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0168-9002/$ - se

doi:10.1016/j.ni

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1Present addr

Nuclear Instruments and Methods in Physics Research A 556 (2006) 146–157

www.elsevier.com/locate/nima

Characterization of ETL 9357FLA photomultiplier tubes forcryogenic temperature applications

A. Ankowskia, M. Antonellob, P. Aprilic, F. Arneodoc, A. Badertscherd, B. Baiboussinove,M. Baldo Ceoline, G. Battistonif, P. Benettig, M. Bischofbergerd, A. Borio di Tiglioleg,

R. Brunettig,1, G. Bucciarellic, A. Buenoh, E. Calligarichg, F. Carbonarai, M.C. Carmonah,F. Cavannab, P. Cenninij, S. Centroe, A. Cesanak, D.B. Clinel, K. Cieslikm, A.G. Coccoi,

Z. Daid, C. De Vecchig, A. Dabrowskam, A. Di Ciccoi, R. Dolfinig, A. Ereditatoi, A. Ferellab,A. Ferrarij, G. Fiorilloi, D. Garcıa-Gamezh, Y. Ged, D. Gibine, A. Gigli Berzolarig,

I. Gil-Botellad,n, K. Graczyka, L. Grandig, A. Guglielmie, J. Holeczeko,C. Juszczaka, D. Kie"czewskap, J. Kisielo, T. Koz"owskiq, M. Laffranchid,

J. Łagodap, B. Lisowskil, J. Lozanoh, M. Markiewiczm, A. Martınez de la Ossah,C. Mattheyl, F. Maurig, A.J. Melgarejoh, A. Menegollig, G. Menge,

M. Messinad, C. Montanarig, S. Murarof, S. Navas-Conchah, J. Nowaka,S. Otwinowskil, O. Palamarac, L. Perialer, G. Piano Mortarib, A. Piazzolig,

P. Picchir, F. Pietropaoloe, W. Po"ch"opeks, M. Pratag, M.C. Pratag,P. Przewlockiq, A. Rappoldig, G.L. Rasellig,�, E. Rondioq, M. Rossellag,

A. Rubbiad, C. Rubbiag, P.R. Salaf, R. Santorellii, D. Scannicchiog, E. Segretob,Y. Seol, F. Sergiampietrit, J. Sobczyka, S. Stacho, J. Stepaniakq, R. Suleju,

M. Szeptyckaq, M. Szarskam, A. Szelcm, M. Terranik, F. Varaninie,S. Venturae, C. Vignolig, H. Wangl, X. Yangl, A. Zalewskam

aInstitute of Theoretical Physics, Wroc!aw University, Wroc!aw, PolandbDipartimento di Fisica, Universita dell’Aquila and INFN, L’Aquila, Italy

cLaboratori Nazionali del Gran Sasso (LNGS) INFN, Assergi, ItalydInstitute for Particle Physics, ETH Honggerberg, Zurich, SwitzerlandeDipartimento di Fisica, Universita di Padova and INFN, Padova, ItalyfDipartimento di Fisica, Universita di Milano and INFN, Milano, Italy

gDipartimento di Fisica Nucleare e Teorica, Universita di Pavia and INFN, via Bassi 6, I-27100 Pavia, ItalyhDepartamento de Fısica Teorica y del Cosmos and Centro Andaluz de Fısica de Partıculas Elementales (CAFPE), Universidad de Granada, Granada, Spain

iDipartimento di Scienze Fisiche, Universita Federico II di Napoli and INFN, Napoli, ItalyjCERN, Geneve, Switzerland

kDipartimento di Ingegneria Nucleare, Politecnico di Milano and INFN, Milano, ItalylDepartment of Physics and Astronomy, University of California, Los Angeles, USA

mH. Niewodniczanski Institute of Nuclear Physics, Krakow, PolandnCIEMAT, Departamento de Investigacion Basica, Madrid, Spain

oInstitute of Physics, University of Silesia, Katowice, PolandpInstitute of Experimental Physics, University of Warszawa, Poland

qA.So!tan Institute for Nuclear Studies, Warszawa, PolandrLaboratori Nazionali di Frascati (LNF) INFN, Frascati, Italy

e front matter r 2005 Elsevier B.V. All rights reserved.

ma.2005.10.108

ing author. Tel.: +390382 987410; fax: +39 0382 423241.

ess: gianluca.raselli@pv.infn.it (G.L. Raselli).

ess: INFN, Torino, Italy.

