Very High QE bialkali PMTs

Mª Victoria Fonseca

University Complutense, Madrid, SpainUniversity Complutense, Madrid, Spain

How a classical PMT is operating

photons

Quantum EfficiencyQuantum efficiency (QE) of a sensor Quantum efficiency (QE) of a sensor

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

process (and not poisson !)process (and not poisson !)

Assume Assume N photonsN photons are impinging onto a are impinging onto a photocathode and every photon has the same photocathode and every photon has the same probability Pprobability P to kick out one ph.e.to kick out one 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 N x P x (1 –– P)P)

Signal to noise ratioThe signalThe signal--to noise ratio (SNR) of a given photocathode to noise ratio (SNR) of a given photocathode with QE=P can be calculated aswith QE=P can be calculated as

SNR = SNR = √√ [N x P/(1 [N x P/(1 -- P)]P)]For example, for N = 1 (single impinging photon):For example, for N = 1 (single impinging photon):

PP 0.10.1 0.30.3 0.90.9 0.950.95 0.990.99

SNRSNR 0.330.33 0.650.65 33 4.44.4 9.99.9

Short Historical Excursion1889: 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

Short Historical ExcursionImproved materials discovered later as a Improved materials discovered later as a combination of good luck with combination of good luck with „„intelligent intelligent guessingguessing““A very important step was to realize that the A very important step was to realize that the photocathode materials are SEMICONDUCTORSphotocathode materials are SEMICONDUCTORSMetallic versus Nonmetallic materials: Metallic versus Nonmetallic materials:

yield of metallic photocathodes is very low yield of metallic photocathodes is very low because of very high reflectivitybecause of very high reflectivitysemiconductors have less reflection lossessemiconductors have less reflection losses

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

Short Historical Excursion

1910: Photoelectric effect on K1910: Photoelectric effect on K--Sb compound was Sb compound was found (Pohl & Pringsheim).found (Pohl & Pringsheim).1923: found that thermionic emission of W is 1923: 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 above It was found that the work function in the above case was lower than of Cs metal in bulk.case was lower than of Cs metal in bulk.1936: discovered high efficiency of Cs1936: discovered high efficiency of Cs--Sb (GSb (Göörlich).rlich).

QE of Metals

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

QE: Short Historical Excursion

19551955--1958 Sommers found the 1958 Sommers found the „„multialkalimultialkali““ effect: effect: combination of combination of CsCs--KK--NaNa--SbSb has high QE in the has high QE in the visible spectrum.visible spectrum.Also were discoveredAlso were discovered

CsCs33SbSb on MnO (S11, on MnO (S11, ⎣⎣peakpeak @400nm, QE ~ 20%)@400nm, QE ~ 20%)(Cs)Na(Cs)Na22KSbKSb (S20, (S20, ⎣⎣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%)

Typical Quantum Efficiencies

Boost of the QE of Bialkali PMTsIn recent years we were intensively working with In recent years we were intensively working with the wellthe well--known PMT manufacturers trying to known PMT manufacturers trying to boost QE of bialkali PMTs. boost QE of bialkali PMTs. Over past 40 years Over past 40 years there was no progress.there was no progress.

After several iterations there was success.After several iterations there was success.Already 2 years ago PMTs with peak QE values Already 2 years ago PMTs with peak QE values in the range of 32in the range of 32--35 % became available.35 % became available.These new high QE PMTs are used in the These new high QE PMTs are used in the imaging camera of the MAGICimaging camera of the MAGIC--II telescopeII telescope

How is it possible to boost the QE and who is interested in it ?

Use of highly purified materials for the photo Use of highly purified materials for the photo cathode (change from 99.999 cathode (change from 99.999 99.9999 or even 99.9999 or even of higher purity; will provide less scattering of higher purity; will provide less scattering length for elength for e-- (low recombination probability)(low recombination probability)Optimal tuning of the photo cathode thicknessOptimal tuning of the photo cathode thicknessOptimal tuning of the material composition Optimal tuning of the material composition Optimal tuning of the antiOptimal tuning of the anti--reflective layerreflective layerOptimal tuning of the Cs layer thicknessOptimal tuning of the Cs layer thickness

QE of our „old champion“ 2‘PMT from Hamamatsu

Mirzoyan, et al., NIM A (proc. Beaune‘05)

PMTs of MAGIC-IPMTs of MAGIC-I

QE by a diffuse scattering coating, +WLS

Effective QE ~ 15 %

WLSmilky layereffect

(D. Paneque, et al., 2002)

Recent PMTs Electron TubesRecent PMTs Electron TubesDifferent 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)

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

Charge collection efficiencyQE alone is not a very meaningful parameter for a QE alone is not a very meaningful parameter for a PMT. The charge collection efficiency (CE) is an PMT. The charge collection efficiency (CE) is an equally important parameter.equally important parameter.

