highest qe‘s measured so far razmick mirzoyan, max-planck-institute for physics munich, germany

Highest QE‘s Measured So Far

Razmick Mirzoyan,

Max-Planck-Institute for PhysicsMax-Planck-Institute for Physics

Munich, GermanyMunich, Germany

29th of June 20012, Univ. Chicago

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

Quantum Efficiency

Quantum efficiency (QE) of a sensor is defined Quantum efficiency (QE) of a sensor is defined as the ratioas the ratio

QE = N(ph.e.) : N(photons)QE = N(ph.e.) : N(photons) Conversion of a photon into ph.e. is a purely Conversion of a photon into ph.e. is a purely

binomial process (and not poisson !)binomial process (and not poisson !) Assume Assume NN photons photons are impinging onto a are impinging onto a

photocathode and every photon has the photocathode and every photon has the same same probability probability PP to kick out a ph.e.. to kick out a ph.e..

Then the Then the meanmean number of ph.e.s is number of ph.e.s is N x PN x P and the and the VarianceVariance is equal to is equal to N x P x (1 – P)N x P x (1 – P)

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Differences between binomial and poisson distributions

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Signal to noise ratio

The signal-to noise ratio of the photocathode can The signal-to noise ratio of the photocathode can be calculated asbe calculated as

SNR = [N x P/(1 - P)]SNR = [N x P/(1 - P)]1/21/2

For example, if N = 1 (single impinging For example, if N = 1 (single impinging photon):photon):

PP 0.10.1 0.30.3 0.90.9

SNRSNR 0.330.33 0.650.65 33

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P 0.1 0.3 0.9

SNR 1.5 2.9 13.4

SNR = [N x P/(1 - P)]SNR = [N x P/(1 - P)]1/21/2

For N = 20 impinging photons:

Signal to noise ratio

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Light conversion into a measurable

Visible light can react and become measurable by:Visible light can react and become measurable by: Eye Eye (human: QE ~ 3 % &(human: QE ~ 3 % & animal), plants, animal), plants,

paints,... paints,... Photoemulsion Photoemulsion (QE ~ 0.1 – 1 %)(QE ~ 0.1 – 1 %) (photo- (photo-

chemical)chemical) Photodiodes (photoelectrical, evacuated)Photodiodes (photoelectrical, evacuated)

Classical & hybrid photomultipliers Classical & hybrid photomultipliers (QE ~ 25 (QE ~ 25 %)%)

QE ~ 45 % (HPD with GaAsP photocathode)QE ~ 45 % (HPD with GaAsP photocathode) PhotodiodesPhotodiodes (QE ~ 70 – 80 %)(QE ~ 70 – 80 %) (photoelectrical)(photoelectrical)

PIN diodes, Avalanche diodes, SiPM,...PIN diodes, Avalanche diodes, SiPM,... photodiode arrays like CCD, CMOS photodiode arrays like CCD, CMOS

cameras,...cameras,...

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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

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Short Historical Excursion Improved materials were discovered later on but it Improved materials were discovered later on but it

was a combination of a good luck with „intelligent was a combination of a good luck with „intelligent guessing“guessing“

A very important step was to realize that the A very important step was to realize that the photocathode materials are SEMICONDUCTORSphotocathode materials are SEMICONDUCTORS

MetallicMetallic versus versus Nonmetallic Nonmetallic materials: materials: yield of metallic photocathodes is very low yield of metallic photocathodes is very low

because of very high reflectivitybecause of very high reflectivity semiconductors have less reflection lossessemiconductors have less reflection losses

The main loss process in metals is the e- The main loss process in metals is the e- scattering; => e- escape depth of only few atomic scattering; => e- escape depth of only few atomic layers is possiblelayers is possible

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Short Historical Excursion

The losses in Semiconductors because of The losses in Semiconductors because of phonon scattering (interaction with lattice) are phonon scattering (interaction with lattice) are much less, i.e. e- from deeper layers can reach much less, i.e. e- from deeper layers can reach the surface the surface

