Andrea Di Simone CERN PH/ATC and INFN-CNAF On behalf of ATLAS RPC groups:

1. Ageing test of ATLAS RPCs at X5 Andrea Di Simone CERN PH/ATC and INFN-CNAF On behalf of ATLAS RPC groups: Lecce, Napoli, Protvino, Roma2

2. Outline Ageing effects in bakelite RPCs Experimental setup Plate resistivity increase Current monitoring Damage recovery Conclusions

3. Ageing effects in bakelite RPCs • Long time operation of resistive plate chambers is known to produce two main ageing effects: • gradual increase of the electrode resistivity (i.e. reduced rate capability) under very high working currents. This effect, however, is known not to be relevant for the ATLAS experiment: • previous tests showed that after an ageing of ~10 ATLAS years, chamber performance remains above the ATLAS requirements. • degradation of the inner surface of the plates due to operation with fluorine-rich gas mixtures, leading to an increase of the noise in the detector

4. Plate resistivity The plate resistivity is known to be related to environmental parameters such as temperature and relative humidity: Higher T  Lower r; Higher RH Lower r Plates kept under high current densities (hundreds of mA/m2) for long periods, show a gradual increase in resistivity which is found to be faster when the plates are operated at lower RH values. This effect is selective wrt the working voltage polarity, i.e. any change in environmental RH is more effective when applied to the anode side of the plate. both the gas mixture and the external environment need to be humidified in order to operate the anode sides of the two plates in proper RH conditions.

5. Experimental setup 3 standard production chambers(BML-D) in the area. 6 gaps under ageing test. Beam 137Cs source (20 Ci); 660 keV photons

6. Ageing status

7. Plate resistivity measurements (1) First method: the chambers are filled with Ar, and operated above 2kV, where the voltage drop across the gas remains constant. In these conditions, the I-V curve is linear and the ratio V/I gives the value of the resistance of the bakelite. Linear increase dominated by bakelite resitivity I-V characteristic in pure Ar

8. Plate resistivity measurements (2) Second method: the efficiency plateaus with full and closed source are compared. The voltage difference between the two plateaus is due to the gap current, which produces a voltage drop across the bakelite plates. From the voltage drop and the measured current we calculate the plates resistivity. Vgas=Vgap- RbakIgap No source full source After correction for resistivity No correction

9. Plate resistivity evolution OFF ext RH control ON

10. Plate resistivity evolution

11. Plate resistivity evolution (2) Each detector layer consist of two gas gaps with the gas flowing serially from the lower to the upper ones Only the 6 lower gaps were kept at the working point The upper ones are normally kept at HV=0 After ~2 years of operation, the plate resistivities of the upper chambers are consistent with the initial values The operating current is the primary cause of the observed increase in plate resistivity

12. Current monitoring Chamber currents have been continuously monitored during the test: currents at working point ohmic currents @ 5kV Both current have proved to be an important tool for diagnostics of the gap operation: Ohmic currents are an indicator of the presence of pollutants on the plate surface, rather than of an actual damage of the surface, and are very sensitive to any problem related to the recirculation system’s filters. Working currents are sensitive to gas mixture problems and to filter exhaustion

13. Current monitoring (2) Wrong mixture Filter exausted

14. Current monitoring (3)

15. Detector noise and surface damage Fluorine rich gas mixtures produce, under electrical discharge, F- ions which can damage the inner surface of the gas gaps. This results in an increase of the detector noise. This type of damage can, to some extent, be recuperated by operating the chamber at lower voltage, large gas flow and possibly with isobutane enriched mixtures (see G. Aielli's presentation, session N29). We illustrate in the following a significant example of damage/recovery.

16. Surface damage (1) A major malfunctioning of the recirculated gas system occurred at an integrated charge corresponding to 7 ATLAS years (safety factor 5). At the same time, the DCS system has not been able to shut down the HV. The chambers have continued operating at working point, under full irradiation, without any gas flow. This lead to a damage to the internal surface of the plates, detectable from an increase (by a factor 2) of the working currents at closed source. Moreover, the presence of pollutants on the surface caused an increase by a factor 4 of the ohmic currents of the gaps.

17. Surface damage (2)

18. Surface damage (3) (full source)

19. Damage recovery Increasing the isobutane concentration in the gas mixture has shown in the past to be very effective in the recovery of damaged bakelite RPCs. The isobutane component was raised from 5% to 15% Besides the recovery process, the performance of the chambers under this new gas mixture has also been studied.

20. Damage recovery - results gap 1 gap 2 gap 3 gap 4 gap 5 gap 6 T Chambers were kept at 7kV We observed a decrease of the working currents on all the chambers The ohmic current showed also a steady and regular decrease. The ageing at normal working point has now restarted. If no current increases are observed, the standard ATLAS mixture will be restored Working currents I(mA) T (°C) gap 1 gap 2 gap 3 gap 4 gap 5 gap 6 T Ohmic currents T (°C) I(mA)

21. Damage recovery – results (2) Current evolution isotherms gap 1 gap 2 gap 3 gap 4 gap 5 gap 6 T gap 1 gap 2 gap 3 gap 4 gap 5 gap 6 T I(mA) I(mA) T (°C) T (°C) Ohmic currents Working currents

22. Conclusions RPC operation with proper relative humidity in both the gas and the environment limits (and could eliminate) the increase of the plate resistivity under high operating currents, which is one of the dominant ageing effects in bakelite RPCs All along the test, chamber performance (efficiency, cluster size, rate capability) remained largely above the ATLAS requirements. After a serious damage to the inner gap surface due to a problem with the gas system, the gaps have been completely recovered using an isobutane enriched gas mixture.