1. Highest QE‘s Measured So Far Razmick Mirzoyan, Max-Planck-Institute for Physics Munich, Germany

2. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes Quantum Efficiency Quantum efficiency (QE) of a sensor is defined as the ratio 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 !) 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.. 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) Then the mean number of ph.e.s is N x P and the Variance is equal to N x P x (1 – P) 2

3. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes Differences between binomial and poisson distributions 3

4. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes Signal to noise ratio The signal-to noise ratio of the photocathode can be calculated as SNR = [N x P/(1 - P)] 1/2 SNR = [N x P/(1 - P)] 1/2 For example, if N = 1 (single impinging photon): For example, if N = 1 (single impinging photon): P0.10.30.9 SNR0.330.653 4

5. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes P0.10.30.9 SNR1.52.913.4 SNR = [N x P/(1 - P)] 1/2 For N = 20 impinging photons: Signal to noise ratio 5

6. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes Light conversion into a measurable Visible light can react and become measurable by: 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,... 6

7. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 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 1 st important application: reproduce sound for film 7

8. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes Short Historical Excursion Improved materials were discovered later on but it was a combination of a good luck with „intelligent guessing“ 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 A very important step was to realize that the photocathode materials are SEMICONDUCTORS Metallic versus Nonmetallic materials: 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 The main loss process in metals is the e- scattering; => e- escape depth of only few atomic layers is possible 8

9. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 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 The losses in Semiconductors because of phonon scattering (interaction with lattice) are much less, i.e. e- from deeper layers can reach the surface 9

10. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes Short Historical Excursion MetalSemiconductor Photon  e- conversion High reflectivity Low efficiency Low reflectivity High efficency e- motion Low efficiency: e- e- scattering High efficiency low phonon loss Surface barrier Work function > 2 eV Determined by e- affinity 10

11. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes Short Historical Excursion 1910: Photoelectric effect on K-Sb compound was found (Pohl & Pringsheim). 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). 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. 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). 1936: discovered high efficiency of Cs-Sb (Görlich). 11

12. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes QE of Metals For photon energies > 12 eV QE of 1-10 % were reported for For photon energies > 12 eV QE of 1-10 % were reported for Ni, Cu, Pt, Au, W, Mo, Ag and Pd Ni, Cu, Pt, Au, W, Mo, Ag and Pd (1953, Wainfan). (1953, Wainfan). 7% for Au @ 15 eV 7% for Au @ 15 eV 2% for Al@ 17 eV 2% for Al@ 17 eV 12

13. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes Escape Depth Escape depth can be defined as the thickness above which the photoemission becomes independent on thickness (in reflective mode) 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). The measured escape depth was 10-20 atomic layers for K, Rb, Cs (1932). 13

14. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes QE: Short Historical Excursion 1955-1958 Sommers found the „multialkali“ effect: combination of Cs-K-Na-Sb has high QE in the visible spectrum. 1955-1958 Sommers found the „multialkali“ effect: combination of Cs-K-Na-Sb has high QE in the visible spectrum. Also were discovered Also were discovered  Cs 3 Sb on MnO (S11, peak 400nm, QE ~ 20%)  (Cs)Na 2 KSb (S20, peak 400nm, QE ~ 30%)  K 2 CsSb ( peak 400nm, QE ~ 30%)  K 2 CsSb(O) ( peak 400nm, QE ~ 35%) 14

15. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes Typical Quantum Efficiencies 15 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

16. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes Boost of the QE of Bialkali PMTs In recent seevral years we were intensively working with the well-known PMT manufacturers looking into the possible boost of the QE of bialkali PMTs. Over past 40 years there was no progres reported. In recent seevral years we were intensively working with the well-known PMT manufacturers looking into the possible boost of the QE of bialkali PMTs. Over past 40 years there was no progres reported. After several iterations success could be reported. After several iterations success could be reported.  PMTs with peak QE values in the range of 32- 35 % became available.  These QE boosted PMTs are used in the imaging camera of the MAGIC telescopes 16

17. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes How it shall be possible to boost the QE and who is interested in it ? Use of highly purified materials for photo cathode (provides lower scattering for e - (low recombination probability) Use of highly purified materials for photo cathode (provides lower scattering for e - (low recombination probability)  → e- kicked out from deeper (top) layers can reach photo cathode-vacuum junction ¬_ and „jump“ into it (→ thicker cathode is possible). Optimal tuning of the photo cathode thickness Optimal tuning of the photo cathode thickness ……of the structure and material composition ……of the structure and material composition ……of the anti-reflective layer ……of the anti-reflective layer ……? ……? 17

