1. 1 A.Di Ciaccio LHC upgrade Working Group November 2005 The ATLAS RPC trigger system  The entire stand-alone muon system has been designed to handle background rates up to 5 times the LHC expectation with minor degradation of performances  At the SLHC at L= 10 35 cm -2 s -1 there will be an obvious increase of the background counting rate (naively a factor 10).  ATLAS has already set-up a shielding radiation task force group to study a background reduction.  Question for the RPC system:  After 10 year of operation at the LHC the detector performance are still acceptable at the particle rate of the SHLC??  Understanding of the RPC ageing essential to answer such a question

2. 2 A.Di Ciaccio LHC upgrade Working Group November 2005 Trigger chambers-3D view ATLAS RPC system: 1116 camere 7000 m 2 di rivelatore 8500 strip panels 350000 read-out channels Resistive Plate Chambers(RPC) (|η| < 1.05)

3. 3 A.Di Ciaccio LHC upgrade Working Group November 2005 ATLAS barrel toroid 656 muon stations (of which 380 MDT/RPC) embedded inside the 8 coils

4. 4 A.Di Ciaccio LHC upgrade Working Group November 2005 The RPC muon system Three RPC detector layers: – 2 in the middle station, 1 in the outer Each layer: – 2 gas gaps and 4 readout planes for each detector element – Eta and Phi read-out copper strips panels, pitch ranging from 26.4 to 33.9 mm Each gap: – 2 mm gas gap with bachelite electrodes – bakelite resistivity : ~ 1-4x10 10  cm – Gas mixture: (C 2 H 2 F 4 ) 94.7% - (C 4 H 10 ) 5% - (SF 6 ) 0.3% Performance: – RPCs working in avalanche mode – Efficiency: > 98% – Time resolution: ~ 1-2 ns – Spatial resolution: 5-10 mm – Rate capability: ~ 1000 Hz/cm 2 Three RPC detector layers: – 2 in the middle station, 1 in the outer Each layer: – 2 gas gaps and 4 readout planes for each detector element – Eta and Phi read-out copper strips panels, pitch ranging from 26.4 to 33.9 mm Each gap: – 2 mm gas gap with bachelite electrodes – bakelite resistivity : ~ 1-4x10 10  cm – Gas mixture: (C 2 H 2 F 4 ) 94.7% - (C 4 H 10 ) 5% - (SF 6 ) 0.3% Performance: – RPCs working in avalanche mode – Efficiency: > 98% – Time resolution: ~ 1-2 ns – Spatial resolution: 5-10 mm – Rate capability: ~ 1000 Hz/cm 2 ATLAS air-core toroid

5. 5 A.Di Ciaccio LHC upgrade Working Group November 2005 First level muon trigger in the barrel RPC LVL1 Muon trigger: ● Six programmable thresholds ● Low-pT trigger (RPC1+RPC2) 6 GeV/c ● High-pT trigger (low- pT+RPC3) 10 Gev/c – Fast programmable geometrical coincidence (programmable cone opening) – Algorithm performed separately in eta and phi – Region of Interest: D  x D  = 0.1 x 0.1 The RPC system provides: ● LVL1 muon trigger ● Bunch Crossing Identification ● Second coordinate measurements The RPC system provides: ● LVL1 muon trigger ● Bunch Crossing Identification ● Second coordinate measurements

6. 6 A.Di Ciaccio LHC upgrade Working Group November 2005 ATLAS shielding system Radiation shielding has been carefully studied and optimized to reduce background hall at a reasonable level

7. 7 A.Di Ciaccio LHC upgrade Working Group November 2005 Muon background counting rate @L=10 34 Background rate < 20 Hz/cm**2 in the barrel

8. 8 A.Di Ciaccio LHC upgrade Working Group November 2005 Muon background counting rate @L=10 35  At L=10 34 cm -2 s -1 the total counting rate (from Fluka/GCALOR MC) in the RPC system is ~10-20 Hz/cm 2 MC assumption have to be validate at the LHC start  Preliminary Montecarlo study at L=10 35 cm -2 s -1 (M.Shupe et al) indicates that the background rate in the barrel muon system can be reduced at least of a factor 2 with: a berillium beam pipe up to z=16m  Additional shielding, modification of the forward layout are also investigated to further decrease the background rate  The expected rate in the Barrel muon system could be estimated ~50-100 Hz/cm 2

9. 9 A.Di Ciaccio LHC upgrade Working Group November 2005 Beam pipe upgrade MC studies M.Shupe et al:ATL-Tech-2004-003

