RPC2010- Darmstadt- 9/12-Feb-2010. p.1M. Abbrescia – University and INFN Bari

New gas mixtures for Resistive Plate Chambers

operated in avalanche mode

New gas mixtures for Resistive Plate Chambers

operated in avalanche mode

M. Abbrescia, V. Cassano, S. Nuzzo, G. Piscitelli, D. Vadruccio, F. Zaza

RPC2010- Darmstadt- 9/12-Feb-2010. p.2M. Abbrescia – University and INFN Bari

The problem

RPC are operated (e.g. at the large LHC experiments) with the so-called “standard” mixture

C2H2F4/i-C4H10/SF6 96.2/3.5/0.3

Quite successful mixture but: operating voltage close to 10 kV;

expensive power suppliers, connectors, etc.; production of HF during operation;

issues for aging; isobutane content close to flammability limit;

need for sensors and external storage.

RPC2010- Darmstadt- 9/12-Feb-2010. p.3M. Abbrescia – University and INFN Bari

The “ideal” solution

It is well known that avalanche processes in RPCs depend on environmental conditions:i.e. temperature and pressure

because they change gas density

STD

STDappleff T

T

p

pHVHV

Ideally, operating RPC a ½ Atm would reduce of a factor 2 the operating voltage and meet at least 2 of the wishlist

Not practically feasible in large systems

M. Abbrescia et al., NIM A 359 (1995), 603-609

RPC2010- Darmstadt- 9/12-Feb-2010. p.4M. Abbrescia – University and INFN Bari

The “smart” solution

Helium is a noble gas with no vibrational or rotational degrees of freedom, with an ionization potential greater than C2H2F4

Undergoes mainly elastic scattering with electrons;Takes part only partially in the avalanche processes;In first approximation behaves only like a space holder;

Essentially reduces the partial pressure of the active mixture

Effect similar (in first approximation) to operate at a reduced pressure

RPC2010- Darmstadt- 9/12-Feb-2010. p.5M. Abbrescia – University and INFN Bari

New mixtures containing He

Standard mixture used as a reference… then“A” mixture

62.5/2.3/0.2/35 C2H2F4 /C4H10/SF6/He

“B” mixture 48.1/1.75/0.15/50 C2H2F4 /C4H10/SF6/He

“C” mixture 48/1.7/0.3/50 C2H2F4 /C4H10/SF6/He

“A” and “B” essentially standard mix. + Helium

“C” increased SF6 fraction

RPC2010- Darmstadt- 9/12-Feb-2010. p.6M. Abbrescia – University and INFN Bari

Experimental set-up

10 × 10 cm2 × 2 mm bakelite RPC

The trigger and the entire system

HV and electronics

RPC2010- Darmstadt- 9/12-Feb-2010. p.7M. Abbrescia – University and INFN Bari

Data acquisition and analysis

Induced signals readout by oscilloscope and analyzed with MatLab dedicated software

Induced signals readout by oscilloscope and analyzed with MatLab dedicated software

Signal

Noise

Trigger

RPC2010- Darmstadt- 9/12-Feb-2010. p.8M. Abbrescia – University and INFN Bari

Noise analysisConfronto del rumore medio (RMS)

0,4

0,45

0,5

0,55

0,6

0,65

0,7

0,75

0,8

6000 6500 7000 7500 8000 8500 9000 9500 10000 10500Hveff (V)

RM

S (

mV

)

Standard mixture

A mixture (35% He)

B mixture (50% He)

C mixture (50% He + 0.3 SF6)

Substantially independent of mixture and operating voltage

RMS ~ 0,6 mV

Noise determined using the first 50 ns of each

waveforms

Gaussianly distributed

Average noise

RPC2010- Darmstadt- 9/12-Feb-2010. p.9M. Abbrescia – University and INFN Bari

Induced charge (1/2)

A mixture (35% He)HV = 7700 V

A mixture (35% He)HV = 7700 V

A mixture (35% He)HV = 8200V

A mixture (35% He)HV = 8200V

A lot (~hundred) of charge distributions taken,

at different HVwith different gas mixtures

Interesting “by themselves”whole information from RPC storedUseful for any kind of analysis

“Avalanche” events“Avalanche” events

“Streamer” events“Streamer” events

Threshold chosenThreshold chosen

RPC2010- Darmstadt- 9/12-Feb-2010. p.10M. Abbrescia – University and INFN Bari

Induced charge (2/2)

0

2

4

6

8

10

12

6300 6800 7300 7800 8300 8800 9300 9800 10300HVeff (V)

Qin

d (

pC

)

Standard mixture A mixture (35% He)

B mixture (50% He) C mixture (50% He + 0.3% SF6)

0

2

4

6

8

10

12

6300 6800 7300 7800 8300 8800 9300 9800 10300HVeff (V)

