The Resistive Plate Chamber detectors at the Large Hadron Collider experiments Roberto Guida Paolo Vitulo PH-DT-DI Univ. Pavia CERN EDIT school 2011

1949: Keuffel  first Parallel Plate Chamber 1955: Conversi used the “PPC idea” in the construction of the flash chambers 1980: Pestov  Planar Spark chambers – one electrode is resistive – the discharge is localised 1982: Santonico  development of the Resistive Plate Chamber – both electrode are resistive RPC applications: ‘85: Nadir (n-n\bar oscillation) – 120 m 2 (Triga Mark II – Pavia) ‘90: Fenice (J/   n-n\bar) – 300 m 2 (Adone – Frascati) ‘90: WA92 – 72 m 2 (CERN SPS) ‘90: E771– 60 m 2 ; E831 – 60 m 2 (Fermilab) 1992: development of RPC detector suitable to work with high particle rate  towards application at LHC : L3 – 300 m 2 (CERN-LEP) : BaBar – 2000 m 2 (SLAC) Some history

Identikit of RPC detectors for LHC Basic parameter for a detector design:  Gap width  Single gap/double gap/multi gap design  Gas mixture  Gas flow distribution  Bakelite bulk resistivity  Linseed oil electrode coating es

The RPC detector  Gap width 2 mm: Good compromise between good efficiency, time resolution and rate capability  More gaps: Increase time resolution and efficiency  Double gap design: Best ratio induced/drift charge, therefore best signal/charge ratio  Freon based mixture: Higher efficiency (at the same gas gain) and lower streamer probability  Bakelite bulk resistivity =  cm: Good compromise between high rate capability and low current and noise  Linseed oil treatment: Lower current and noise rate. No ageing effect observed

Why the RPC? Drift chambers (cylindrical geometry) have an important limitation: Primary electrons have to drift close to the wire before the charge multiplication starts  limit in the time resolution  0.1  s  Not suitable for trigger at LHC + In a parallel plate geometry the charge multiplication starts immediately (all the gas volume is active). + much better time resolution (  1 ns) + less expensive (  100 €/m 2 ) However: -Smaller active volume -Electrical discharge may start more easily -Relatively expensive gas mixture -Quite sensitive to environmental conditions (T and RH)

Lab. activity: Switch on the RPCs  HV scan  Pulse height  Pulse charge

Avalanche signal Streamer signal Towards a new operation regime Originally RPC were operated in Streamer mode:  Ar-based mixture  Higher signal (100 pC) but also high current in the detector  Voltage drop at high particle rate  loss of efficiency  poor rate capability (< 100 Hz/cm 2 ) Operation with high particle rate possible in Avalanche mode:  Freon-based mixture  lower signal (  pC) but also lower current in the detector  Less important high voltage drop at high particle rate  good rate capability (  1 kHz/cm 2 ) R. Santonico et al. ATLAS Muon TDR

Ionizing particles passing through the gas are producing primary ionization Primary electrons accelerated in the electric field will start to produce further ionization n total : total number e - /Ion  E: total energy loss W i : /(total number e - /Ion) Working principle

Gain Primary e - Charge multiplication  = first Townsend coefficient  attachment coefficient  =  -  = effective Townsend coefficient  x  20  M  10 8 Reather breakdown limit

Time constant for charge development is related to drift velocity and multiplication Time constant for recharge the elementary cell is related to the RC  discharge <<  recharge Are the most important improvement with respect to previous generations Since  recharge >>  discharge the arrival of the electrons on the anode is reducing the electric field and therefore the discharge will be locally extinguished.  the electrode are like insulator after the first charge development  Self-extinguish mechanism HV  discharge = 1/  v d ~ 10 ns  recharge =  ~ 10 ms Why resistive electrodes? 

The RPC detector: gap width The gap width is affecting both time performance charge distribution Narrow gap  better time resolution, but “more critical” charge spectrum (R seems to be the driving parameter)  reduce number of primary electrons R =  R > 1 R < 1 cluster density (primary clusters)  g =const = 18 M. Abbrescia et al. NIMA

The RPC detector: single vs double gap The double gap layout:  allow to operate at lower  (i.e. lower high voltage)  best ration q ind /q tot (single 0.7; double 1.4; 3 gaps 0.8)  Better charge distribution  Twice expensive per unit of active area

Formation of the induced signal Equivalent circuit of double RPC near the discharge region: Ratio between total charge and induced charge on the strips:

The RPC detector: gas mixture Key parameters: cluster density (primary clusters) – only gas dependent  first Townsend coefficient (charge multiplication) – gas, HV dependent  attachment coefficient – gas (electronegative and/or quencher), pressure dependent  effective Townsend coefficient =  -   High gas: high efficiency and low streamer probability at the same  ! i.e. for avalanche regime: better Freon ( = 5.5 mm -1 ) than Ar ( = 2.5 mm -1 )  Voltage increasing