ARTICLE IN PRESSA. Ankowski et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 146–157 147

sDepartment of Electronics, AGH University of Science and Technology, Krakow, PolandtINFN, Pisa, Italy

uInstitute of Radioelectronics, University of Warsaw, Warszawa, Poland

Received 8 September 2005; accepted 29 October 2005

Available online 1 December 2005

Abstract

We carried out a careful evaluation of the performance of the large cathode area ETL 9357FLA photomultiplier tube operating at

cryogenic temperature. The measurements were focused on evaluating the parameters which mainly characterize the operating

performances of the device down to 77K and the spread of the distinctive features over 54 samples assembled in the ICARUS apparatus.

The results that we obtained demonstrate that the photomultiplier is suited for light detection in such unconventional operating

conditions, certifying this device for the direct measurement of scintillation light coming from noble-gas liquids in detectors dedicated to

neutrino physics and dark matter research.

r 2005 Elsevier B.V. All rights reserved.

PACS: 85.60.H; 29.40.M; 07.02.M

Keywords: Photomultiplier tubes; Scintillation detectors; Cryogenics

1. Introduction

The detection of scintillation light coming from noble-gas liquids has been proposed in some experimentsdedicated to neutrino physics and dark matter research.In these experiments photomultiplier tubes (PMTs) directlyimmersed in liquid phase and operating at cryogenictemperatures are often adopted as photon detectors. Incase of glass-window PMTs, the sensitivity to ultravioletlight is achieved by the use of a wavelength shifterdeposited on the PMT surface. This is the case of theICARUS T600 apparatus that employs 74 PMTs to detectthe scintillation light at 128 nm wavelength of 600 tons ofliquid argon [1].

In this paper we report the results of the test of about 50large cathode area Electron Tubes Ltd (ETL) 9357FLAphotomultipliers specifically manufactured for theICARUS T600 liquid argon detector. Our measurementswere focused on evaluating the parameters which mainlycharacterize the operating performances of the devices atcryogenic temperature and the spread of the distinctivefeatures over different samples. In the present work we donot take into consideration the presence of any wavelengthshifter on the PMTs’ surface, which will be the subject ofcoming papers. In order to minimize the costs of the testactivity, all the cryogenic measurements were carried out inliquid nitrogen, the temperature of which (77K) is veryclose to that of the liquid argon (87K).

In particular we inquired into the following character-istics:

(1)

photocathode behaviour; (2) shape of the anode pulse; (3) single-electron response of the anode pulse;

Fig. 1. The ETL 9357FLA photomultiplier with blasted glass window.

(4) gain;

(5)

single-electron transit time features (spread, pre andlate pulse presence);

(6)

after-pulses; (7) dark count rate and spectrum.

The present paper is organized as follows: in Section 2we present the general characteristics of the ETL PMTmodel; Section 3 is dedicated to the behaviour at cryogenictemperature of the photocathode type adopted in thisdevice; Section 4 is dedicated to the characterization atcryogenic temperature of 54 PMT samples; conclusions arereported in Section 5.

2. The ETL 9357FLA photomultiplier

The ETL 9357FLA is a 12-stage LF-dynode PMT withan hemispherical glass window of 200mm (8-in.) diameter

ARTICLE IN PRESS

Table 1

Nominal physical and electric characteristics of the Electron Tubes

9357FLA photomultiplier [2]

Type and serial number 9357FLA; 1116

Length and diameter 293mm; 203mm

Cathode size 190mm

Cathode type K2CsSb on Pt layer

Spectral response 3002500nm

Dynodes 12LF CsSb

SER peak-to-valley 2:3Rise time and FWHM 5ns; 8 ns

Maximum gain 5� 107

Quantum efficiency (blue) � 20%

HV for 107 gain 1500V

A. Ankowski et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 146–157148

(Fig. 1). The bialkali photocathode ðK2CsSbÞ is depositedon a Pt under-layer in order to extend the range oftemperatures the device can be operated at. The PMT isotherwise very similar to the parent model ETL 9353B.One single device (no. 1116) was used as reference for thetest measurements. Its main physical and electric char-acteristics, as provided by the manufacturer, are summar-ized in Table 1 [2].