The convolution of the QE with the CE is the real The convolution of the QE with the CE is the real important parameter, the photon detection efficiency, important parameter, the photon detection efficiency, PDE . This is what one needs to measurePDE . This is what one needs to measure

While an absolute measurement is not easy, a While an absolute measurement is not easy, a comparative, relative measurement can be easily comparative, relative measurement can be easily performed.performed.

Optical axis is adjusted

PDE measurements

PMTs were observinga piece of white wall of 2.1 m size. The fullobserving anglewas set to 120°.

2.1

m

0.6 m

0.6 m laser

PDE measurements

We performed Photon Detection Efficiency (PDE) measurements for Hamamatsu and ET PMTs. 80 ps flashes from a laser @ 400 nm were illuminating a white wall in a storage room.

ET142 Hamamatsu3352

-- 1400v

-- 1300v

-- 1200v

-- 1100v

PDE measurements

Single photoelectrons

Final PMT selection

Our measurements show that Hamamatsu PMT have on average 20 % higher photon detection efficiency (PDE) than ET PMTs(this essentially reflects the existing differences between the QE’s).

The QE of a not coated PMT from Hamamatsu is comparable to the QE of a milky coated ET.

The coating of Hamamatsu PMT is increasing its effective QE by ~10 %. => select Hamamatsu PMTs for the M-II camera.

Short before we were making the order for the selected PMTs for the M-II camera to Hamamatsu, they released news on their very recent developments, Ultrabialkali PMTs:

Recent Surprises• All the 3 PMT manufacturers could report enhanced QE values, the bestbeing Hamamatsu, who gave it the name „Super-bialkali“ (QE~ 33-36 %).

• One year ago Hamamatsu claimed toproduce PMTs with peak QE of 43-45 % ! (once the djinn comes out of the lamp you cannot control it anymore) ;-)

• Recently also Photonis joined club of„Ultra-bialkali“. Moreover, it pushed the QE values even higher up !

The very recent 3‘ Photonis PMTs:QE peak values in excess 50 % !

Preliminary !

Afterpulse

PMT SelectionMAGIC-II

time

Afterpulse

NSB@La Palma ~ 130 MHzNSB@La Palma ~ 130 MHz

If Afterpulse rate is highIf Afterpulse rate is high> 1 %, the afterpulses > 1 %, the afterpulses will dominate MAGIC will dominate MAGIC trigger. trigger.

H+H+

Phe.Phe.HH22OO

photocathodephotocathode

2ed dynode2ed dynode1st dynode1st dynode

Ions fly back and hit the Ions fly back and hit the cathode then produce cathode then produce photoelectrons againphotoelectrons again. .

Secondary eSecondary e--

Afterpulse Timing Spectrum

PMT SelectionMAGIC-II

Assume the electric field between cathode and 1st dynode is E

q*E = m*a a = q*E/mS = 0.5*a*t²; t² = 2*S/a

t = sqrt( 2*S*m/(q*E))

The time between the main pulse and the afterpulse is about the transit time of the ion from the 1st dynode to the cathode.

The ions may come from The ions may come from HH22O, CHO, CH44 and Heand He

nsns

Main pulsesMain pulses

Afterpulse Rate

Most of the PMTs have afterpulse rate 0.2~0.8% (@4phe) , Most of the PMTs have afterpulse rate 0.2~0.8% (@4phe) , even for SBA PMT from Hamamatsu even for SBA PMT from Hamamatsu

NO correlation between QE and afterpulse rate !! NO correlation between QE and afterpulse rate !!

PMT SelectionMAGIC-II

Aging Test: “PMT never die; they just fade away”

Due to long time illumination => the dynodes become “tired” (fatigue) => gain drops.

The PMTs were run totally for 10150 minutes (7 days), under the constant illumination that induced an initial anode current of 150 µA.(the NSB@La Palma is about 1 µA). So we illuminate with stronger light intensity to speed up the measurement.

PMT SelectionMAGIC-II

Aging Test

Light intensity variations are taken into account by usinga monitoring PIN diode.

MAGIC PMTs are operated under moon observation. Assume average DC is 2 uA.

1 year operation : 2000 h * 3600 s * 2uA ~ 15 C.=> For h-xc3348: gain drop to 90 %

H-xc-3348

H-xc-3344

ET-2325

ET-10001

Total charge, C

Pulse Width, Rising and falling time

An fast oscilloscope (1.5 GHz bandwidth), 5 G/s sampling rate, followed by a 2 GHz bandwidth amplifier (gain x 100) is used.