The RPC detector: gas mixture Key parameters: Moreover quencher (iC 4 H 10 ) and electronegative (SF 6 ) gases will help in containing the charge development and reducing the streamer probability Charge distribution: avalanche streamer Fraction of streamer vs High voltage: M. Abbrescia et al. NIMA R. Santonico Scientifica Acta Pavia

The RPC detector: gas mixture Key parameters: Large are giving also a better charge distribution (constant  ) ≤ 4 mm -1 spectrum monotonically decreasing Charge spectrum vs Streamer probability M. Abbrescia et al. NIMA

The RPC detector: gas flow distribution Experience says that the gas flow has to be at least  0.3 vol / h even without radiation Is the gas distribution inside the gap the origin of this limit? Is there a space for future improvements? Some preliminary results coming from a finite element simulation: Waldemar Maciocha Antonio Romanazzi The velocity field shows areas in which the gas is hardly moving Thanks to Gas velocity (m/s) inlets

RPC detector: bakelite resistivity The detector rate capability is strongly dependent on the bakelite resistivity. At high particle rate (r) the current through the detector can become high enough to produce an important voltage drop (V d ) across the electrode: s: electrode thickness : average pulse charge  : bakelite resistivity In order not to lose efficiency  V d <  10 V Therefore   ~  cm The time constant of an elementary cell is lower at lower resistivity: the cell is recovering faster (it is quicker ready again) after a discharge took place inside it.

RPC detector: linseed oil surface treatment RPC electrodes are usually treated with linseed oil:  better quality of the internal electrode surface  it acts as a quencher for UV photons  better detector performance …but…  More time needed during construction  Ageing problems? (Not observed) What is the linseed oil:  Drying oil (consists basically of triglycerides)  Drying is related to C=C group in fatty acid  Cross-linking (polymerization) in presence of air (O 2 play important role) due to C=C

RPC detector: linseed oil surface treatment Few SEM photos (S.Ilie, C.Petitjean EST/SM-CP EDMS ): Defect on Bakelite surface possibly covered with linseed oil Thickness of the layer: ~ 5  m The linseed oil layer is damaged by a surface outgassing

RPC detector: linseed oil surface treatment Effect on UV photons hitting the electrode internal surface: Linseed oil absorbance 8 eV 5.6 eV UV sensitivity for coated and non coated Bakelite C.Lu NIMA P.Vitulo NIMA

RPC detector: linseed oil surface treatment Chamber Performance: With linseed oil coated electrodes Lower current (  1/10) Lower noise rate (  1/10) M. Abbrescia et al. NIMA

RPCs for LHC experiments Why RPCs for application in LHC experiments need a particular “care”?  Huge (  5000 m 2 of sensitive area) and very expensive ( CHF) systems (for comparison BaBar was about 2000 m 2 )  Very long period of operation expected (at least 10 years)  Very high level of background radiation expected  Integrated charge never reached before: 50 mC/cm 2 for ALICE and CMS 500 mC/cm 2 in ATLAS  Large detector volume  basically impossible to operate the gas system in open mode  closed loop operation  gas mixture quality

RPCs for LHC experiments Where are the RPCs systems at LHC? ATLAS experiment: - Active surface 4000 m % C 2 H 2 F 4 ; 5% iC 4 H 10 ; 0.3 % SF 6 - Gas Volume 16 m % Rel.Humidity - Expected rate ~ 10 Hz/cm 2 - Closed loop operation CMS experiment:

Closed loop gas circulation  Large detector volume (~16 m 3 in ATLAS and CMS)  use of a relatively expensive gas mixture  closed-loop circulation system unavoidable. Nowadays with 5-10 % of fresh gas replenishing rate  cost is  700 €/day But….  Several extra-components appear in the return gas of irradiated RPCs  Detector performances can be affected if impurities are not properly removed Purifiers:

Gas analysis results: chromatography Many extra components identified in the return mixture from detector  Operated with open mode gas system  Under high gamma radiation Concentration of the order of  10 ppm Mainly hydrocarbons other Freon

With respect to reference bakelite surface:  High fluorine concentration  Na signal appeared  N signal disappeared   Linseed oil and melamine layer etched: Na is used as a catalyser for phenolic resin (bulk) Normal surface layer (made on melamine resin) contain N Bakelite SEM results We analyzed few bakelite samples from an RPC with relatively high current. The visual inspection of the surface shows at least two different kinds of surface defects: Reference bakelite

Operation of RPC detectors  The operation of a large area detector is never simple.  “Second order” problems may come from anywhere and anytime. Few example:  Gas quality is a crucial issue for all gaseous detectors (therefore also for RPC)  Environmental conditions (like temperature and relative humidity) are affecting the detector performances (complex network of sensors is needed in order to understand behaviours)  Gas leaks in the detector (unfortunately is a weak point) In the following, for time reason, I will discuss only an example concerning the gas quality

RPC detectors at LHC: some data A CMS event with muon track reconstructed Lab. activity: Switch on the RPCs

Lab. activity: Pulse charge spectra Avalanche vs. Streamer  Signals  Time behavior  Charge  Noise charge spectra

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