In the ICARUS T600 apparatus a total of 74 photo-multipliers are used to detect the light coming from liquidargon scintillation. The samples tested for the present workconstitute the set of 54 photomultipliers mounted in thesecond module of the ICARUS detector2. All the devices,coming from the same production batch, were selected atmanufacture by requesting a uniform gain and peak-to-valley ratio. Moreover sand blasting of the windows wasrequested in order to improve the adhesion to thewavelength shifter coating (TPB TetraPhenyl-Butadiene)adopted in the experiment to convert the 128 nm liquidargon scintillation light to photons in the blue region [3].

VACUUMPUMP

VACUUMGAUGE

LN2

N2

PM

CATHODE

PT1000

OPTICAL FIBRE

PULSER

LED/LASER

Trigger Signal

H.V. 100VAnode + Dynodes

PREAMP.

AMPLIFIER

PC

I/O BOARD

Cathode

PT1000

3. Photocathode behaviour at cryogenic temperature

It is well-known that the spectral response of aphotocathode depends on the operating temperature: atlow temperature the sensitivity usually increases in theshortest wavelength region and it gets worse in the longestwavelength one, near the cut-off [4–7].

In addition, because of the semiconductor nature ofphotosensitive material, the photocathode exhibits a rapidincrease of resistivity at low temperature [4–7]. Thisparameter is very important because when the sensitivearea is illuminated with an intense flux, the current in thecathode induces a local voltage drop with respect to thepower supply; the area becomes positively charged andonly few photoelectrons will overcome the electric barrier

2In the first ICARUS T600 module a minimal light detection system was

set-up with only 20 PMTs. These devices were not extensively tested before

the final installation inside the apparatus due to the forthcoming test run

of the module in summer 2001 [1]. So their characteristics are not taken

into account in the work presented in this paper.

near the surface. In this situation, if the maximumemission-current is not limited, a non-linear behaviour ofthe cathode response with respect to light flux isexperienced.At room temperature multialkali photocathodes are

fairly conductive so that good collection efficiencies canbe achieved even for cathode emission-current values ofhundreds of nanoamperes. On the other hand, theresistivity of bialkali photocathodes is very high causingpoor collection efficiencies even for cathode emission-currents as low as few nanoamperes. At low temperaturethis effect becomes remarkable. In case of a high pulserepetition rate, the resistivity induces a non-linear responseof the cathode with respect to the pulse frequency; due tothe resistivity, the local charge losses cannot be restored bythe power supply resulting in an average voltage dropbetween cathode and first dynode.The drawbacks due to the photocathode resistivity can

be avoided at manufacture by the use of nearly transparentconductive under-coatings. The 9357FLA photomultiplieradopts a bialkali K2CsSb photocathode deposited on a Ptunder-layer. The use of this PMT model to detect thescintillation light coming from noble-gas liquids requires adetailed knowledge of its photocathode behaviour atcryogenic temperature.

3.1. The experimental set-up

The comparison of the photocathode properties betweenroom and cryogenic temperature required dedicated tests.In order to simplify the experimental set-up we used two 2-in. PMTs specially designed by ETL: one with a standardK2CsSb photocathode and one with a bialkali photo-cathode on the Pt under-layer ðK2CsSbþ PtÞ.The measurements were carried out using a stainless-

steel vacuum chamber designed to house the PMT undertest (see Fig. 2). The chamber was placed in a dewar that

T

FEEDTROUGH

GPIB

DIGITALOSCILLOSCOPE

Fig. 2. Experimental set-up for the PMTs characterization at cryogenic

temperature.

