Efficient ion blocking in gaseous detectors and its application to visible-sensitive

1. Efficient ion blocking in gaseous detectors and its application to visible-sensitive gas-avalanche photomultipliers A. Lyashenko, A. Breskin and R. Chechik Weizmann Institute of Science, Rehovot, Israel And J.M.F. dos Santos, F.D. Amaro and J.F.C.A. Veloso University of Coimbra, Portugal

2. Secondary effects in gaseous detectors Time Projection Chamber (TPC) Gaseous Photo-Multiplier (GPM) Ions   secondary e emission ion feedback pulses gain & performance limitations Ions  dynamic track distortions

3. 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. Visible-sensitive GPM: Ion-feedback development if - stable operation of visible sensitive GPM Ar/CH4 (95/5),γeff+~0.03, Gain ~ 105 => IBF < 3.3*10-4 Visibile-sensitive gas photomultiplier review: M. Balcerzyk et al., IEEE Trans. Nucl. Sci. Vol. 50 no. 4 (2003) 847

5. IBF in cascaded GEM GPMs (high Edrift) High Edrift (>0.5 kV/cm)needed to efficiently extract photoelectrons Bachman et al. NIMA438(1999)376 5% @ 0.5kV/cm, Gain ~105 Breskin et al. NIM A478(2002)2252-5%@ 0.5kV/cm, Gain ~105 Bondar et al. NIM A496(2003)3253% @ 0.5kV/cm, Gain ~ 105 Need another factor of 100!!!

6. The Microhole & Strip plate (MHSP). hv photocathode E drift VA-C VC-T A C E trans cathode mesh Two multiplication stages on a single, double-sided, foil R&D: Weizmann/Coimbra ~80% of avalanche ions are trapped by cathode strips and plane Veloso et al. Rev. Sci. Inst. A 71 (2000) 237.

7. The benefit of MHSP in a cascade. 3GEMs+MHSP 4GEMs IBF: 20% @ Gain > 105 IBF: 3% @ Gain > 105 7 times lower than with cascaded GEMs Maia et al. IEEE NS49 (2002) Maia et al. NIM A504(2003)364 Mörmann et al. NIM A516 (2004) 315

8. Reverse-biased MHSP (R-MHSP) concept Ions are trapped by negatively biased cathode strips R-MHSP Flipped-R-MHSP Can trap only ions from successive stages Can trap its own ions Roth, NIM A535 (2004) 330 Breskin et al. NIM A553 (2005) 46 Veloso et al. NIM A548 (2005) 375 Lyashenko et al., JINST (2006) 1 P10004 Lyashenko et al., JINST (2007) 2 P08004

9. BETTER ION BLOCKING: “COMPOSITE” CASCADED MULTIPLIERS: 1st R-MHSP or F-R-MHSP: ion defocusing (no gain!) Mid GEMs:gain Last MHSP: extra gain & ion blocking R-MHSP/GEM/MHSP F-R-MHSP/GEM/MHSP

10. IBF in “composite” micro-hole multipliers IBF measured with 100% e-collection efficiency Gas PMT conditions (high drift field) TPC conditions (low drift field) IBF=1.5*10-4 @ Gain=104 IBF=3*10-4 @Gain=105 IBF is100 times lower than with 3GEMs Lyashenko et al., JINST (2007)2 P08004

11. New ideas for ion blocking Example (R&D in course @WEIZMANN/COIMBRA )  NEW!“COBRA”: GEM-LIKE PATTERNED ION-SUPPRESSING ELECTRODES (R. d’Oliveira, CERN)

12. IBF suppression with “Cobra” IBF=2.7*10-5 Gain=104 IBF=3*10-6 Gain=105 IBF 1000 times lower than with GEMs, best results ever achieved Though, presently at the expense of electron collection (~20%)

13. IBF reduction summary * Reflective PC **Gated mode

14. Visible-sensitive GPM Test detector setup UHV compatible materials Sealed detector Bi-alkali PC Base plate made in Novosibirsk

15. Gmeas G Visible-sensitive GPM: Gain Divergence 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. Gated operation of visible-sensitive GPM Ion gating electrode Gain~106 GATED MULTI-GEM GAIN: ~100 in DC mode (ion feedback limit),IBF~10% ~106 in ion-gatingmode; IBF~10-4 A.Breskin et al. NIM A553 (2005) 46-52

17. Flipped Cobra + 2GEMs K-Cs-Sb K-Cs-Sb Gain~105 CsI DC Gain limit~100 in cascaded GEMs DC operation of visible-sensitive GPM Gain >105 in DC mode  single photon sensitivity First evidence of DC high gain operation of visible-sensitive GPM

18. DC operation of visible-sensitive GPM 2GEMs + Cobra + GEM K-Cs-Sb Gain~104 CsI Gain ~104 at full collection efficiency for photoelectrons IBF=7*10-3@Gain=104 was not optimized

19. Summary Cascaded Patterned Hole Multipliers (PHM)  significant improvement in ion blocking in gaseous detectors with MHSP/GEM-based CASCADED MULTIPLIERS • 100 times lower IBF than with cascaded GEMs with full efficiency for collecting primary electrons! • Not yet investigated with visible-sensitive photocathodes with Cobra/GEM-based CASCADED MULTIPLIERS • 1000 times lower IBF than with cascaded GEMs • with so-far reduced efficiency for collecting primary electrons • Gain >105 reached with visible-sensitive K-Cs-Sb PC • with full efficiency for collecting primary electrons • Gain ~104reached with visible-sensitive K-Cs-Sb PC First evidence of high-gain DC operation of visible-sensitive GPM Further work: • Operation of MHSP/GEM -based cascaded multiplier with visible PC

20. Additional slides

21. Auger neutralization process • Eiis the potential energy of the ion • Epethe photoemission threshold, • E1 and E2 are the potential energy of the photocathode electrons that participate in theprocess, and • Ekinthe kinetic energy of the emitted secondary electron. Condition for the secondary electron emission: Ei>2Epe Epe (K2SbCs)=2eV, while Ei=(CH4)=12.6 eV

22. Charge exchange in 700 Torr Ar/CH4(95/5) 0.5cm It takes 100 – 1000 collisions for Ar + + CH4 Ar + CH4+ Mean free path ~10-5cm at normal conditions Only CH4+ remain after 10-3/p – 10-2/p cm (p=0.05 => 0.02 – 0.2 cm) of drift

23. K-Cs-Sb stability in gas