18. QE Measuring Device @ MPI 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 18

19. Test Setup for Parametrising 7-PMT Clusters 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 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 19

20. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes QE of „champion“ 2´´ PMT from Hamamatsu Mirzoyan, et al., (2006) (proc. Beaune‘05) 20

21. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes QE of another 2´´ PMT from Hamamatsu a new process they are able to produce PMTs with peak QE of 33.7 % on average (Yoshizawa, 2005). Hamamatsu made a statement at Beaune-05 conference that by using a new process they are able to produce PMTs with peak QE of 33.7 % on average (Yoshizawa, 2005). 21

22. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes QE of a champion 2´´ PMT from Photonis According to Photonis the peak QE should have been 38 %; we meaured 34 % 22

23. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes QE of a 3´´ PMT from Electron Tubes 23

24. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes Electron Tubes Optimising the QE of PMTs Different batches show different behaviour Different batches show different behaviour QE is high (~30% !!) QE is high (~30% !!) Peak @ ~ 350 nm Peak @ ~ 350 nm Low QE at long (> 450 nm) Low QE at long (> 450 nm) 24

25. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes PMTs for MAGIC-I QE  by coating with a diffuse scattering layer Effective QE  ~ 15 % WLS milky layer effect (D. Paneque, et al., 2002) 25

26. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes QE of 3 PMTs (2 Hamamatsu + 1 ET) before and after coating with milky layer 26

27. MAGIC-I imaging camera PMTs (blue) compared to MAGIC-II PMTs (red) 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 27

28. Peak QE of MAGIC-II PMTs @ 350 nm = 32 % 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 28

29. M-I Upgrade camera PMTs: peak QE @350nm and QE( ) 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes = 33.9 % 29

30. QE of several tested PMTs 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 30

31. 1.5‘ CTA candidate PMT Hamamatsu R11920-100 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 31

32. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 32

33. QE of CTA candidate PMT from Hamamatsu 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 33

34. Collection Efficiency of the 1.5‘ CTA candidate PMT 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 34

35. We (CTA) expect that a similar product will be soon (still during this year) provided by the ET Enterprises 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 35

36. Maximum Peak QE of 3‘ Photonis PMT 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 36

37. Minimum Peak QE 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes Photonis 3‘ 37

38. Peak QE Map Scanned with a ~3mm Beam 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 38

39. PMTs with reflective mode photo cathode 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 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% 39

40. Photo cathode light reflection and absorption 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes blue: ETL 9102B; green: ETL 9902B Motta, Schönert (2005) 40

41. Light-induced afterpulsing 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 41

42. 2-types of afterpulsing 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 42

43. Light-emission microscopy @ MPI 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 43

44. PMT light emission 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 44

45. The higher QE @ corners are due to the mirror reflection 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 45

46. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes Other strongly competing ultra-fast, LLL sensors with single ph.e. resolution Two types of ultra-fast response LLL sensors, providing good single ph.e. resolution, start to strongly compete with the classical PMTs. Two types of ultra-fast response LLL sensors, providing good single ph.e. resolution, start to strongly compete with the classical PMTs. These are These are  HPDs with GaAsP photocathode  SiPM (and its variations) 46

47. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes 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) 47

48. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes SiPM: making its own way MEPhI-MPI-EXCELITAS 48

49. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes SiPM: making its own way: the mean number of measured ph.e. is 96 49

50. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes Conclusions In recent years on our request the main PMT manufacturers have been working on boosting the QE of classical PMTs In recent years on our request the main PMT manufacturers have been working on boosting the QE of classical PMTs As a result bialkali PMTs of 1-3´´ size with ~ 35 % peak QE („superbialkali“) became commercially available (~ 40% boost!) As a result bialkali PMTs of 1-3´´ size with ~ 35 % peak QE („superbialkali“) became commercially available (~ 40% boost!) The PMTs continue becoming better and better The PMTs continue becoming better and better The last word is not yet said The last word is not yet said 50

51. 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck- Munich: Highest QE PhotoCathodes HPD Output Signal FWHM ～ 2.7 ns Time [ns] 0 2 4 6 8 10 12 14 16 51