GEM-based gaseous Photomultipliers for UV and visible photon imaging. Dirk Mörmann Amos Breskin Rachel Chechik Marcin Balcerzyk Bhartendu Singh

Transcription

1 GEM-based gaseous Photomultipliers for UV and visible photon imaging Dirk Mörmann Amos Breskin Rachel Chechik Marcin Balcerzyk Bhartendu Singh

2 Gaseous Photomultiplier State of the art: Advantages: large areas, flat geometry operation in magnetic fields sensitivity to single photons spectral range from UV to visible fast (ns range) high localization accuracy (sub-mm range) Applications: RICH *, Calorimetry, Astrophysics, Medical applications,... * F. Piuz et al. NIM A 433 (1999) 178

3 Photoelectron backscattering Reason: elastic collisions with gas molecules Depends on ration σ(elastic)/σ(ineleastic); field dependent Choice of gas mixture with low σ(elastic) Solution: increase field on photocathode surface

4 Photon feedback Prevents operation in some gases (e.g. noble gases) Prevents operation with some pc s (sensitive to the emission w.l. of the gas) Limits maximum gain imaging distortions 2V/div 50ns/div t corresponds to electron drift time

5 Ion feedback limits operation with some pc s imaging distortions Limits maximum gain may damage photocathode 100mV/div 400µs/div t corresponds to ion drift time

6 Most gas PMTs are wire chambers coupled to a photocathode operating in an open geometry High feedback (ions & photons) gain & localization limits Solution: CLOSED GEOMETRY

7 Gas Electron Multiplier (GEM) F. Sauli NIM A 433 (1997) e - in Photo of a GEM Electron Microscope view of a GEM Electric field in the holes Typical parameters: 50µm Kapton metal coated Ø60-100µm holes µm pitch 80% opacity 10 3 e - out

8 Multi-Gem photomultiplier drastically reduces ion feedback photon feedback This leads to higher gain operation in pure noble gases longer lifetime of pc A. Buzulutskov et al. NIM A 443 (2000)164

9 Some results with the GEM-PM Gain with 3 GEMs time resolution e - σ=2.1ns Ar/N 2 (98/2) 10 1, CsI High gain Limit for Wire chambers 10e - σ=0.92ns 150e - σ=0.33ns Excellent time resolution

10 GEM-PMT with reflective photocathode Reflective photocathode: Higher QE Easier to produce Detector: Photon feedback eliminated Higher gain D. Mörmann et al. NIM A 478 (2002) 230

11 High Quantum Efficiency QE in vacuum: 2500Å CsI on Au-coated GEM 23 % is not sensitive due to holes But: Still higher QE than semi-transp.

12 Photon feedback suppression a) GEM + CsI a) CH 4 1atm Single e - spectra Gain 10 6 b) b) Excess of high pulses due to photon feedback No photon feedback visible

13 Efficient electron transfer Optimization of parameters: Fields Gas mixture geometry D. Mörmann et al. NIM A 478 (2002) 230

14 Some results with reflective PC High gain Excellent time resolution

15 The multi-gem-pmt is a mature technique for gas-flow operation with UV-photocathodes Real challenge: gaseous PMTs for the visible range! Photocathodes (e.g. bi-alkali) are very chemically reactive. Solutions: 1. Coating => low QE 2. bare photocathodes => sealed mode

16 Preparation of a sealed detector for the visible range -tin 28x28mm Au-coated GEM Assembled detector ready for sealing Empty package

17 Multi-chamber UHV setup sealing to package under gas p.c. deposition evaluation QE measurement in vacuum Load-lock substrate baking Bi-alkali pc production (QE nm in vacuum for Semi-transparent pc) Hot Indium sealing to => critical for pc

18 Sealed detector in package Sealing of 3 Kapton-GEMs + K-Cs-Sb photocathode Accepted leak rate: <10-12 mbar liter /sec

19 QE stability in Ar Good 150 C Stable operation of bi-alkali photodiode for more than half a year in gas BUT: non-optimized sealing temp. for high QE High backscattering in Ar (ε~40%) => Low QE

20 Sealed GEM-PMT detector in Argon QE 4% 3% 2% 1% PC died after ~35days PC death probably due to a micro-leak Slow photocathode decay: Day 24 Day 35 0% time, days nm 435nm 405nm 365nm 312nm

21 Sealed detector operated in Argon Ion feedback: Dependence on photocathode: 100mV/div 400µs/div 300V Gain ~30 K-Cs-Sb: Current deviates from exponential => feedback!

22 Ion feedback: dependence on gas Ion feedback depends on gas mixture! Not due solely to kinetic energy Not due solely due ion species Further investigations are on the way

23 Solution: Ion gating Open gate Voltage pulse on parallel wires Block ions before they reach the pc Established technology in TPCs Ion drift lines May reduce rate capability Closed gate

24 Summary: Photo feedback suppression High QE with reflective PC directly on GEM Successful sealing of a photodiode in Argon Promising sealing of a 3-GEM PMT Problematic: Ion feedback Current steps: Ion gating measurements Improvement of sealing technology Investigations of mixtures to reduce ion feedback clean GEMs from glass, ceramic etc. Single-photon imaging