least in PPACs)?


If we succeed this will clarify the physical nature of the rate-induced breakdown process.

The result may be useful also for other detectors.

We measured the maximum current in a 3 mm gap PPAC for the following combinations of parameters

Anode or cathode copper plate or mesh

150 m or 50 m mesh wires

Check if the effect depends on the ion density over the cathode surface

Oxidized or clean copper

High (ethane) or low (methanol) ionization potential ions in gas

Presence or absence of a drift gap

The current ranges from 100 to 200 nA (3 mm2 beam)independently of any of these parameters

but… it jumps to 1 A if the gap width is reduced to 0.6 mm!


SetupSingle gap

10 mm

0.6 mm

Drift

Thin amp. gap

300 pF

Slow current amp

Metal plate

Wire meshes


100 mm

Collimator (2 mm diam.=3.1 mm2)

Fast current amp

5 M

-HV

Pre-amplified thin gap

10 mm

0.6 mm

Drift

Thin amp. gap

300 pF

Slow current amp

Metal plate

Wire meshes


100 mm

Collimator (2 mm diam.=3.1 mm2)

Fast current amp

5 M

-HV

2 mm Transfer

2 mm Thick preamp. gap

Electronics

Slow current amp. (DC coupled): sensitivity 1 or 10 V/A averaging time 5 ms

Fast current amp. (AC coupled): sensitivity 0.1 V/A averaging time 200 ns

Thin gap PPAC


Gain calibration

Example of gain calibration curve

1.0E-01

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

0 500 1000 1500 2000

Applied voltage (V)

Ga

in

Ionization region (current)

High gain region (to be fitted)

Counting mode points (check)

Fitted exponential

Ion pulse shape

The gap current was measured by the slow current amp., calibrated against a Keithley 414s picoammeter.

200 nA

1 sQion 1.6 pC

Qelectrons Qion/(ln(G)-1) Qion/10 160 fC

velectrons 5 cm/s Pulse widthelectrons gap/velectrons 12 ns

Pulse heigthelectrons (Q/ Pw)electrons 13 A

G Qion / (e N0)

Gap field30 kV/cm

Thin gap PPAC


Rate-gain capabilities

Mesh pitch = 500 m

Gap thickness = 600 m

Thin gap PPAC

1,0E+02

1,0E+03

1,0E+04

1,0E+05

1,0E+06

1,0E+02 1,0E+03 1,0E+04 1,0E+05 1,0E+06 1,0E+07 1,0E+08

Rate/mm2

Ga

in

ABCD (pre-amp gain=30)Thick gap (3 mm)MSGC (NIM 1996-1998)Micromegas (1997 data)GEM+MSGC*

0

* V.Peskov, Wien WCC, 1998


10

100

1000

10000

1,00E+02 1,00E+03 1,00E+04 1,00E+05 1,00E+06

Gain

Imax

(n

A)

3 mm gap

0,6 mm gap

From the point of view of current

Thin gap PPAC


Discharges

1 A

20 ms

Qdischarge 20 nC

but in the readout capacitor there were

QC 1.5 kV 300 pF = 450 nC

For some (yet unknown) reason in thin gaps full sparks don’t develop and the discharge is self-limited!

0.5 A

500 ms

Rate-induced spark in a thick gap chamber using the same amplifier as the previous one

Thin gap PPAC


0

20

40

60

80

100

120

1

15 29 43 57 71 85 99

113

127

141

155

169

183

197

PH(arb. units)

Cou

nts

050

100150200250300350400

1

11

21

31

41

51

61

71

81

91

10

1

11

1

12

1

PH(arb. units)

Co

un

ts

0

20

40

60

80

100

120

1

17

33

49

65

81

97

11

3

12

9

14

5

16

1

17

7

19

3

PH(arb. units)

Co

un

ts

Detector “B”

Detector “C”

Detector “D” (preamp)

Energy resolution

•The mesh pitch is comparable to the gap width causing the field in the gap to be non uniform•May be improved by a finer mesh

5.9 keVX-rays

5.9 keVX-rays

5.9 keVX-rays

Thin gap PPAC


Position resolution

From the literature on thick-gap PPACs and RPCs: about 100 m.

Quadratic contributions to the distributionbeam width:100 mbeam divergence: 20 melectronics noise: 22 mdetector: 48 m

Our own measurements usinga 9 mm gap RPC and a collimatedX-ray beam

Timing accuracy

From the literature on PPCs and RPCs: better than 1 ns .

(Arefiev et al.)

Thin gap PPAC


Thin gap parallel mesh chamber summary

Negative points

Mesh pitch comparable with the gap width

Bad energy resolution (50%) when comparedwith the best resolution of PPACs (14%).

Intrinsic granularity

May be solved (or not) by using a finer mesh

Probably affects negatively the timing accuracy

Detector physics not fully understood (but quite a lot is know about parallel geometry chambers)

Discharges are not totally avoided.

Thin gap PPAC


Thin gap PPAC summary

Positive points

Large current capability (essentially gainrate)

Made with standard stainless steel wire mesh

Mechanically robust

No melting

Can be strongly stretchedto achieve good parallelism

Free of defects(spikes, etc..)

Large maximum(low-rate) gain

107 Hz/mm2 @ gain 1000

105 Hz/mm2 @ gain 104

Very mild discharges + strong electrodes virtually indestructible

“Macro”-technology cheap and easy to build

Free of dielectrics nocharging-up effects

Good timing and position resolution expected (from experience with thick gap PPCs and RPCs)

Thin gap PPAC

Documents

Spark-protected high-rate...P.Fonte CERN 1998 a1 Spark-protected high-rate parallel geometry gas chambers P.Fonte Laboratório de Instrumentação e Física