Final results from an extensive aging test on bakelite Resistive

Plate Chambers

Stefano de Capua

University of Rome II “Tor Vergata”

and INFN

Outlook

VII RPC Workshop - October 21, 2003 2/15 Stefano de Capua

• Rate Capability vs. bakelite resistivity

• Aging studies at the Gamma Irradiation Facility (CERN)

• Measurements of the bakelite resistivity

• Effects of humidity on bakelite resistivity

• Conclusions

Historical review

VII RPC Workshop - October 21, 2003 3/15 Stefano de Capua

1998. Resistive Plate Chambers were proposed in the LHCb Muon Detector, in the regions with a maximum flux density of 750 Hz/cm2.

1999. Two RPC prototypes (A and B) were built with identical characteristics:

- bakelite electrodes with bulk resitivity ~ 1010 cm

- sensitive area ~ 50x50 cm2

2000. The rate capability of these detectors was initially measured to be Rcap > 3 kHz/cm2 (NIM A 456 (2000) 95)

2001. An extensive test started at the Gamma Irradiation Facility to study aging effects on the Rate Capability.

Rate Capability vs. bakelite resistivity

VII RPC Workshop - October 21, 2003 4/15 Stefano de Capua

We defined a RPC detector capable to stand a given rate if:- efficiency > 95% (trigger requirement)

- at least 400 V plateau (safety requirement)- HV < 11000 (streamer limitation)

August 2001

ρA = 39 x 1010 Ωcm @ 20 oC

Rcap ~ 640 Hz/cm2 @ 20 oC

July 2002

ρA = 110 x 1010 Ωcm @ 20 oC

Rcap ~ 200 Hz/cm2 @ 20 oC

Testbeam measurements (X5 muons beam)

T=25.0 oC T=24.5 oC

Monitoring the RPC resistivity during irradiation

VII RPC Workshop - October 21, 2003 5/15 Stefano de Capua

• We used a simple method to measure the bakelite resistivity in the detector continuosly during the GIF test (G. Carboni et al., NIM A 498 (2003) 135)

• The model is based on the hypothesis that all the physical properties of an RPC must depend on the quantity

Vgap = V0 – RI

and it requires the detector to be exposed at a large flux of radiation.

dS

ρS

2dR

Current saturation with Current linearity with HV

If

then R = V/I

is strongly affected by temperature:

20 = e(T-20)

radiation

Aging test in 2001

VII RPC Workshop - October 21, 2003 6/15 Stefano de Capua

• The gas mixture was C2H2F4/i-C4H10/SF6 (96/4/1) @ 1l/h (steel tubes).

• During the first seven months the irradiated detector (RPC A) accumulated a charge Qint = 0.4 C/cm2 (~3.5 LHCb years).

• The reference dectector (RPC B) accumulated only 0.05 C/cm2.

RPC A RPC B

Date Qint (C/cm2)

R20 (MΩ)

ρ20 (1010 Ωcm)

Qint (C/cm2)

R20 (MΩ)

ρ20 (1010 Ωcm)

oct 99 0 <3 <2 0 ~5 ~3

jan 01 0.076 10.6 6.6 - - -

mar 01 0.11 13.6 8.5 - - -

jul 01 0.361 42.0 26 - - -

aug 01 0.42 62.6 39 0,05 20 13

dec 01 0.42 111 69 - - -

irradiation

• Large resistance increase for RPC A

• Evidence of increase not related to irradiation for both

• The temperature coefficient was measured (~0.12) and found in agreement with our other measurements performed with different bakelite samples. This result confirm the validity of the model.

Aging test in 2002

VII RPC Workshop - October 21, 2003 7/15 Stefano de Capua

• Both detectors now installed close to the source to measure ρ continuously

• Only ~0.05 C/cm2 accumulated charge

• Both detectors show a steady increase of ρ with time

RPC A RPC B

Hypothesis: resistivity increasing due to drying up of bakelite

Aging test in 2003: effects of humid gas

VII RPC Workshop - October 21, 2003 8/15 Stefano de Capua

RPC A RPC B

• humid flow: 1.2% of vapor H2O added to the usual gas mixture

• Clear effect of humid gas, but:on RPC B there is a sharp decrease of resitivity

on RPC A the effect is much reduced

• But: ρ rapidly restores to old values when dry gas is flowed

Study of bakelite resistivity

VII RPC Workshop - October 21, 2003 9/15 Stefano de Capua

In 2003 a test on bakelite resistivity was performed. The test started in August and went on ~60 days.

A thermostatic chamber was used to control the temperature

A vacuum pump was used to keep samples in a vacuum bell.Courtesy of G. Passaleva

Two bakelite samples 10x10 cm2 were tested

Temperature coefficient (1)

VII RPC Workshop - October 21, 2003 10/15 Stefano de Capua

0.140.02

0.12 0.02

(Gcm)

T(oC)

(Gcm)

(Gcm)

T(oC)

(Gcm)Sample A Sample B

The temperature was increased from 15 oC up to 30 oC in not conditioned enviroment (humidity~50%).

was measured and we found a consistent value with values previously measured.

days days

Temperature coefficient (2)

VII RPC Workshop - October 21, 2003 11/15 Stefano de Capua

The temperature was increased from 15 oC up to 40 oC in 100% humid air enviroment.

was measured and we found a different value.

Sample A(Gcm)

T(oC)

(Gcm)

(Gcm)

T(oC)

(Gcm)

Sample B

0.100.02

0.09 0.02

Temperature correction

VII RPC Workshop - October 21, 2003 12/15 Stefano de Capua

Temperature was increased from 15 oC up to 40 oC. Each step lasted about 1 day.100% humid air enviroment.

Resistivity corrected using:

A = 0.14

Still depending on temperature

Resistivity corrected using:

A = 0.10

Clear dependance of the temperature coefficient on humidity percentage(see also Arnaldi et al., NIM A 456 (2000) 142)

Hypothesis: since the dependance on T was corrected, the resistivity decrease depended on the progressive water absorption.

Sam

ple

A

~35% less in 2 days

Effects of humidity

VII RPC Workshop - October 21, 2003 13/15 Stefano de Capua

Sample A left in the thermostatic chamber with 100% humid airand constant T decreased rapidly

But, when the thermostatic chamber was opened, the previous value was restored

Sample A Sample B

Drying up in vacuum

VII RPC Workshop - October 21, 2003 14/15 Stefano de Capua

When the vacuum was created, increased rapidly .A silica gel was used to absorb humidity.

The bakelite resistivity depends strongly on the percentage of water in its enviroment.

Sample A Sample B

Conclusions

VII RPC Workshop - October 21, 2003 15/15 Stefano de Capua

• Aging effects on bakelite RPCs have been extensively studied for 3 years on two identical detectors with resistivities around 1010Ωcm.

• After 2 years operation, ρ increased to ~100 x 1010Ωcm reaching the value of ~200 x 1010Ωcm at the end of the third year.

• Although irradiation (0.4 C/cm2) contributes to the resistivity increase, the effect is mainly related to dry gas flow.

• Humid gas has been flowed with different response: - RPC B shows a sharp decrease of resistivity- RPC A shows an effect very much reduced

• Restoring dry gas flow has resulted again in a fast resistivity increase.

• A study of resistivity was performed on two bakelite samples.

• The temperature coefficient showed a clear dependance on humidity percentage.

• The measurements confirmed the strong dependance on the enviroment conditions of the bakelite resistivity, especially on the humidity percentage.

• Flow of humid gas does not appear to be a practical method to recover detector performances. A better solution could be a continous flow of humid gas.