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ƒ)

A. Ankowski et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 146–157 149

was filled with liquid nitrogen before the measurements at77K, leaving the upper part of the apparatus at roomtemperature. The light from an external pulsed source wasbrought to the photocathode by means of a 3m long, 1mmcore-diameter optical fibre through a vacuum feed-throughmounted in the higher part of the apparatus at roomtemperature. The fibre length (1m) inside the vacuumchamber and its position far from the cathode (20mm)were chosen in order to guarantee the projection of thelight spot on the PMT sensitive area, even in the case ofthermal contractions of the system during the coolingdown. In this situation any thermal shock on the lightsource and on the feed-through was prevented and,considering the light source intensity stable, a constantlight intensity on the cathode was assured during the fulltime of the measurement.

The chamber was filled with nitrogen gas during thecooling phase in order to enhance the heat exchangebetween the thermal bath and the PMT. The temperaturewas continuously monitored during the tests by means oftwo PT1000 platinum sensors connected to a PC acquisi-tion board. The vacuum condition was restored at the endof the cooling phase before starting any measurement.

The PMTs were operated as photodiodes. To thispurpose all the dynodes and the anode were tied togetheras a single electrode, kept at a positive voltage of about100V, enough to collect all the photoelectrons emitted bythe cathode. Signals from the cathode were amplified bymeans of a CANBERRA 2005 charge preamplifier, thenthey were shaped at 2ms by means of an ORTEC 570amplifier and finally they were fed to a digital oscilloscope;the average pulse peak was recorded by a personalcomputer by means of a GPIB board.

We used different laser diodes and LEDs as light sourcesin order to cover the whole photocathode spectral range(Table 2). They were operated in pulse mode by means of aLeCroy 9210/9214 fast pulser. The pulse width (in all casesless than 500 ns) and the amplitude were selected in orderto get from the cathode (at room temperature) a charge perpulse ranging from 10 fC to 10 pC. The pulse repetition ratevaried from about 10�3 Hz to 100 kHz.

Table 2

Light source characteristics

Source type Emission

wavelength

(nominal) (nm)

Emission

wavelength

(measured)

(nm)

FWHM

(measured)

(nm)

LED 430 444 75

LED 470 474 34

LED 525 515 40

LED 571 568 28

LED 590 591 17

Laser diode 635 629 16

The measurements were carried out using an Ocean Optics Inc. spectro-

meter.

3.2. Measurement and results

As a first step we measured the cathode signal amplitudeSK ðT ¼ 300K; l; f Þ at room temperature as a function ofthe light repetition rate f. To this purpose, once defined theworking wavelength l, we adjusted the light pulse durationand its intensity in order to get from the cathode 1 pC perpulse. Tests carried out with different light intensities didnot lead to different results. The light intensity was thenmonitored for a few hours in order to verify its stability;the variations observed in 1 day were within 1%.The chamber was then cooled down by pouring liquid

nitrogen inside the dewar. The temperature on thephotomultiplier surface was continuously monitored. Weexperimentally verified that at least 6 hours must be waitedbefore starting any measurement in order to let the cathodesurface thermalize.Afterwords we measured the cathode signal SK ðT ¼

77K; l; f Þ at cryogenic temperature using the same lightstimulation set at room temperature.The relative sensitivity Rðl; f Þ of the photocathode,

defined as

Rðl; f Þ ¼SK ðT ¼ 77K; l; f Þ

SK ðT ¼ 300K; l; f Þ(1)

was used to describe the behaviour of the photocathode atT ¼ 77K with respect to the room temperature.The relative sensitivity studied as a function of the light

pulse rate f is presented in Fig. 3 for the two photo-cathodes. The measurements were carried out using a LEDwith central emission wavelength of l ¼ 470 nm. It isevident that the ratio R of the standard bialkali ðK2CsSbÞphotocathode rapidly decreases as the pulse rate increases;at 1Hz, the electrostatic field distortions near the cathodesurface are already so high as to reduce the photoelectronyield of a factor 10, making this device useless at cryo-genic temperature. On the contrary the K2CsSbþ Pt

0.01

0.1

1

0.001 0.1 10 1000 105

Rel

ativ

e S

ensi

tivity

R (

λ=47

0 nm

,

Pulse Rate (Hz)

K2CsSb

K2CsSb+Pt

Fig. 3. Relative sensitivity vs. the light pulse rate for K2CsSb and

K2CsSbþ Pt photocathodes and l ¼ 470nm light source. The vertical

bars indicate the statistical spread (sigma) of each measurement.

