1. Divide and Gain! Multi-gap RPC detectors B.Satyanarayana DHEP/TIFR DHEP/TIFR Moon Moon Devi INO/HBNI

2. Wilson’s Cloud Chamber (1894) B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 20142 Contributed to the discoveries of e+, µ and K.

3. Geiger–Müller tube (1928) B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 2014 Cylindrical single-wire counters working in a saturated discharge mode (“Geiger mode”).

4. Georges Charpak’s MWPC (1968) B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 20144 The first modern electronically-readout gaseous detector. Contributed to the discoveries of W, Z, c and t.

5. Single-gap RPC (1981) B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 2014 Yu.N. Pestov & G.V. Fedotovich (1978) Keuffel, J.W.; Parallel-Plate Counters Rev. Sci. Inst. 20 (1949) 202 Contributed to the discovery of H.

6. Plan of the talk  Principle of operation of Single-gap RPC  Development and characterisation  Status of deployment in ICAL experiment  Principle of operation of Multi-gap RPC  Design of 6-gap MRPC and its optimization  Experimental setup and data acquisition  Characterization of the device  Concluding remarks and future plans B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 20146

7. Schematic of a basic SRPC B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 2014  Resistive materials like glass or bakelite for electrodes  Special paint mixture (developed locally) for semi-resistive coating  Plastic honey-comb laminations used as readout panel  Special plastic films for insulating the readout panels from high voltages  Two modes of operation: Avalanche (R134a:Isobutane:SF 6 ::95.5:4.2:0.3) and Streamer (R134a:Isobutane:Ar::56:7:37)  Resistive materials like glass or bakelite for electrodes  Special paint mixture (developed locally) for semi-resistive coating  Plastic honey-comb laminations used as readout panel  Special plastic films for insulating the readout panels from high voltages  Two modes of operation: Avalanche (R134a:Isobutane:SF 6 ::95.5:4.2:0.3) and Streamer (R134a:Isobutane:Ar::56:7:37) 7

8. Principle of SRPC operation  Electron-ion pairs produced in the ionisation process drift in the opposite directions.  All primary electron clusters drift towards the anode plate with velocity v and simultaneously originate avalanches.  A cluster is eliminated as soon as it reaches the anode plate.  The charge induced on the pickup strips is q = (-e Δ x e + e Δ x I )/g.  The induced current due to a single pair is i = dq/dt = e(v + V)/g ≈ ev/g, V « v.  Prompt charge in RPC is dominated by the electron drift. B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 20148

9. Two modes of SRPC operation B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 2014  Gain of the detector « 10 8  Charge developed ~1pC  Needs a preamplifier  Longer detector life  Typical gas mixture R134a:iB:SF 6 ::94.5:4:0.5  Moderate purity of gases is fine!  Higher counting rate capability  Gain of the detector  10 8  Charge developed ~100pC  No need for a preamplier  Relatively shorter detector life  Typical gas mixture R134a:iB:Ar::62.8:30  High purity of gases expected  Low counting rate capability Avalanche mode Streamer mode 9

10. Control of avalanche process  Role of RPC gases in avalanche control  R134a is the ionising gas (83 clusters/cm, compare with Argon’s 30 clusters/cm).  R134a also captures free electrons and localise avalanches. e - + X  X - + h (Electron attachment) e - + X  X - + h (Electron attachment)  Isobutane to stop photon induced streamers. X+ + e-  X + h (Recombination) X+ + e-  X + h (Recombination)  SF 6 for preventing streamer transitions.  Growth of the avalanche is governed by dN/dx = α N.  The space charge produced by the avalanche, shields (at about α x = 20) the applied field and avoids exponential divergence.  Townsend equation should be dN/dx = α (E)N. B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201410

11. The avalanche growth  An ionising particle crossing the gas gap g produces ng free electrons, n being the average number per unit length.  Number of drifting electrons at the time t after gas ionisation is N(t) = n(g-vt)e  t  Current induced on the pickup electrodes, i = eN(t)v/g = evn(1-vt/g)e  t  The integral q =  idt between 0 and t max (= g/v ) is the prompt charge, i.e. q = Ie  g /(  g) 2  I = eng is the electron charge delivered by the incoming particle.  Total charge delivered in the gas is Q = en  Ie  x dx = Ie  g /(  g) (integrated between 0 and g)  The ratio of prompt to total charge q/Q = 1/  g is « 1, as  g  20 (the limit of the avalanche to streamer transition).  This is due to the fact that most free electrons are produced very near to the anode. B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201411

12. Typical expected parameters  No. of clusters in a distance g follows Poisson distribution with an average of  Probability to have n clusters  Number of electrons reaching the anode  Intrinsic efficiency  So  max depends only on gas and gap  Intrinsic time resolution  So  t doesn’t depend on the threshold  Area of signal pickup spot (  counting rate capability) B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 2014  Gas: 96.7/3/0.3 (R134a/iB/SF 6 )  Electrode thickness: 2mm  Gas gap: 2mm  HV: 10.0KV (E = 50KV/cm)  Relative permittivity (  ): 10  Mean free path ( ): 0.104mm  Avg. no. of electrons/cluster: 2.8  Drift velocity (V D ) = 130mm/ns  Townsend coefficient (  ): 13.3/mm  Attachment coefficient (  ): 3.5/mm  Total charge (q tot ): 200pC  Induced charge (q ind ): 6pC  Charge threshold: 0.1pC  Efficiency (  max ): 90%  Time resolution(  t ): 950pS  Signal pickup spot (S) = 0.1mm 2 12

