Report on rpc ageing studies

1. Report on rpc ageing studies G. Aielli Wg4 meeting 3/2/2014

2. Common Ageing test on rpcs • The RPC of Alice, Atlas and Cms share almost all the construction procedures and material features as well as the production site. • Common ageing studies from 1998 to 2005 (Alice, Atlas, Cms) • Carried out at the GIF under a the supervision of a constituted rpc ageing studies taskforce (chaired by A. Sharma) • Long term irradiation of prototypes, module-0 and production chambers • Each experiment tried to accumulate the hits expected in 10 years of LHC operation at nominal luminosity with a given safety factor • Alice: 50 mC/cm2 • Atlas: 400 mC/cm2 • CMS: barrel (100 mC/cm2)EC 400 mC/cm2

3. Test procedure • The procedure can be different from case to case but in general included: • Monitoring of the integrated current • Monitoring and control of environmental conditions • Periodic measurement of the efficiency and rate capability • Periodic measurement of the electrode resistivity • Periodic measurement of the dark current • The choice of the working point and the acceleration factor is critical for the reliability of the results

4. Observed Ageing effects on rpcs • Detector lifetime from the electric point of view (from ageing tests) • Depends on the stability of the electrode conduction properties and surface integrity • Two potential problems have been pointed out: • Increase of the electrode resistivity due to the loss of ionic carriers. This would harm the rate capability by means of a non negligible voltage drop across the electrode plates • Damaging of the inner surface by means of HF deposit from the discharge. This increases the dark current and the noise rate and if left without control my seriously harm the detector • Risk factors are influenced by • Total charge per count  integrated charge and the local discharge probability • Local counting rate  integrated charge and HF production rate • Local gas change rate and composition  HF production and removal rate • Gas and environment humidification  electrode resistivity control • Temperature  electrode resistivity, increase of the noise, acceleration of the HF damage • Gap features (plate resistivity oiling type…) endurance in harsh conditions

5. Main control nodes Analysis of the risk factors - Graph Local discharge Local HF production Gas composition Working point temperature Surface damage Total current Global HF production • Resistivity increase can be controlled through the RH and has a negative feedback • Noise induced by HF can be controlled by lowering the current. It has a positive feedback HF accumulation Low resistivity Gas change rate Counting Rate Surface quality initial resistivity background Resistivity increase Voltage drop Gas RH supply

6. Analysis of the risk factors • Given an amount of integrated charge it gives a very different effect depending on how it is generated depending on: • Working point: the lower the better  many small avalanches are better than few big ones • Rate (duration): the lower (long) the better if the environmental condition are controlled • Temperature: the lower the better • Relative humidity: best value 40-45% for stable resistivity around 5*1010W cm • Resistivity: the higher the better compatibly with the rate expectations • Oiling: a stronger coating increase the detector endurance • Gas composition: complex answer see later • Gas change rate: the higher the better • It turned out that in case of detector internal damage a current generated in ideal condition can heal the problem instead of worsening it!

7. The atlas ageing case • Recovery after extreme damage due to the recirculation (and dcs) failure • Increase of the noise current up to 20 A source off. Also the ohmic current increased • Recovery monitored at constant temperature • Deep cleaning with pure argon discharge • Operation at reduced voltage (7000V) • 15% C4H10 • 2 change/h • Extra current disappeared completely and the detector performance was as before

8. ATLAS BARREL background map from currents • ATLAS barrel middle layer RPCs. Eta vs Phi • Measured at L=6*1033cm-2s-1 @ 8 TeV • Extrapolated at L=1034cm-2s-1 @ 14 TeV • Average value 18 Hz/cm2 • Max=44 Hz/cm2 • Min=5 Hz/cm2 • Data within prediction including the 1.6 enery factor…(20 Hz/cm^2) • the spread is a factor of 9. The peak respect to the average is a factor > 2.4 • In phase 1 the safety factor will be almost fully used • In phase 2 we will be running beyond the design specification and after 20 years from the construction and 14 years of operation • The detector can not be replaced but a further layer can be added • This point out the necessity of increasing the redundancy to overcome the danger of ageing effects

9. The atlas extrapolation The plot shows the RPC average current density vs. instantaneous luminosity at beam dump. The measurements refer to 2010 and 2011 data and span over a range of more than 4 decades. Data fit a straight line with a slope of 0.312 ± 0.001 nA m-2 /1030cm-2s-1 and a negligible intercept. Extrapolating the fit to L = 1034 cm-2s-1 and converting the current in counts with a 30 pC/count experimental factor, an average rate of ~10 Hz/cm2 is predicted.

10. CMS background Linear Extrapolation @ L = 1035 cm-2 s-1 Rate = 50 Hz/cm2 (average RB4 & RB1) Rate = 100 Hz/cm2 (RB4 sector 4) RB2 and RB3 at least a factor 5 less • CMS has chamber both in endcap and barrel thus there is a large spread in expected rate • Estimated rate at L = 1035 cm-2s-1 • Has been included the energy effect (factor 1.6) ?? • In the worst case Q = 0.36 C/cm2 @ Lint = 3000 fb-1 • Total surfaceabout 10000 m2 • 1000 A ?? @ 1030cm-2s-1 • 0.1 nA/1030 cm-2 s-1 • In ATLAS 0.3 • Numbers to be confirmed Linear Extrapolation @ L = 1035 cm-2 s-1 Rate = 100 to 200 Hz/cm2

11. Conclusions • More ageing data from CMS and ALICE needed • Clear definition of the Ph1 and Ph2 needs and expectations • Started a new common effort on the gas study at the old GIF (waiting for the new one) • aim to monitor all the detector parameters including HF production • Aim to change the mixture composition and test new components vs. HF and performance • To be added: studies about the time driven and mechanical ageing • Proposal to include not only the failure induced by individual detectors ageing but also the system obsolescence induced by the increase of performance need: • Redundancy • Selectivity • Fake rejection • coverage