1. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs Efficient ion blocking in gaseous detectors and its application to visible-sensitive gaseous photomultipliers A. Breskin, A. Lyashenko and R. Chechik Weizmann Institute of Science, Rehovot, Israel & J.M.F. dos Santos, F.D. Amaro and J.F.C.A. Veloso University of Coimbra, Portugal

2. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs Secondary effects in gaseous detectors Gaseous Photo-Multiplier (GPM) Time Projection Chamber (TPC) Ions   secondary e emission  ion feedback pulses  gain & performance limitations Ions   dynamic track distortions

3. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs IBF: Ion Back-Flow Fraction IBF: The fraction of avalanche-generated ions back- flowing to the drift region or to the photocathode  Major efforts to limit ion backflow 1. GATING  operation in “gated-mode”  deadtime, trigger - - 2. NEW e - - MULTIPLIERS  operation in DC mode (cascaded-GEM*, MICROMEGAS…&: OTHERS)  Challenge: BLOCK IONS WITHOUT AFFECTING ELECTRON COLLECTION *GEM: Gas Electron Multiplier - Sauli, NIM A 386, (1997) 531.

4. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs Visible-sensitive gaseous photomultipliers: Ion-feedback development Visibile-sensitive gas photomultiplier review: M. Balcerzyk et al., IEEE Trans. Nucl. Sci. Vol. 50 no. 4 (2003) 847 - stable operation of visible sensitive GPMif Ar/CH 4 (95/5), γ eff + ~0.03, Gain ~ 10 5 => IBF < 3.3*10 -4  eff : ion feedback coeff.

5. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs IBF in cascaded GEM GPMs (high E drift ) High E drift (>0.5 kV/cm) needed to efficiently extract photoelectrons Bachman et al. NIMA438(1999)376 5% @ 0.5kV/cm, Gain ~10 5 Breskin et al. NIM A478(2002)225 2-5%@ 0.5kV/cm, Gain ~10 5 Bondar et al. NIM A496(2003)325 3% @ 0.5kV/cm, Gain ~ 10 5 Need another factor of 100!!!

6. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs The Microhole & Strip plate (MHSP) ~80% of avalanche ions are trapped by cathode strips and plane Two multiplication stages on a single, double-sided, foil R&D: Weizmann/Coimbra photocathode cathode mesh hv V C-T V A-C E trans E drift CA Veloso et al. Rev. Sci. Inst. A 71 (2000) 237.

7. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs IBF: 3% @ Gain > 10 5 IBF: 20% @ Gain > 10 5 The benefit of MHSP in a cascade Maia et al. IEEE NS49 (2002) Maia et al. NIM A504(2003)364 Mörmann et al. NIM A516 (2004) 315 3GEMs+MHSP 4GEMs 7 times lower than with cascaded GEMs

8. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs Reverse-biased MHSP (R-MHSP) concept Flipped-R-MHSPR-MHSP Can trap its own ions Ions are trapped by negatively biased cathode strips Lyashenko et al., JINST (2006) 1 P10004 Lyashenko et al., JINST (2007) 2 P08004 Roth, NIM A535 (2004) 330 Breskin et al. NIM A553 (2005) 46 Veloso et al. NIM A548 (2005) 375 Can trap only ions from successive stages

9. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs 1st R-MHSP or F-R-MHSP: ion defocusing (no gain!) Mid GEMs: gain Last MHSP: extra gain & ion blocking BETTER ION BLOCKING: “COMPOSITE” CASCADED MULTIPLIERS: R-MHSP/GEM/MHSPF-R-MHSP/GEM/MHSP

10. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs TPC conditions (low drift field) Gas PMT conditions (high drift field) IBF=1.5*10 -4 @ Gain=10 4 IBF=3*10 -4 @ Gain=10 5 Lyashenko et al., JINST (2007) 2 P08004 IBF in “composite” micro-hole multipliers IBF measured with 100% e-collection efficiency IBF is 100 times lower than with 3GEMs

11. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs R&D in course @ WEIZMANN/COIMBRA ) NEW! “COBRA”: GEM-LIKE PATTERNED ION-SUPPRESSING ELECTRODES (R. d’Oliveira, CERN) New ideas for ion blocking 

12. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs IBF suppression with “Cobra” IBF=2.7*10 -5 Gain=10 4 IBF=3*10 -6 Gain=10 5 IBF 1000 times lower than with GEMs, best results ever achieved Though, presently at the expense of electron collection (~20%)

13. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs IBF reduction summary TPC (E drift =0.1-0.2kV/cm, Gain=10 4 ) GPM (E drift =0.5kV/cm, Gain=10 5 ) Detector type IBFCollection efficiency IBFCollection efficiency 3GEM0.5%100%5% (20%) * 100% 4GEM100%2% (0.01%) ** 100% R-MHSP/ GEM/MHSP 0.08%100%0.1%100% F-R-MHSP/ GEM/MHSP 0.015%100%0.03%100% “Cobra”/ 2GEM 0.0027%20%0.0003%20% * Reflective PC **Gated mode

14. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs Sealed detector Test detector setup Base plate made in Novosibirsk Visible-sensitive GPM UHV compatible materials Bi-alkali PC

15. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs Visible-sensitive GPM: Gain Divergence G G meas K-Cs-Sb, Na-K-Sb, Cs-Sb : Current deviates from exponential Max Gain ~ few 100, IBF~10% D. Mörmann et al.,NIM A 504 (2003) 93

16. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs A.Breskin et al. NIM A553 (2005) 46-52 Gated operation of visible-sensitive GPM GATED MULTI-GEM Gain~10 6 GAIN: ~100 in DC mode (ion feedback limit),IBF~10% ~10 6 in ion-gating mode; IBF~10 -4 Ion gating electrode

17. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs DC operation of visible-sensitive GPM Gain >10 5 in DC mode  single photon sensitivity But: e - collection efficiency ~ 20% K-Cs-Sb Gain~10 5 K-Cs-Sb CsI DC Gain limit~100 in cascaded GEMs Flipped Cobra + 2GEMs

18. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs DC operation of F-R-MHSP/GEM/MHSP with K-Cs-Sb photocathode with K-Cs-Sb photocathode Gain ~10 5 at full photoelectron collection efficiency First evidence of DC high gain operation of visible-sensitive GPM

19. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs Summary Cascaded Patterned Hole Multipliers (PHM)  significant improvement in ion blocking in gaseous detectors of importance in: Tracking detectors (TPC) & Gaseous Photomultipliers with MHSP/GEM-based CASCADED MULTIPLIERS 100 times lower IBF than with cascaded GEMs with full efficiency for collecting primary electrons! Gain >10 5 reached with visible-sensitive K-Cs-Sb PC with Cobra/GEM-based CASCADED MULTIPLIERS 1000 times lower IBF than with cascaded GEMs with so-far low e - collection efficiency –Gain >10 5 reached with visible-sensitive K-Cs-Sb PC First evidence of high-gain DC operation of visible-sensitive GPM Further work: Optimization of COBRA Double-sided patterned hole-multipliers

20. A. Breskin RD51 Amsterdam 4/08 ION BLOCKING & visible-sensitive gas-PMs K-Cs-Sb stability in gas