13. Deployment scenario of RPCs B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201413

14. INO Laboratory Complex B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 2014 Basic features 14

15. ICAL detector and construction B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 2014 Magnet coils RPC handling trolleys Total weight: 50Ktons Height of a 5-storey building Particles produced in the neutrino interactions pass through alternating layers of iron plates and RPCs, leaving tracks in the latter. Tracks bend as per the charge of the produced particles, due to the ICAL’s magnetic field. 15

16. Factsheet of ICAL detector B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 2014ParameterICALICAL-EM No. of modules31 Module dimensions16.2m × 16m × 14.5m8m × 8m x 2m Detector dimensions49m × 16m × 14.5m8m × 8m x 2m No. of layers15020 Iron plate thickness56mm Gap for RPC trays40mm Magnetic field1.3Tesla RPC dimensions1.95m × 1.91m × 30mm Readout strip pitch30mm No. of RPCs/Road/Layer84 No. of Roads/Layer/Module84 No. of RPC units/Layer19216 No. of RPC units28,800 (107,266m 2 )320 (1,192m 2 ) No. of readout strips3,686,40040,960 16

17. V-I characteristics SRPC characterisation B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 2014 Efficiency plateau Charge Timing 17

18. Prototyping of ICAL detector B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 2014 2m  2m RPC test stand in TIFR 1m  1m RPC stack in TIFR 18 1m  1m RPC stack in Madurai1m  1m RPC stack in VECC

19. Results from prototype stacks B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201419

20. Chamfering and engraving of glass B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201420

21. Painting/curing of glass plates B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201421

22. Automation of RPC gap making B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201422

23. R&D to … B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201423

24.  Scaling up and automation of procedures for large scale production of RPC detectors is completed.  Order for 400 RPC gas gaps required for the ICAL Engineering Module is about to be placed.  Local industry is a key partner in building the ICAL detector. Industrial production of SRPCs B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201424

25. Closed loop gas system B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 2014 About 200,000 litres (60 LPG cylinders) of gas will be circulating in the ICAL detector all the time. Regenerate, Recycle and Reuse 25

26. Multi-gap RPC (1996) B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201426 M.C.S. Williams

27. Main concepts of MRPC  The internal resistive plates are all electrically floating. They take the correct voltage initially due to electrostatics; however they are kept at the correct voltage due to the flow of electrons and positive ions created by the avalanches in the gaps. The stable state is equal gain in all gaps.  The resistive plates are transparent to the fast signals generated by the avalanches inside each gas gap. The induced signal on the external electrodes is the sum of the activities of all the gaps.  Thin sub-gaps, optimised gas parameters, controlled avalanche and differential signal readout are the key design parameters for their exceptional performance. B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201427

28. Performance of ALICE’s MRPCs B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201428

29. MRPCs for PET imaging B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201429  Detector with good efficiency, spatial, timing and energy resolutions.  Low system dead time (lower dose)  Rejection of scattered, random or multiple events.  Scintillation crystals (Bismuth Germanium Oxide (BGO), Gadolinium Oxyorthosilicate (GSO), Lutetium Oxyorthosilicate (LSO), etc.) current choices.  MRPC is a cheaper, works on direct detection, with higher FOV  Gamma sensitivity saturates at a thickness of 400µm for bakelite, 200µm common glass and 150µm for lead glass.  Standard electrodes are coated with with high Z material acting as  -e converter. Maria Necchi

30. Backup Slides

31. RPC simulations  Primary cluster positions: Poisson statistics  Avalanche growth: Exponential law  Avalanche gain fluctuations: Polya distribution  Induced charge: Ramo theorem B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201431

32. The choice of detector: ICAL  Use (magnetised) iron as target mass and RPCs as active detector elements.  Atmospheric neutrinos have large L and E range. So ICAL has large target mass: 50kton in its current design.  Nearly 4  coverage in solid angle (except near horizontal).  Upto 20 GeV muons contained in fiducial volume; most interesting region for observing matter effects in 2–3 sector is 5–15 GeV.  Good tracking and energy resolution.  ns time resolution for up/down discrimination; good directionality.  Good charge resolution; magnetic field ∼ 1.5 Tesla.  Ease of construction (modular; 3 modules of 17 kTons each).  Note: ICAL is sensitive to muons only, very little sensitivity to electrons; Electrons leave few traces (radiation length 1.8 (11) cm in iron (glass)). B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201432

33. RPC characteristics and merits  Large detector area coverage, thin (~10mm), small mass thickness  Flexible detector and readout geometry designs  Solution for tracking, calorimeter, muon detectors  Trigger, timing and special purpose design versions  Built from simple/common materials; low fabrication cost  Ease of construction and operation  Highly suitable for industrial production  Detector bias and signal pickup isolation  Simple signal pickup and front-end electronics; digital information acquisition  High single particle efficiency (  95%) and time resolution (~1nSec)  Particle tracking capability; 2-dimensional readout from the same chamber  Scalable rate capability (Low to very high); Cosmic ray to collider detectors  Good reliability, long term stability  Under laying Physics mostly understood! B.Satyanarayana & Moon Moon Devi ASET Colloquium September 12, 201433