ARTICLE IN PRESS

0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

e S

ensi

tivity

R (

λ , ƒ

)

400 450 500 550 600 650 700

Wavelength (nm)

Fig. 4. Relative sensitivity of the K2CsSbþ Pt photocathode vs. the light

wavelength. The horizontal bars represent the wavelength spread

(FWHM) of the light sources; the vertical bars indicate the statistical

spread (sigma) of each measurement.

PULSER

H.V. (VA)

H.V. (VK)

LASER

Anode Signal

Trigger Signal

LN2

PMT

DEWAR

OPTICALFIBRE

Fig. 5. Experimental set-up for the gain measurements.

A. Ankowski et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 146–157150

photocathode shows a quite flat reduction of about 20% atvery low pulse rates up to 100 kHz. This effect could beascribed to a worsening at low temperature of the opticalmatching between cathode and under-layers.

Considering the available light sources (see Table 2), weinvestigated the behaviour at cryogenic temperature of theK2CsSbþ Pt photocathode as a function of the lightwavelength l.

The results, presented in Fig. 4, are characterized by asmall reduction of the photocathode sensitivity forwavelength values from 400 to 500 nm and by a continuousdecrease above this range. Tests carried out with differentlight intensities (10 fC to 10 pC per pulse) and repetitionrates (1Hz to 100 kHz) did not show different results.

4. Devices characterization at cryogenic temperature

A test was performed on 54 ETL 9357FLA PMTs tocertify the devices before the wavelength shifter depositionand the final assembling in the ICARUS T600 module. Inthe following we describe the methods and the measure-ments carried out for each PMT sample in order to ratethe characteristics which are temperature dependent. Theobtained results assume statistical significance of theoperating properties of this PMT model at cryogenictemperatures.

4.1. Experimental set-up

The test set-up made use of a dedicated sustainingstructure in which we mounted the PMT under test, withits voltage divider, an optical fibre light guide and twoelectric cables, as shown in Fig. 5. The structure was insertedin a stainless-steel cylindrical dewar (30 cm internal diameter,80 cm high) which was filled with liquid nitrogen during themeasurements at cryogenic temperature. The thermalinsulation and the darkness conditions inside the dewar were

preserved by means of a proper cap, through which theelectrical cables and the optical fibre were allowed to enter.The PMT was provided with a voltage divider (base)

able to distribute the proper electrical potential to theelectrodes both at room and at cryogenic temperature. Thedividers were realized using thick-film resistors ðR ¼ 1MOÞmounted on printed-circuit boards which were soft-soldered directly on the PMT output leads. We conformedthe voltage distribution to the following outline:

�

ground on the first dynode VD1 ¼ 0V; � fixed negative voltage on the cathode V K ¼ �600V; � changeable positive voltage on the anode VA ¼

80021100V;

� voltage on the dynode-chain distributed by the divider

suggested by the manufacturer ð2RðD1�D2Þþ2R ðD2�D3Þ þ RðD3�D4Þ þ RðD4�D5Þ þ RðD5�D6ÞþRðD6�D7ÞþRðD7�D8Þ þ RðD8�D9Þ þ RðD9�D10ÞþRðD10�D11ÞþRðD11�D12Þþ RðD12� AÞÞ;

� voltage on the focusing grid VG ¼ VD3.