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Short Historical ExcursionMetalMetal SemiconductSemiconduct

oror

Photon Photon e- e-

conversionconversionHigh High reflectivityreflectivity

Low efficiencyLow efficiency

Low reflectivityLow reflectivity

High efficencyHigh efficency

e- motione- motion Low efficiency:Low efficiency:

e- e- scatteringe- e- scatteringHigh efficiencyHigh efficiency

low phonon low phonon lossloss

Surface barrierSurface barrier Work functionWork function

> 2 eV> 2 eVDetermined by Determined by e- affinitye- affinity

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Short Historical Excursion

19101910: Photoelectric effect on K-Sb compound : Photoelectric effect on K-Sb compound was found (Pohl & Pringsheim).was found (Pohl & Pringsheim).

19231923: found that thermionic emission of W is : found that thermionic emission of W is greatly enhanced when exposed to Cs vapour greatly enhanced when exposed to Cs vapour (Kingdon & Langmuir). (Kingdon & Langmuir).

It was found that the work function in the It was found that the work function in the above case was lower than of Cs metal in bulk.above case was lower than of Cs metal in bulk.

19361936: discovered high efficiency of Cs-Sb : discovered high efficiency of Cs-Sb (Görlich).(Görlich).

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QE of Metals

For photon energies > 12 eV QE of 1-10 % For photon energies > 12 eV QE of 1-10 % were reported for were reported for

NiNi, , CuCu, Pt, , Pt, AuAu, W, Mo, Ag and Pd , W, Mo, Ag and Pd

(1953, Wainfan). (1953, Wainfan).

7% for Au @ 15 eV7% for Au @ 15 eV

2% for Al2% for Al @ 17 eV @ 17 eV

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Escape Depth

Escape depth can be defined as the thickness Escape depth can be defined as the thickness above which the photoemission becomes above which the photoemission becomes independent on thickness (in reflective mode)independent on thickness (in reflective mode)

The measured escape depth was 10-20 atomic The measured escape depth was 10-20 atomic layers for K, Rb, Cs (1932).layers for K, Rb, Cs (1932).

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QE: Short Historical Excursion

1955-19581955-1958 Sommers found the „multialkali“ Sommers found the „multialkali“ effect: combination of effect: combination of Cs-K-Na-SbCs-K-Na-Sb has high QE has high QE in the visible spectrum.in the visible spectrum.

Also were discoveredAlso were discovered CsCs33SbSb on MnO ( on MnO (S11S11, , peakpeak 400nm, QE ~ 20%) 400nm, QE ~ 20%) (Cs)Na(Cs)Na22KSbKSb ( (S20S20, , peakpeak 400nm, QE ~ 30%) 400nm, QE ~ 30%) KK22CsSbCsSb ( (peakpeak 400nm, QE ~ 30%) 400nm, QE ~ 30%) KK22CsSb(O) (CsSb(O) (peakpeak 400nm, QE ~ 35%) 400nm, QE ~ 35%)

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Typical Quantum Efficiencies

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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



Boost of the QE of Bialkali PMTs

In recent seevral years we were intensively In recent seevral years we were intensively working with the well-known PMT working with the well-known PMT manufacturers looking into the possible boost manufacturers looking into the possible boost of the QE of bialkali PMTs. of the QE of bialkali PMTs. Over past 40 years Over past 40 years there was no progres reported.there was no progres reported.

After several iterations success could be After several iterations success could be reported.reported. PMTs with peak QE values in the range of PMTs with peak QE values in the range of

32-35 % became available.32-35 % became available. These QE boosted PMTs are used in the These QE boosted PMTs are used in the

imaging camera of the MAGIC telescopesimaging camera of the MAGIC telescopes

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How it shall be possible to boost the QE and who is interested in it ? Use of highly purified materials for photo cathode Use of highly purified materials for photo cathode

(provides lower scattering for e(provides lower scattering for e- - (low (low recombination probability)recombination probability) →→ e- kicked out from deeper (top) layers can e- kicked out from deeper (top) layers can

reach photo cathode-vacuum junction reach photo cathode-vacuum junction ¬¬__ and and „jump“ into it („jump“ into it (→→ thicker cathode is possible). thicker cathode is possible).

