1. RICH04 Mexico A. Breskin Ion-induced effects in GEM & GEM/MHSP- gaseous photomultipliers for the UV & visible spectral range http://www.weizmann.ac.il/home/detlab A. Breskin, D. Mörmann, A. Lyashenko and R. Chechik Department of Particle Physics, The Weizmann Institute of Science 76100 Rehovot, Israel F.Amaro, J.Maia, J.Veloso and J.dos Santos Physics Dept., University of Coimbra, 3004-516 Coimbra, Portugal

2. RICH04 Mexico A. Breskin Gaseous Photomultipliers (GPM) Gas 1 atm (F.Piuz et al) Problems with wire chambers: open geometry open geometry  - Photon and ion feedback  gain limitations - Damage to the photocathode CsI on readout pads photocathode I will discuss only our work! Possible solution: closed geometry Cascaded GEM & others 

3. RICH04 Mexico A. Breskin Multi-GEM GPM largely reduced photon feedback compared to “open” geometry no photon feedback thick pc: easier production low sensitivity to charged particles! Semitransparent Photocathode A. Buzulutskov et al. NIM A 443 (2000)164D. Mörmann et al. NIM A 478 (2002) 230 higher QE! high 2D precision [0.1-0.2 mm] high gain [>10 5 ]  single photon sensitivity! fast signals [ns]  good timing Reflective Photocathode

4. RICH04 Mexico A. Breskin Multiplication in multi-GEM structures For a given total gain: a larger number of GEMs permits operation @ lower V GEM  HIGHER STABILITY D. Mörmann et al. WIS

5. RICH04 Mexico A. Breskin Electron transmission into holes 0.7 1.0 1.4 1.9 2.6 3.7 5.2 7.3 10 14 20kV/cm  V GEM =500V  V GEM =300V  V GEM =100V Good extraction E drift E GEM high  V GEM high surface field low backscattering => optimal operation at high  V GEM E>2

6. RICH04 Mexico A. Breskin - Reflective PC ( compared to ST ), higher QE, low sensitivity to ionizing BG radiation - High optical opacity of multi-GEM, no photon-feedback - Reduced ion back-flow ( compared to MWPC ) - Reduced secondary effects  high gains 10 6 - 10 7 - Operation with large variety of gases, noble gases, CF 4, etc - Fast: with CF 4   = 1.6ns w\single electrons    = 0.33ns w\150 electrons GPMs: highlights e-e- 50mV 10ns gain 10 5, no photon feedback. Refl. CsI, 1 atm CF 4 3 GEM, single electron pulses. With reflective PC: low sensitivity to ionizing background radiation

7. RICH04 Mexico A. Breskin Examples of GEM-GPM applications Hadron-Blind Detector (HBD) for PHENIX (I. Tserruya et al. Weizmann) UV imaging detectors of LXe scintillators for Dark-Matter experiments (XENON, E. Aprile et al. Columbia Univ.) UV imaging detectors for a fast LXe Gamma-camera for PET ) D. Thers - Nantes/A.B.-Weizmann) GPM LXe LIQUID Xe Xe GAS

8. RICH04 Mexico A. Breskin Visible-range Gaseous Photomultipliers Real challenge: GPMTs for the visible range! Photocathodes(e.g. bi-alkali) are very chemically reactive. Cannot operate in flow-mode! Solution: Visible-range GPMTs => sealed mode D. Mörmann et al. NIM A504 (2003) 93 UV visible UV: established technique various ~“air-stable” photocathodes

9. RICH04 Mexico A. Breskin GPMT for visible light sealed 3 Kapton-GEMs & KCsSb PC Sealing in gas: In/Sn; 130-150 0 C D. Mörmann et al. NIM A504 (2003) 93 M.Balcerzyk et al. IEEE TNS 50 (2003) 847 QE in Ar/CH4 (95/5) ~ 70% of QE in vacuum (backscattering)  best expected ~20% @360-400 nm QE in transmissive mode Ar/CH4 95/5% Wavelength [nm] 300400500600 15 10 5 Q E % 13% = best QE measured after sealing. 2 weeks stability under development: Silicon, ceramic Expected higher stability Sealed detector package with semitransparent K-Cs-Sb PC Best sealed GPMT: QE = 6% @ 365nm stable for 1 month ~2”