10. 10 A.Di Ciaccio LHC upgrade Working Group November 2005 Background rate: preliminary studies It is possible to have the same rate in the barrel muon with a increase of 2 in luminosity changing to a Berillium pipe up to z=16m

11. 11 A.Di Ciaccio LHC upgrade Working Group November 2005 The ATLAS RPC performance  Cosmic ray test performance  What we have learnt after the ageing tests at X5/ GIF on:  Modulo 0  Production chamber tests

12. 12 A.Di Ciaccio LHC upgrade Working Group November 2005 Cosmic ray tests:RPC performance Efficiency vs high voltage Counting rate vs HV Cluster size Nstrip=1.6 BOS unit : dimension 1.1 x3.9 m 2 4 gas volumes: 8 read-out strip panels BOS unit : dimension 1.1 x3.9 m 2 4 gas volumes: 8 read-out strip panels Hz/cm 2  = 2 ns Time distribution

13. 13 A.Di Ciaccio LHC upgrade Working Group November 2005 A 15 months ageing test performed on module-0 at the GIF – X5 CERN irradiation facility.  Average expected counting rate in the ATLAS barrel ~100 Hz/cm 2 ( including a safety factor ~ 5)  Total counts expected in 10 ATLAS years: 10 10 /cm 2  Total delivered charge: 0.3 C/cm 2 ( (100 Hz/cm 2 x 10 8 x30pC/count) First RPC Ageing test at GIF GIF-X5: Uniform irradiation with low energy gamma Source: 740 GBq 137 Cs  = 0.662 MeV RPC module-0 NIM A478 (2002) 271

14. 14 A.Di Ciaccio LHC upgrade Working Group November 2005  The RPC module-0 after 12Y ageing shows a rate capability of ~300Hz/ cm 2  The ageing effect can be described in term of an increase of the operating voltage, due to an increase of the electrode total resistence (bakelite+graphite layer)  The RPC module-0 after 12Y ageing shows a rate capability of ~300Hz/ cm 2  The ageing effect can be described in term of an increase of the operating voltage, due to an increase of the electrode total resistence (bakelite+graphite layer) First ageing test results Efficiency vs HV after Q = 0.36 C/cm 2 (12 ATLAS years ) This test was complemented with more ageing tests in Roma2 (with 60 Co source) for better understanding of the electrode ageing. -> a major deterioration of the graphite layer was observed.  The graphite layer was increased (keeping the same surface resistivity)

15. 15 A.Di Ciaccio LHC upgrade Working Group November 2005 More ageing test at GIF Beam 3 standard production chambers(BML-D) in the area. 6 gaps under ageing test. 137 Cs source (20 Ci); 660 keV photons

16. 16 A.Di Ciaccio LHC upgrade Working Group November 2005 Plate resistivity study 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

17. 17 A.Di Ciaccio LHC upgrade Working Group November 2005 Plate resistivity measurements 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 resistivity Linear increase dominated by bakelite resistivity I-V characteristic in pure Ar

18. 18 A.Di Ciaccio LHC upgrade Working Group November 2005 Ageing status at GIF Integrated charge : 240mC/cm 2 corrisponding to:8 year ATLAS with a safety factor ~5

19. 19 A.Di Ciaccio LHC upgrade Working Group November 2005 Understanding of ageing effects  Long time operation of RPCs is known to produce two main ageing effects: 1. Gradual increase of the total electrode resistivity (i.e. reduced rate capability) under high working currents. 2. 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

20. 20 A.Di Ciaccio LHC upgrade Working Group November 2005 How to reduce the increase of resistivity?  The on-going X5/GIF ageing test showed that it is possible to ‘moderate’ the ageing, precisely the increase of the bakelite resistivity  it can be significantly reduced if the gas as well as the air in the environment humidity are kept at the level of 40-50% RH

21. 21 A.Di Ciaccio LHC upgrade Working Group November 2005 Plate resistivity evolution (1) ext RH control ON OFF  (20)=  (T)/exp(0.1(20-T))

22. 22 A.Di Ciaccio LHC upgrade Working Group November 2005 RPC efficiency after 7 ATLAS year Full source : rate ~500 Hz/cm**2 on station 1

23. 23 A.Di Ciaccio LHC upgrade Working Group November 2005 Effect of F - production  The RPCs make large use of electronegative gases to control and limit the discharge process (C 2 H 2 F 4 and SF 6 )  The decomposition of such a gases under electrical discharge produces a significant concentration of fluoride radicals that can be detected in the RPC exhausted gas.  The F radicals may easily produce HF. Due to its high chemical reactivity, this represents a possible cause of the inner surface damaging if it is not quickly removed by the gas flow. – HF is an aggressive acid: it can harm the inner surface finding the eventual weak points of the oil protective coating.