Optimal tuning of the photo cathode thicknessOptimal tuning of the photo cathode thickness …………of the structure and material compositionof the structure and material composition …………of the anti-reflective layerof the anti-reflective layer …………??

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QE Measuring Device @ MPI



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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

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QE of „champion“ 2´´ PMT from Hamamatsu

Mirzoyan, et al., (2006) (proc. Beaune‘05)

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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 a new process they are able to produce PMTs with peak QE of 33.7 % on average (Yoshizawa, 2005). peak QE of 33.7 % on average (Yoshizawa, 2005).

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QE of a champion 2´´ PMT from Photonis

According to Photonis the peak QE should have been 38 %; we meaured 34 %

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QE of a 3´´ PMT from Electron Tubes

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Electron Tubes Optimising the QE of PMTsElectron Tubes Optimising the QE of PMTs

Different batches show Different batches show different behaviour different behaviour

QE is high (~30% !!)QE is high (~30% !!)

Peak @ ~ 350 nmPeak @ ~ 350 nm

Low QE at long Low QE at long (> 450 nm)(> 450 nm)

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PMTs for MAGIC-IPMTs for MAGIC-I

QE by coating with a diffuse scattering layer

Effective QE ~ 15 %

WLSmilky layereffect

(D. Paneque, et al., 2002)

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QE of 3 PMTs (2 Hamamatsu + 1 ET) before and after coating with milky layer

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



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Peak QE of MAGIC-II PMTs @ 350 nm

<QE> = 32 %



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



<QE> = 33.9 %

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QE of several tested PMTs



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1.5‘ CTA candidate PMT Hamamatsu R11920-100



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QE of CTA candidate PMT from Hamamatsu



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Collection Efficiency of the 1.5‘ CTA candidate PMT



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We (CTA) expect that a similar product will be soon (still during this year) provided by the ET Enterprises



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Maximum Peak QE of 3‘ Photonis PMT



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Minimum Peak QE



Photonis 3‘

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Peak QE Map Scanned with a ~3mm Beam



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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 practiceone finds values of ~40%

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Photo cathode light reflection and absorption



blue: ETL 9102B; green: ETL 9902B

Motta, Schönert (2005)

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Light-induced afterpulsing



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2-types of afterpulsing



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Light-emission microscopy @ MPI



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PMT light emission



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The higher QE @ corners are due to the mirror reflection



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Other strongly competing ultra-fast,LLL sensors with single ph.e. resolution

Two types of ultra-fast response LLL sensors, providing Two types of ultra-fast response LLL sensors, providing good single ph.e. resolution, start to strongly compete with good single ph.e. resolution, start to strongly compete with the classical PMTs.the classical PMTs.

These are These are HPDs with GaAsP photocathodeHPDs with GaAsP photocathode SiPM (and its variations) SiPM (and its variations)

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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)

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SiPM: making its own way

MEPhI-MPI-EXCELITAS

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SiPM: making its own way: the mean number of measured ph.e. is 96

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Conclusions

In recent years on our request the main In recent years on our request the main PMT manufacturers have been working PMT manufacturers have been working on boosting the QE of classical PMTson boosting the QE of classical PMTs

As a result bialkali PMTs of 1-3´´ size with As a result bialkali PMTs of 1-3´´ size with ~ 35 % peak QE („superbialkali“) became ~ 35 % peak QE („superbialkali“) became commercially available (~ 40% boost!) commercially available (~ 40% boost!)

The PMTs continue becoming better and The PMTs continue becoming better and betterbetter

The last word is not yet saidThe last word is not yet said

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HPD Output Signal

<pulse height distribution>

<pulse shape>

FWHM ～ 2.7 ns

Time [ns]0 2 4 6 8 10 12 14 16

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highest qe‘s measured so far razmick mirzoyan, max-planck-institute for physics munich, germany

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