Qin

d (

pC

)

Standard mixture A mixture (35% He)

B mixture (50% He) C mixture (50% He + 0.3% SF6)

Very similar trend (slope) of Qind vs. HV for different mixtures where HV @ equal Qind scales down with He increase

Induced charge reported here for avalanche events only

Earlier streamer “onset” with B and C mixtures

RPC2010- Darmstadt- 9/12-Feb-2010. p.11M. Abbrescia – University and INFN Bari

Efficiency

0

10

20

30

40

50

60

70

80

90

100

5000 6000 7000 8000 9000 10000

HVeff (V)

% e

ffic

ienz

a e

stre

amer

Standard mixture

A mixture

B mixture

C mixture

0

10

20

30

40

50

60

70

80

90

100

5000 6000 7000 8000 9000 10000

HVeff (V)

% e

ffic

ienz

a e

stre

amer

Standard mixture

A mixture

B mixture

C mixture

~800 V

~600 V

~400 V

Operating point and useful plateau width scale down with increased He percentageScaling only roughly proportional to He percentage for high He fraction;Useful plateau wider increasing SF6 fraction

RPC2010- Darmstadt- 9/12-Feb-2010. p.12M. Abbrescia – University and INFN Bari

Time properties

178

180

182

184

186

188

190

192

6000 6500 7000 7500 8000 8500 9000 9500 10000 10500HVeff (V)

Tim

e d

elay

(n

s)

Standard mixture A mixture (35% He)

B mixture (50% He) C mixture (50% He + 0.3 SF6) A mixtureHV = 7.7 kVRMS = 1.8 ns

0

2

4

6

8

10

12

14

16

18

6000 6500 7000 7500 8000 8500 9000 9500 10000 10500

HVeff (V)

Tem

po

(n

s)

Standard mixture

A mixture (35% He)

B mixture (50% He)

C mixture (50% He + 0.3 SF6)

0

2

4

6

8

10

12

14

16

18

6000 6500 7000 7500 8000 8500 9000 9500 10000 10500

HVeff (V)

Tem

po

(n

s)

Standard mixture

A mixture (35% He)

B mixture (50% He)

C mixture (50% He + 0.3 SF6)

Gaussian distribution of time responseTime delays and resolution (roughly) scale with He percentageTime resolution at plateau around 1.4 ns

1.4 ns

1 ns/100 V

RPC2010- Darmstadt- 9/12-Feb-2010. p.13M. Abbrescia – University and INFN Bari

Correlation studies (1/2)

0

2

4

6

8

10

12

14

16

18

25 35 45 55 65 75 85 95

Efficiency(%)

Cha

rge

(pC

)

Standard mixture

A mixture (35% He)

B mixture (50% He)

C mixture (50% He + 0,3% SF6)

0

2

4

6

8

10

12

14

16

18

25 35 45 55 65 75 85 95

Efficiency(%)

Cha

rge

(pC

)

Standard mixture

A mixture (35% He)

B mixture (50% He)

C mixture (50% He + 0,3% SF6)

Induced charge directly related to efficiency (rather obvious…) and…

To better compare performance with different mixture use efficiency as independent variable

Identical behaviour

=Curves superimposed(regardless the HV)

Identical behaviour

=Curves superimposed(regardless the HV)

Qind linearly increasing as efficiency increases

Qind vs. efficiency

All events over thr., constant eff , Qind increas

RPC2010- Darmstadt- 9/12-Feb-2010. p.14M. Abbrescia – University and INFN Bari

Correlation studies (2/2)

Time properties substantially independent of gas mixtures once the operating point has

been set

Time resolution vs. efficiency

Time resolution vs. efficiency

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

0 10 20 30 40 50 60 70 80 90 100

Efficiency (%)

Tim

e r

es

olu

tio

n (

ns

)

Standard mixture

A mixture (35% He)

B mixture (50% He)

C mixture (50% He + 0,3 % SF6)

178

180

182

184

186

188

190

0 10 20 30 40 50 60 70 80 90 100

Efficiency (%)

Tim

e (n

s)

Standard mixture

A mixture (35% He)

B mixture (50% He)

C mixture (50% He +0,3% SF6)

Time delay vs. efficiencyTime delay vs. efficiency

At full efficiency all mixtures with time resolution in the 1.4-1.7 ns range

RPC2010- Darmstadt- 9/12-Feb-2010. p.15M. Abbrescia – University and INFN Bari

Conclusions

It is a “smart” solution and works general idea verified corrections at high He content

inelastic scattering, Penning effect, etc.

New gas mixtures allow: reduction of the operating voltage (2-3 kV at least) reduced flammability problems (increase i-C4H10?) good (> 95%) efficiency and time resolution (< 2ns)

Good starting point for studies in this field

definition for a new “standard” mixture