Highest QE‘s Measured So Far

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

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

Differences between binomial and poisson distributions
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 For example, if N = 1 (single impinging photon): P 0.1 0.3 0.9 SNR 0.33 0.65 3 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

Signal to noise ratio SNR = [N x P/(1 - P)]1/2 For N = 20 impinging photons: P 0.1 0.3 0.9 SNR 1.5 2.9 13.4 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: 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,... 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 1st important application: reproduce sound for film 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

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

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

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

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). 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 Ni, Cu, Pt, Au, W, Mo, Ag and Pd (1953, Wainfan). 7% for 15 eV 2% for 17 eV 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) The measured escape depth was atomic layers for K, Rb, Cs (1932). 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

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

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 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. After several iterations success could be reported. PMTs with peak QE values in the range of % became available. These QE boosted PMTs are used in the imaging camera of the MAGIC telescopes 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) → 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 ……of the structure and material composition ……of the anti-reflective layer ……? 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

QE Measuring Device @ MPI

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

QE of another 2´´ PMT from Hamamatsu
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). 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 % 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

QE of a 3´´ PMT from Electron Tubes

Electron Tubes Optimising the QE of PMTs
Different batches show different behaviour QE is high (~30% !!) ~ 350 nm Low QE at long l (> 450 nm) 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
milky layer effect WLS (D. Paneque, et al., 2002) Effective QE  ~ 15 % 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 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

MAGIC-I imaging camera PMTs (blue) compared to MAGIC-II PMTs (red)

Peak QE of MAGIC-II PMTs @ 350 nm

M-I Upgrade camera PMTs: peak QE @350nm and QE(l)

QE of several tested PMTs

1.5‘ CTA candidate PMT Hamamatsu R11920-100

QE of CTA candidate PMT from Hamamatsu

Collection Efficiency of the 1.5‘ CTA candidate PMT

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

Maximum Peak QE of 3‘ Photonis PMT

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

Peak QE Map Scanned with a ~3mm Beam

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

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

Light-induced afterpulsing

2-types of afterpulsing

Light-emission microscopy @ MPI

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

The higher QE @ corners are due to the mirror reflection

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. These are HPDs with GaAsP photocathode SiPM (and its variations) 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) 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

SiPM: making its own way
MEPhI-MPI-EXCELITAS 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

Conclusions 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!) The PMTs continue becoming better and better The last word is not yet said 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

HPD Output Signal <pulse height distribution> <pulse shape> Time [ns] FWHM～2.7 ns 29th of June 20012, Univ. Chicago Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

Highest QE‘s Measured So Far

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