10. RICH04 Mexico A. Breskin Ion feedback: photocathode-dependent (band gap, electron affinity) gas-dependent (ion species, PC surface processes) field-dependent (ion velocity) No significant feedback observed with CsI Significant with efficient secondary electron emitters, e.g. visible photocathodes Gain limitation by ion-feedback K-Cs-Sb: Current deviates from exponential 100mV/div 400  s/div Recently measured: SEE Probability = 0.05 – 0.5 electron/ion in Ar/CH 4 mixtures (Gas dependent, Ar is worst) 1 atm Ar, 1 GEM ST PC

11. RICH04 Mexico A. Breskin Drawbacks of ion-photocathode interaction Secondary avalanches due to ion feedback  gain limits, imaging problems (observed in K-Cs-Sb) Photocathode damage due to ion sputtering observed in both: CsI and K-Cs-Sb Major efforts to limit ion backflow

12. RICH04 Mexico A. Breskin Ion back-flow in multi-GEM Tracking detectors & TPCs Electron’s pathIon’s back-flow Back-flowing ions Distort the E-field E d can be kept relatively LOW  reduces ion back flow to a few % levels E d cannot be too low  keep e-diffusion low  localization resolution EdEd S.Bachman et al. NIMA438(99)376: 5% @ 0.5kV/cm A.Breskin et al. NIM A478(2002)225 2-5%@ 0.5kV/cm A.Bondar et al. NIM A496(2003)325 3%@ 0.5; 0.5% @ 0.1 kV/cm (GEMs with small holes)

13. RICH04 Mexico A. Breskin Electron’s path Ion’s back-flow Ion back-flow in multi-GEM Attempts to reduce the ion back-flow: variables  V GEM ; E trans ; E ind  E @ photocathode must be high for good e-extraction; best @ high V GEM  Ion back-flow 10-20% at best…! - (without affecting e - transfer) Detectors with solid converters D. Mörmann et al. NIM A516 (2004) 315

14. RICH04 Mexico A. Breskin The Microhole & Strip Plate (MHSP)  Foil:5  m copper on both sides of 50  m Kapton  Bi-conical holes: 50/70  m (inner/outer) diameter  Anode-strip pattern:175  m pitch /15  m strips  Production: similar to GEM technology (CERN) Two multiplication stages on a single, double-sided, foil J.M.Maia et al. IEEE NS49 (2002) J.M.Maia et al. NIM A504(2003)364 R&D in course: Weizmann/Coimbra

15. RICH04 Mexico A. Breskin Ion back-flow: MHSP vs GEM All ions flow backSome ions flow back but others flow towards the strip cathodes and bottom cathode Multi-GEMGEM & MHSP anodebottom cathode A C GEM & MHSP: ion flow reduced to 2-3% levels! J.Maia et al. NIM A523(2004)334

16. RICH04 Mexico A. Breskin MHSP simulation Bouianov Simulations:Oleg Bouianov photocathode cathode mesh hv V C-T V A-C E trans E drift C A

17. RICH04 Mexico A. Breskin The multi-GEM & MHSP photomultiplier J. Maia et al. NIM A523(2004)334 High gain and low ion back flow: 2-3% 0.01 0.1 1 1E+021E+031E+041E+051E+06 Effective Gain Ion back-flow ratio  E T1 =1.0 kV/cm E T2 =E T3 =0.25 kV/cm E ind =- 5.0 kV/cm Ar/5%CH 4 p=760 Torr V GEM1 =350 V V GEM2 =V GEM3 250 V 300 V 280 V 350 V V GEM3 315 V V hole 2-3%

18. RICH04 Mexico A. Breskin Ion back-flow reduction: reversed-MHSP & GEM J.Veloso et al. WIS/Coimbra IEEE 2004 IBF R&D IN PROGRESS! R-MHSP MHSP   MHSP: gain & ion blocking R-MHSP: ion defocusing * WIS/Coimbra IBF: Ion Backflow Reduction  R-MHSP Gain=30 ~1200 ions/e ~300 ions/e 30 x gain 4 x ions!  R-MHSP: Roth, Vienna 04  C  C C 