24. 24 A.Di Ciaccio LHC upgrade Working Group November 2005 Effect of F - production: noise increase  Electrode surface damages by HF produces an increase of the noise rate and of the noise current.  It is enhanced by insufficient gas flow rate a flow rate up to 1 Vol/h is required in Atlas Recycled gas system mandatory with a purifier/filter to trap the impurities  A method for monitoring the Fluorine production in the gas has been developed in Roma2

25. 25 A.Di Ciaccio LHC upgrade Working Group November 2005 F - Measurement setup TISAB + H2O fluoride probe DAQ pH meter Gas system Gas T,RH probe RPC current SCALER: Singles doubles Magnetic stirrer Gas in Gas out (teflon) To exit bubbler Teflon container F˙ is measured by an Ion selective electrode probe(0.02 ppm F˙ sensitivity), read by a PHmeter (used as impedance converter and prompt monitor) and recorded by DAQ Serial line  The F- can be measured by trapping the ions e.g. by bubbling the gas in water where the fluorine is detectable as Fluoride.  The TISAB (Total Ionic Strength Adjousting Buffer) neutralizes the effect of the electrode interfering substances such as OH- or metal traces. It keeps the PH around 5.5

26. 26 A.Di Ciaccio LHC upgrade Working Group November 2005 More ageing test results  We have also seen that the i-Butane has a strong effect in reducing the Fluorine production this suggests to maximize the i-Butane concentration in the RPC gas we are however already close to the flammability limits of the mixture (with 5% i-buthane) At SHLC could we change the mixture with more isobuthane?  Operation at low environment temperature, typically <24 C, is also essential to keep the noise at low level

27. 27 A.Di Ciaccio LHC upgrade Working Group November 2005 T=21C T=31C Noise increases with Temperature 10250V 9900V 10500V 10200V 9600V 9900V HV corrected =9350V HV corrected =9600V chamber 4 gap 2 efficiency Working point From T= 21 C to T=31 C  a factor 10 increase in the noise observed at the working point

28. 28 A.Di Ciaccio LHC upgrade Working Group November 2005 Current increase with temperature Both ohmic and multiplicative part of Igap increase with Temperature T=21C T=31C Lecce group,Seoul 2004

29. 29 A.Di Ciaccio LHC upgrade Working Group November 2005 A test of robustness after a major accident  A major malfunctioning of the recirculated gas system occurred at an integrated charge corresponding to ATLAS years  The chambers have continued operating at working point, under full irradiation, without any gas flow >>>the DCS system was not been able to shut down the HV  This lead to a damage to the internal surface of the plates, detectable from an increase 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.

30. 30 A.Di Ciaccio LHC upgrade Working Group November 2005 Ohmic current increase

31. 31 A.Di Ciaccio LHC upgrade Working Group November 2005 Working current increase (full source)

32. 32 A.Di Ciaccio LHC upgrade Working Group November 2005 Increase isobuthane in the mixture  Increasing the isobutane concentration in the gas mixture to study a possible recovery of damaged bakelite RPCs. The isobutane component was raised from 5% to 15% the performance of the chambers under this new gas mixture has also been studied.

33. 33 A.Di Ciaccio LHC upgrade Working Group November 2005 Damage recovery – results (1) 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. After the standard ATLAS mixture has been restored and ageing restarted Working currents Ohmic currents I(  A) T (°C) 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

34. 34 A.Di Ciaccio LHC upgrade Working Group November 2005 Damage recovery – results (2) Current evolution isotherms Working currents Ohmic currents I(  A) T (°C) I(  A) T (°C) 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

35. 35 A.Di Ciaccio LHC upgrade Working Group November 2005 Conclusions (1) All along the test (8 ATLAS year, safety factor 5), chamber performance (efficiency, cluster size, rate capability) have remained largely above the ATLAS requirements. This ageing is equivalent to 16 SHLC year at a rate of 50Hz/cm**2 (with no safety factor) From the RPC ageing studies we have learned : Temperature, and RH of the enviroment and gas mixture must be kept at a proper value, F- production could be reduced by isobuthane in the mixture

36. 36 A.Di Ciaccio LHC upgrade Working Group November 2005 Conclusions (2) More R & D is necessary mainly on new gas mixture The first 1-2 year of ATLAS are necessary to draw more realistic conclusions on the behaviour of the RPCs at the SHLC For a correct background rate estimation at the SHLC it is important to test the Montecarlo calculation at the LHC start-up.