Fast response : rising time ~ 700 ps, FWHM~ 1.1 nsFast response : rising time ~ 700 ps, FWHM~ 1.1 ns

PMT SelectionMAGIC-II

ET-10015

Single ph.e.Ham.- 3354

Single ph.e.

Requested budget

400 Hamamatsu UBA PMTs: 400 Hamamatsu UBA PMTs: 500500€€/unit /unit 200.000200.000Associated electronics Associated electronics 120.000 120.000 Travel 3 yearsTravel 3 years 30.00030.000OscilloscopeOscilloscope 25.00025.000LaserLaser 12.00012.000OthersOthers 17.00017.000total 400.000 euros total 400.000 euros

Tests using MAGIC I central inner camera

ConclusionsIn recent years on our request the main PMT In recent years on our request the main PMT manufacturers were working on manufacturers were working on boosting the QE of boosting the QE of classical PMTsclassical PMTsAs a result bialkali PMTs of 1As a result bialkali PMTs of 1--33´´´´ size with 32size with 32--35 % 35 % peak QE became commercially available already in peak QE became commercially available already in 2006 2006

(~ 35% boost!)((~ 35% boost!)(they got the name superthey got the name super--bialkali)bialkali)In autumn 2006 we learned from Hamamatsu about In autumn 2006 we learned from Hamamatsu about the sothe so--called called ultraultra--bialkali PMTs with 43bialkali PMTs with 43--45 % peak 45 % peak QEQENow also Now also PhotonisPhotonis could demonstrate on the could demonstrate on the example of example of 33‘‘ PMT peak QE values scattered in the PMT peak QE values scattered in the range 35range 35--55 %.55 %.Together with SiPM the new bialkali PMTs will Together with SiPM the new bialkali PMTs will dominate the market very soon !dominate the market very soon !

THE ENDTHE END

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

In recent times two more types of ultraIn recent times two more types of ultra--fast fast response LLL sensors, providing good response LLL sensors, providing good single ph.e. resolution, start to strongly single ph.e. resolution, start to strongly compete with the classical PMTs.compete with the classical PMTs.These are These are

HPDs with GaAsP photocathodeHPDs with GaAsP photocathodeSiPM (and its variations) SiPM (and its variations)

Calibration with SiPM:

<46> ph.e. are measured

HPD Output Signal

<pulse height distribution><pulse shape>

FWHM～2.7 ns

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

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

QE boost with Wavelength Shifter•WLS–Butyl- PBD(260-340 to 360-460 nm)–POPOP(300-400 to 400-500 nm)–Paraloid B72(n = 1.4)in Toluene

On the Input window

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, paints,... animal), plants, paints,... Photoemulsion Photoemulsion (QE ~ 0.1 (QE ~ 0.1 –– 1 %)1 %) (photo(photo--chemical)chemical)Photodiodes (photoelectrical, evacuated)Photodiodes (photoelectrical, evacuated)Classical & hybrid photomultipliers (QE ~ 25 %)Classical & hybrid photomultipliers (QE ~ 25 %)

QE ~ 45 % (HPD with GaAsP photocathode)QE ~ 45 % (HPD with GaAsP photocathode)PhotodiodesPhotodiodes (QE ~ 70 (QE ~ 70 –– 80 %)80 %) (photoelectrical)(photoelectrical)PIN diodes, Avalanche diodes, SiPM,...PIN diodes, Avalanche diodes, SiPM,...photodiode arrays like CCD, CMOS cameras,...photodiode arrays like CCD, CMOS cameras,...

Short Historical Excursion

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

Short Historical ExcursionMetalMetal SemiconductorSemiconductor

Photon Photon ee--conversionconversion

High reflectivityHigh reflectivityLow efficiencyLow efficiency

Low reflectivityLow reflectivityHigh efficencyHigh efficency

ee-- motionmotion Low efficiency:Low efficiency:ee-- ee-- scatteringscattering

High efficiencyHigh efficiencylow phonon losslow phonon loss

Surface barrierSurface barrier Work functionWork function> 2 eV> 2 eV

Determined by eDetermined by e--affinityaffinity

Single Photoelectron Spectrum

PMT SelectionMAGIC-II

Require P / V ratio > Require P / V ratio > 1.3 in order to get 1.3 in order to get better Single better Single Photoelectron Photoelectron ResolutionResolution..

ET117

p/v =1p/v =1

DW 139

p/v=3

P/V =1

zl-7016

p/v=1.4

xc-3353P/V =1P/V =1

Keep 1 phe Keep 1 phe events < 5%events < 5%