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Highest QE‘s Measured So Far

Learn about the highest quantum efficiency (QE) measured in photo cathodes and the differences between binomial and Poisson distributions. Explore the signal-to-noise ratio and the various ways in which visible light can be converted into measurable forms. Take a short historical excursion to understand the development of quantum efficiency in photo cathodes.

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Highest QE‘s Measured So Far

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  1. Highest QE‘s Measured So Far RazmickMirzoyan, Max-Planck-Institute forPhysics Munich, Germany

  2. Quantum Efficiency • Quantum efficiency (QE) of a sensor is defined as the ratio QE = N(ph.e.) : N(photons) • Conversion of a photon into ph.e. is a purely binomial process (and not poisson !) • Assume N photons are impinging onto a photocathode and every photon has the same probability P to kick out a ph.e.. Then the mean number of ph.e.s is N x P and the Variance is equal to N x P x (1 – P) Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  3. Differences between binomial and poisson distributions Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  4. Signal to noise ratio The signal-to noise ratio of the photocathode can be calculated as SNR = [N x P/(1 - P)]1/2 For example, if N = 1 (single impinging photon): Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  5. Signal to noise ratio SNR = [N x P/(1 - P)]1/2 For N = 20 impingingphotons: Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  6. Light conversion into a measurable • Visible light can react and become measurable by: • Eye (human: QE ~ 3 % & animal), plants, paints,... • Photoemulsion (QE ~ 0.1 – 1 %) (photo-chemical) • Photodiodes (photoelectrical, evacuated) • Classical & hybrid photomultipliers (QE ~ 25 %) QE ~ 45 % (HPD with GaAsP photocathode) • Photodiodes(QE ~ 70 – 80 %)(photoelectrical) • PIN diodes, Avalanche diodes, SiPM,... • photodiode arrays like CCD, CMOS cameras,... Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  7. Short Historical Excursion • 1889: Elster and Geitel discovered that in alkali metals a photo-electric effect can be induced by visible light (the existence of the e- was yet unknown) • 1905: Einstein put forward the concept that photoemission is the conversion of a photon into a free e- • Until ~1930 QE of available materials was < 10-4 • 1929: discovered Ag-O-Cs photo-emitter (Koller; Campbell) improved the QE to the level of ~ 10-2 • 1st important application: reproduce sound for film Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  8. Short Historical Excursion • Improved materials were discovered later on but it was a combination of a good luck with „intelligent guessing“ • A very important step was to realize that the photocathode materials are SEMICONDUCTORS • Metallic versus Nonmetallic materials: • yield of metallic photocathodes is very low because of very high reflectivity • semiconductors have less reflection losses • The main loss process in metals is the e- scattering; => e- escape depth of only few atomic layers is possible Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  9. Short Historical Excursion • The losses in Semiconductors because of phonon scattering (interaction with lattice) are much less, i.e. e- from deeper layers can reach the surface Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  10. Short Historical Excursion Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  11. Short Historical Excursion • 1910: Photoelectric effect on K-Sb compound was found (Pohl & Pringsheim). • 1923: found that thermionic emission of W is greatly enhanced when exposed to Cs vapour (Kingdon & Langmuir). • It was found that the work function in the above case was lower than of Cs metal in bulk. • 1936: discovered high efficiency of Cs-Sb (Görlich). Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  12. QE of Metals • For photon energies > 12 eV QE of 1-10 % were reported for Ni, Cu, Pt, Au, W, Mo, Ag and Pd (1953, Wainfan). • 7% for Au @ 15 eV • 2% for Al @ 17 eV Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  13. Escape Depth • Escape depth can be defined as the thickness above which the photoemission becomes independent on thickness (in reflective mode) • The measured escape depth was 10-20 atomic layers for K, Rb, Cs (1932). Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  14. QE: Short Historical Excursion • 1955-1958 Sommers found the „multialkali“ effect: combination of Cs-K-Na-Sb has high QE in the visible spectrum. • Also were discovered • Cs3Sb on MnO (S11, lpeak 400nm, QE ~ 20%) • (Cs)Na2KSb (S20, lpeak 400nm, QE ~ 30%) • K2CsSb (lpeak 400nm, QE ~ 30%) • K2CsSb(O) (lpeak 400nm, QE ~ 35%) Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  15. Typical Quantum Efficiencies While selecting ¾‘ EMI 9083A PMTs for the HEGRA imaging Cherenkov telescopes in ~1996 I found 3 out of ~200 PMTs that showed very high „corning blue“ value Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  16. Boost of the QE of Bialkali PMTs • In recentseevralyearswewereintensivelyworkingwiththe well-known PMT manufacturerslookingintothepossibleboostofthe QE ofbialkali PMTs. Over past 40 yearsthere was noprogresreported. • After severaliterationssuccesscouldbereported. • PMTs withpeak QE values in therangeof 32-35 % becameavailable. • These QE boosted PMTs areused in theimagingcameraofthe MAGIC telescopes Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  17. How it shall be possible to boost the QE and who is interested in it ? • Useofhighlypurifiedmaterialsforphotocathode (provideslowerscatteringfor e- (lowrecombinationprobability) • → e- kicked out fromdeeper (top) layerscanreachphotocathode-vacuumjunction¬_and „jump“ intoit (→thickercathodeispossible). • Optimal tuningofthephotocathodethickness • ……ofthestructureand material composition • ……ofthe anti-reflectivelayer • ……? Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  18. QE Measuring Device @ MPI Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  19. Test Setup for Parametrising 7-PMT Clusters Setup allows in one go (20‘) measuring 1) linearity, 2) afterpulsing, 3) single ph.e. spectra, 4) F-factor, 5) peak-to-valley ratio, 6) gain, 7) HV distribution & 8) doing flat-fielding Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  20. QE of „champion“ 2´´ PMT from Hamamatsu Mirzoyan, et al., (2006) (proc. Beaune‘05) Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  21. QE of another 2´´ PMT from Hamamatsu Hamamatsu made a statementat Beaune-05 conferencethatbyusinga newprocesstheyareabletoproduce PMTs withpeak QE of 33.7 % on average (Yoshizawa, 2005). Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  22. QE of a champion 2´´ PMT from Photonis According to Photonis the peak QE should have been 38 %; we meaured 34 % Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  23. QE of a 3´´ PMT from Electron Tubes Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  24. Electron Tubes Optimising the QE of PMTs • Different batches show different behaviour • QE is high (~30% !!) • Peak @ ~ 350 nm • Low QE at long l(> 450 nm) Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  25. PMTs for MAGIC-I QE by coating with a diffuse scattering layer milky layer effect WLS (D. Paneque, et al., 2002) Effective QE  ~ 15 % Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  26. QE of 3 PMTs (2 Hamamatsu + 1 ET) before and after coating with milky layer Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  27. MAGIC-I imaging camera PMTs (blue) compared to MAGIC-II PMTs (red) Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  28. Peak QE of MAGIC-II PMTs @ 350 nm <QE> = 32 % Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  29. M-I Upgrade camera PMTs:peak QE @350nm and QE(l) <QE> = 33.9 % Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  30. QE of several tested PMTs Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  31. 1.5‘ CTA candidate PMT Hamamatsu R11920-100 Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  32. Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  33. QE of CTA candidate PMT from Hamamatsu Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  34. Collection Efficiency of the 1.5‘ CTA candidate PMT Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  35. We (CTA) expect that a similar product will be soon (still during this year) provided by the ET Enterprises Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  36. Maximum Peak QE of 3‘ Photonis PMT Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  37. Minimum Peak QE Photonis 3‘ Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  38. Peak QE Map Scanned with a ~3mm Beam Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  39. PMTs with reflective mode photo cathode • Such PMTs can provide higher QE than those with transmission mode cathodes • Although in literature one speaks about a peak QE enhancement of • up to x2, in practice • one finds values of ~40% Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  40. Photo cathode light reflection and absorption Motta, Schönert (2005) blue: ETL 9102B; green: ETL 9902B Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  41. Light-induced afterpulsing Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  42. 2-types of afterpulsing Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  43. Light-emission microscopy @ MPI Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  44. PMT light emission Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  45. The higher QE @ corners are due to the mirror reflection Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  46. Other strongly competing ultra-fast,LLL sensors with single ph.e. resolution • Twotypesof ultra-fast response LLL sensors, providinggoodsingleph.e. resolution, starttostronglycompetewiththeclassical PMTs. • These are • HPDs withGaAsPphotocathode • SiPM (anditsvariations) Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  47. 18-mm GaAsP HPD (R9792U-40) (development started in our group ~17 years ago) Designed for MAGIC-II telescope camera; (developed with Hamamatsu Photonics ) (expensive) Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  48. SiPM: making its own way MEPhI-MPI-EXCELITAS Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  49. SiPM: making its own way: the mean number of measured ph.e. is 96 Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

  50. Conclusions • In recentyears on ourrequestthemain PMT manufacturershavebeenworking on boostingthe QE ofclassical PMTs • As a resultbialkali PMTs of 1-3´´ sizewith ~ 35 % peak QE („superbialkali“) becamecommerciallyavailable (~ 40% boost!) • The PMTs continuebecomingbetterandbetter • The last wordis not yetsaid Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

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