We used two independent HV power supplies, one for thecathode and one for the dynode-chain. The high voltageswere brought to the base of the PMT under test by meansof two RG316 cables; the PMT output was directly derivedfrom the line to the anode (A) by means of a 10 nFdecoupling capacitor.We used as light sources a blue LED with a center

emission wavelength of 470 nm for gain measurements anda laser diode emitting in the red region at a nominalwavelength of 635 nm for fast timing measurements (seeTable 2). The diodes were excited using a LeCroy 9210/9214 fast pulser in order to get 50 ns width per light pulsewith the LED and a 300 ps width per light pulse with thelaser. The repetition rate was kept between 1 and 100 kHz.The light was brought to the photocathode of the PMTunder test from the output window of the source by means

ARTICLE IN PRESSA. Ankowski et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 146–157 151

of a multimode optical fibre light guide with a corediameter of 1mm, 2m long.

The measurements at cryogenic temperature were carriedout at 77K with the device under test directly immersed inliquid nitrogen. We adopted a fast cooling procedure inorder to prevent temperature gradients on the PMT bodythat could cause glass cracks or implosions. For thispurpose we inserted manually the sustaining structure inthe dewar previously filled with liquid nitrogen. Thereafter,the device was left 3 days in cryogenic conditions beforebeginning any measurement. Only two PMTs sufferedfrom the thermal shock from ambient to cryogenictemperature, imploding after immersion in liquid nitrogen.Two new samples were purchased in order to keep thenumber of 54 PMTs functioning in the second module ofthe ICARUS detector.

4.2. Anode pulse shape

We recorded the shape of the anode pulses, with the PMToperating at cryogenic temperature, and at a multiplier gainof about 107, feeding directly the anode output to the 50Oinput of a 1GHz oscilloscope; a typical shape for singlephotoelectron illumination (blue LED) is shown in Fig. 6.

A set of single photoelectron pulses were recorded todetermine the signal parameters. The result of thesemeasurements is: a mean amplitude of 12:5� 3:7mV, aleading edge of 3:9� 1:1 ns, a FWHM of 7:3� 1:4 ns and atrailing edge of 10:7� 1:6 ns, in good agreement with thenominal values which characterize the PMT performanceat room temperature (see Table 1). All the tested PMTsshow variation at cryogenic temperature less than 10%from the nominal values declared by the manufacturer.

4.3. Single photoelectron response

The response of a PMT to single photoelectrons (SER),i.e., the charge distribution of the PMT pulses integrated

-12

-10

-8

-6

-4

-2

0

2

0 20 40 60 80 100

Am

plit

ude (

mV

)

Time (ns)

Fig. 6. Typical shape of the anode pulses with the photomultiplier

operating at cryogenic temperature.

over the whole signal shape, is directly related to the PMTgain processes which can be affected by the operatingtemperature. So our SER studies were carried out bymeasuring the charge distribution of the PMT pulsesinduced by single-electron excitation and comparing thedata obtained at room and at cryogenic temperature.The first step of the procedure for the SER measure-

ments required the precise achievement of the actual single-photon illumination conditions of the PMT. This wasdetermined using the following procedure [8,9]: assuming anumber of photoelectrons leaving the PMT cathodedescribed by a Poisson distribution, we can write

Pð2Þ

Pð1Þ¼

m2

where Pð2Þ and Pð1Þ are the probabilities to detect two orone photoelectron, respectively, and m is the mean value ofdetected photoelectrons. In order to keep the PMTmultiphotoelectron probabilities low (less than 1%) it isthus necessary to have mp0:02. This was achieved bycomparing the PMT anode pulse rate with the triggeringrate from the pulser and adjusting the intensity of theexcitation of the light source.We used an electronics set-up in which the PMT output

was integrated by means of a CANBERRA 2005 chargepreamplifier and shaped ð5 msÞ by means of an ORTEC 570amplifier. The output distribution was recorded by meansof a multichannel analyzer.The HV power supply to the anode was adjusted at room

and at cryogenic temperature in order to operate amultiplier gain of about 107, which represents the nominalgain of the photomultiplier. In Fig. 7 typical measuredspectra for two different temperature values are presented.The charge distributions are characterized by peak

profiles, caused by the linear-focusing type structureadopted in this device, which features a high chargeresolution. In order to compare the plots taken at differenttemperature values, the data were fitted by means of ananalytical expression which consists of two parts, assuggested in Ref. [8]:

(1)

a Gaussian convoluted exponential distribution whichtakes into account the PMT dark counts and theelectronic noise affecting the measurement;

(2)

a Gaussian distribution which takes into account theGaussian response of the PMT

SERðX Þ ¼A0

2R1þ ERF

RX 0 � s20ffiffiffi2p

Rs0

� �� �

�EXPs20 � 2RðX � X 0Þ

2R2

� �

þA1ffiffiffiffiffiffi2pp

s12 1þ ERF

X 1ffiffiffi2p

s1

� �� ��1

�EXP �1

2

X � X 0 � X 1

s1

� �2 !

