Long term test of RPCs at Gran Sasso

A. Paoloni (LNF)

LNF-LNGS-Padova

Experimental setup( @ Lab6 of LNGS )

• Scintillator trigger

• Trigger area = (210*28) cm²

• Trigger rate = 15 Hz/m²

• RPC area = (2.9*1.1) m² (full size)

• RPC 1,2,3,4 with xy readout by 3 cm pitch strips

• RPC 2,4,5,6 (with grooves)

• RPC 1,3 (without grooves)

Long term operation test

• RPC 13 old chambers

• RPC 46 produced according to the final procedure (improved oil coating)

• Bakelite resistivity = 5÷10 * 10¹¹ *cm

• Counting rate at Gran Sasso lab surface ≈ 300 Hz/m²

• Counting rate at Gran Sasso hall ≈ 30 Hz/m²

• Ageing: 6 months surface test ≈ 10 years Opera (rate ratio=10, 6/12 months with beam on)

Operating voltage

• Typical operating conditions: Vapplied=7.4 ÷ 7.8 kV, Gas mixture Ar/TFE/i-But=48/48/4 (5 refills/day), T=20÷27 °C, P≈900 mbar

• Different T and P different chamber behaviour at fixed voltage (charge multiplication in gas depends on V/d, where d is the gas density)  

• In our analysis the voltage is rescaled to fixed T0=293 K and P0=1010 mbar according to :

V = Vapplied * (T/T0) * (P0/P)• Moreover, operating current and counting rates

increase with the temperature (change of bakelite resistivity)

Long term test results (old chambers)

• RPC1 shows an increase of the current from day 110

• RPC2 showed an increase already from day 50, reversible by increasing the gas flow from 5 to 10 refills/day (days 105158)

• RPC3 with discharges in HV cabling: now we are not able to read currents, but the chamber is still on

• No loss of efficiency is observed ( ≥ 97% )

RPC 1

RPC 2

RPC 3

T0=1 May 2002

Gas flow

doubled

HV cabling

discharge

Gas failure

Long term test results (RPC1)

Rescaled

operation

voltage

Long term test results (RPC1)

•High currents and rates are correlated (noise)

Long term test results (RPC2)

Rescaled

operation

voltage

Long term test results (RPC2)

Long term test results (RPC3)

Rescaled

operation

voltage

Long term test results (RPC3)

Long term test results (pre-production)

• Currents seem to be more stable

RPC 4

RPC 5

RPC 6

T0=1 August 2002

Gas failure

Long term test results (RPC4)

Rescaled

operation

voltage

Long term test results (RPC4)

Conclusions and outlook

• RPCs after equivalent 10 years of Opera show good efficiency (though with an increase of current and noise)

• This increase can be cured by increasing the gas flow

• Final production procedures seems to improve RPC performances (low currents and noise)

• Ageing tests are still going on at Lab6 of LNGS