19. RICH04 Mexico A. Breskin Gain of 1 st element: 20-30 Other ion-suppression ideas

20. RICH04 Mexico A. Breskin Ion backflow (reflective photocathode) Gain of R-MHSP1 ~ 30

21. RICH04 Mexico A. Breskin 1. Gate open => electron transfer 2. Gate closed, after electron transfer, => ions are stopped Ion Gating E1E1 E2E2 Feedback pulses Non-gated GEM: at best 10% ion-feedback Gated GEM: ion suppression to 10 -4 levels! 10 -4 Problem: dead time! (  s) D. Mörmann et al. NIM A516 (2004) 315

22. RICH04 Mexico A. Breskin Gated GPMT for visible light D. Mörmann et al. WIS 2004 >10 5 GAIN: 100-1000 in DC mode (ion feedback limit) >10 5 in ion-gating mode A breakthrough!

23. RICH04 Mexico A. Breskin Ion-suppression: summary At gain ~ 10 5 Multi-GEM: IBF = 10 -1 – 2 10 -1 Multi-GEM & MHSP: IBF = 2 10 -2 Multi-GEM & MHSP & R-MHSP: IBF = 1-3 10 -3 Gated multi-GEMs: IBF = 10 -4 Photocathode life-time, T PC, depends on the total ion’s accumulated charge on the photocathode:  T PC (GEM-like GPM) = T PC (MWPC GPM) x 1/IBF  Operate at minimal possible gain!

24. RICH04 Mexico A. Breskin K-Sb-Cs photocathode ageing CsI CsBr KCsSb 4-GEM / semitransparent photocathode a small fraction of ions hit the pc  slow aging parallel-grids / semitrans. photocathode all ions hit the pc  faster aging! Aging of K-Cs-Sb under avalanche-ion bombardment in Ar-CH 4 (5%)

25. RICH04 Mexico A. Breskin Summary GEM photomultipliers (GPM): - a mature concept in the UV - important progress in the visible Other advanced “hole-multipliers”: MHSP, TGEM (talk by Rachel Chechik) Ion blocking: cascaded GEM/MHSP/RMHSP – 10 -3 importat also for TPCs!

26. RICH04 Mexico A. Breskin FIN

27. RICH04 Mexico A. Breskin GPM LXe SiO 2 entrance window (3mm) PTFE Wall Thermal Screen Metallic Micromesh Anode plane Cathode wire plane HV 20kV Liquid Xenon Xenon gas 511 keV Gamma ray HV [1-2] kV 9 cm LXe-gaseous PMT gamma-camera for PET SUBATECH-Nantes/WEIZMANN

28. RICH04 Mexico A. Breskin Replace Suggested GEM in the XENON Detector The XENON Dark Matter search: E. Aprile et al. Columbia Univ. astro-ph/0207670 1 ton liquid Xe detector with multi-GEM GAS PHOTOMULTIPLIER Xe GAS LIQUID  Xe Primary scintillation: Photo-effect in LIQUID & GAS Secondary scintillation: Induced by electrons extracted from LIQUID & drifting in GAS WIMPS

29. RICH04 Mexico A. Breskin A single 100 MeV electron identified by a “Cerenkov signal” in the HBD Suggested PHENIX HBD SimulationReal life…. A single event recorded in the STAR TPC showing hundreds of particles: most of them HADRONS e-e- GEM-photodetector  insensitive to particles! HBD

30. RICH04 Mexico A. Breskin TPC/HBD for RHIC-PHENIX Drift regions HV plane (~ -30kV) Grid TPC Readout Plane Readout Pads D R ~ 1 cm f ~ 2 mm Large area UV detector 3-GEM/CsI I. Tserruya et al, WIS GOAL: identification of a few low-mass e-pairs out of hundreds of Hadrons / collision

31. RICH04 Mexico A. Breskin Ion feedback to the ST photocathode Ion feedback as a function of the drift field Dependence on the gain: 3GEM vs 4GEM   TPC Breskin et al. NIM A478(2002)225 GPM “standard” GEMs Breskin et al. NIM A478(2002)225 Bondar et al NIMA  Factor 2-3 IBF reduction

32. RICH04 Mexico A. Breskin Ion back-flow in multi-GEM with reflective pc Variable:  Induction field 10% at best Variable:  GEM voltage  Variable:  transfer field  D. Mörmann et al. NIM A516 (2004) 315