# Mirzoyan VH QE PMT Chicago by fYX8HZg

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```									Highest QE‘s Measured So Far
Razmick Mirzoyan,

Max-Planck-Institute for Physics
Munich, Germany
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)

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Univ. Chicago         Munich: Highest QE PhotoCathodes
Differences between binomial
and poisson distributions

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Signal to noise ratio
The signal-to noise ratio of the photocathode can be
calculated as
1/2
SNR = [N x P/(1 - P)]
For example, if N = 1 (single impinging photon):

P       0.1         0.3         0.9

SNR     0.33       0.65           3

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Univ. Chicago             Munich: Highest QE PhotoCathodes
Signal to noise ratio

1/2
SNR = [N x P/(1 - P)]
For N = 20 impinging photons:

P      0.1        0.3         0.9

SNR    1.5        2.9        13.4

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Univ. Chicago           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,...

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Univ. Chicago         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

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Univ. Chicago         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“
   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
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Univ. Chicago         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

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Univ. Chicago         Munich: Highest QE PhotoCathodes
Short Historical Excursion
Metal                   Semiconductor

Photon  e-              High reflectivity Low reflectivity
conversion               Low efficiency    High efficency
e- motion                Low efficiency:              High efficiency
e- e- scattering             low phonon loss

Surface barrier          Work function                Determined by
> 2 eV                       e- affinity
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Univ. Chicago           Munich: Highest QE PhotoCathodes
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).

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Univ. Chicago         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 Au @ 15 eV

   2% for Al @ 17 eV

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Univ. Chicago         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 10-20 atomic
layers for K, Rb, Cs (1932).

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Univ. Chicago         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.
   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%)

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Univ. Chicago         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,    Razmik Mirzoyan, Max-Planck-
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Univ. Chicago         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 32-
35 % became available.
 These QE boosted PMTs are used in the
imaging camera of the MAGIC telescopes

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

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Univ. Chicago         Munich: Highest QE PhotoCathodes
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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Univ. Chicago         Munich: Highest QE PhotoCathodes
QE of „champion“ 2´´ PMT from Hamamatsu

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

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Univ. Chicago         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).

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Univ. Chicago         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 %

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Univ. Chicago            Munich: Highest QE PhotoCathodes
QE of a 3´´ PMT from Electron Tubes

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Univ. Chicago         Munich: Highest QE PhotoCathodes
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)

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Univ. Chicago         Munich: Highest QE PhotoCathodes
PMTs for MAGIC-I

QE  by coating with a
milky              diffuse scattering layer
WLS                   layer                             (D. Paneque, et al., 2002)
effect

Effective QE  ~ 15 %
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Univ. Chicago            Munich: Highest QE PhotoCathodes
QE of 3 PMTs (2 Hamamatsu + 1 ET) before
and after coating with milky layer

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

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

<QE> = 32 %

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Univ. Chicago         Munich: Highest QE PhotoCathodes
peak QE @350nm and QE(l)

<QE> = 33.9 %

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Univ. Chicago         Munich: Highest QE PhotoCathodes
QE of several tested PMTs

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

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QE of CTA candidate PMT from Hamamatsu

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Univ. Chicago         Munich: Highest QE PhotoCathodes
Collection Efficiency of the 1.5‘
CTA candidate PMT

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Univ. Chicago         Munich: Highest QE PhotoCathodes
We (CTA) expect that a similar product will
be soon (still during this year) provided by the
ET Enterprises

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Univ. Chicago         Munich: Highest QE PhotoCathodes
Maximum Peak QE of 3‘ Photonis PMT

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Minimum Peak QE

Photonis 3‘

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Univ. Chicago           Munich: Highest QE PhotoCathodes
Peak QE Map Scanned with a ~3mm Beam

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

• Such PMTs can
provide higher QE
than those with
transmission mode
cathodes
• Although in
literature one speaks
enhancement of
up to x2, in practice
one finds values of
~40%
29th of June 20012,    Razmik Mirzoyan, Max-Planck-
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Univ. Chicago         Munich: Highest QE PhotoCathodes
Photo cathode light reflection and absorption

Motta, Schönert (2005)

blue: ETL 9102B; green: ETL 9902B

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

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2-types of afterpulsing

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Light-emission microscopy @ MPI

29th of June 20012,    Razmik Mirzoyan, Max-Planck-
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PMT light emission

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The higher QE @ corners are due to the
mirror reflection

29th of June 20012,    Razmik Mirzoyan, Max-Planck-
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Univ. Chicago         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.
   These are
 HPDs with GaAsP photocathode

 SiPM (and its variations)

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

MEPhI-MPI-EXCELITAS

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Univ. Chicago         Munich: Highest QE PhotoCathodes
SiPM: making its own way:
the mean number of measured ph.e. is 96

29th of June 20012,    Razmik Mirzoyan, Max-Planck-
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Univ. Chicago         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
   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,    Razmik Mirzoyan, Max-Planck-
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Univ. Chicago         Munich: Highest QE PhotoCathodes
HPD Output Signal
<pulse height distribution>
<pulse shape>

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

FWHM～2.7 ns
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