ð2Þ

ARTICLE IN PRESS

0

2

4

6

8

10

12

14

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Sam

ples

P/V

300 KTemp.1.52Min3.44Max

54Points2.38Mean0.52RMS

50

2

4

6

8

10

12

14

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Sam

ples

P/V

77 KTemp.0.98Min3.2Max.54Points

1.86Mean0.54RMS

5(a) (b)

Fig. 8. Peak-to-valley ratio P=V distributions at room (a) and at cryogenic temperature (b).

0

200

400

600

800

1000

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 0.5 1 1.5 2 2.5 3 3.5

Cou

nts

Charge (pC)

T = 300 KVK = 600 V

VA = 930 V

Gain (x107)

0

200

400

600

800

1000

0 0.5 1 1.5 2 2.5 3 3.5

Cou

nts

Charge (pC)

T = 77 K VK = 600 V

VA = 1070 V

Gain (x107)

(b)(a)

Fig. 7. Charge distribution of 5� 105 anode signals under single-photon illumination (SER) for two different operating temperatures: (a) 300K; (b) 77K.

A. Ankowski et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 146–157152

where

�

A0 is the number of events (noise) under the exponentialpart of the distribution; � R is the slope of the exponential part of the distribution; � s20 is the pedestal variance due to electronic noise; � X 0 is the pedestal value; � A1 is the number of events (signal) under the Gaussian

part of the distribution;

� s21 is the signal variance; � X 1 is the Gaussian peak position.

The PMT distributions of the peak-to-valley ratio P=V ,defined as the SER peak value divided by the minimumvalue to the left of the peak, and the relative variance to thepeak

ffiffisp¼ ðs1=X 1Þ obtained from the fitting are presented

in Figs. 8 and 9, respectively. The results demonstrate thegood performance in term of SER charge resolution of thePMTs operating at a multiplier gain of about 107, both at

room and at cryogenic temperature. It can be noted a slightlowering of the peak-to-valley ratio at 77K.

4.4. Linearity

The evaluation of the linearity of the PMT response tolight pulses containing more than one photoelectron wascarried out in pulse mode by means of neutral densityfilters assembled in a rotating support positioned betweenthe light source (LED) and the optical fibre light guide. Thelight intensity was set to the maximum excitation value(5V) and the pulse repetition rate was fixed to 10 kHz.Considering the attenuation factors offered by the filters(see Table 3), it was possible to vary the light intensitybrought to the photocathode by three orders of magnitude.As a first step we measured the output charge distribu-

tion using the maximum attenuation filter. An examplecarried out at cryogenic temperature is reported in Fig. 10.Besides the peak of the first photoelectron is clearly visible

ARTICLE IN PRESS

0

200

400

600

800

1000

0 1 2 3 4 5 6 7 8

Cou

nts

Amplitude (phe)

T = 77 K V

K = 600 V

VA = 1070 V

Fig. 10. Anode signal distribution under multiphoton illumination

ðT ¼ 77KÞ.

0

2

4

6

8

10

12

14

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Sam

ples

Rel. Variance

300 KTemp.0.27Min0.69Max

54Points0.43Mean0.10RMS

10

2

4

6

8

10

12

14

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Sam

ples

Rel. Variance

77 KTemp.0.24Min1.70Max

54Points0.42Mean0.21RMS

1(a) (b)

Fig. 9. Relative variance to the peakffiffisp¼ ðs1=X 1Þ distributions at room and at cryogenic temperature.

Table 3

Characteristics of the neutral density filters

Filter no. Optical density Attenuation factor

(nominal) (measured)

0 0.0 1.00

1 0.5 3:19� 0:022 1.0 10:27� 0:113 1.5 31:5� 0:74 2.0 104� 6

5 3.0 1055� 11

The attenuation factor measurements were carried out using a calibrated

photodiode.

A. Ankowski et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 146–157 153

the second photoelectron peak and the bump of the thirdphotoelectron. The charge profile was fitted by means of ananalytical expression in which five additional Gaussianterms were added to Eq. (2) with the following parameter

constraints:

X N ¼ NX 1; sN ¼ s1ffiffiffiffiffiNp

; AN ¼mN

AN�1

where X N is the position of the Nth Gaussian curve withsN width and AN amplitude, m is the mean value ofdetected photoelectrons. The resulting curve is drawn inFig. 10, where the individual photoelectron terms are alsoplotted. The m value resulting from the fit (m ¼ 2:3 phe inthis example) was used to compute the ideal PMT response(ideal signal amplitude) as a function of the light intensity,i.e., the number of photoelectrons detected by a full-lineardevice for each available attenuation filter.The output charge distribution was then acquired

increasing the light intensity and the corresponding meannumber of detected photoelectrons was measured for eachfilter (measured signal amplitude). In Fig. 11 the measuredsignal amplitude is plotted as a function of the ideal one atroom and at cryogenic temperature, using the same PMTsample. The results demonstrate the good performance interm of response linearity up to about 300 photoelectronsof the PMT operating at a multiplier gain of about 107,both at room and at cryogenic temperature. Tests carriedout with different light pulse repetition rates (10Hz to100 kHz) did not show different results.

4.5. Gain

The study of PMT gain dependence on the operatingtemperature was carried out by measuring the SER fordifferent dynode-chain voltages at room and at cryogenictemperature. Data were fitted using expression (2) and thevalue of the peak position X 1 was assumed as the operatinggain value.In Fig. 12 the results obtained at room and at cryogenic

temperature, using the same PMT sample, are plotted. Thesignal amplitude is well represented by an exponentialbehaviour as a function of the anode voltage for both

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106

107

750 800 850 900 950 1000 1050 1100 1150

0.2

0.4

0.60.81

3

5

7

Gai

n

Anode voltage (V)

T = 300 K

T = 77 K

Signal am

plitude (pC)

Fig. 12. Signal amplitude as a function of the anode voltage at room and

at cryogenic temperature. For each measurement the peak position (black

dots) and the SER distribution width (vertical bars) coming from equation

2 fitting are shown.

0

2

4

6

8

10

-100 -90 -80 -70 -60 -50 -40 -30 -20 -10

Sam

ples

Gain variation (%)

-80Min-15Max54Points

-50Mean

0

Fig. 13. Distribution of the gain losses from room to cryogenic

temperature.

1

10

100

1000

104

1 10 100 1000 104

Mea

sure

d si

gnal

am

plitu

de (

phe)

Ideal signal amplitude (phe)

T = 300 K

1

10

100

1000

104

1 10 100 1000 104

Mea

sure

d si

gnal

am

plitu

de (

phe)

Ideal signal amplitude (phe)

T = 77 K

(a) (b)

Fig. 11. Measured PMT signal as a function of the ideal one for two different operating temperatures: (a) 300K; (b) 77K. The horizontal and vertical bars

represent the measurement error induced by the spread of the light source and by the attenuation factor uncertainty.

A. Ankowski et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 146–157154

operating temperatures, with an appreciable reduction ofthe multiplier gain at 77K. Besides, measurements takenafter a few cooling down and warming up of the same PMTsample did not shown hysteresis effects on the multipliergain.The distribution of the reduction coefficient for the

whole set of 54 devices is plotted in Fig. 13. It can be notedthat all the tested PMTs suffered a gain reduction atcryogenic temperature.Similar behaviour was observed for all the tested PMTs.

Taking into consideration a nominal multiplier gain of 107

at room temperature, the samples showed wide variationsof gain losses at cryogenic temperature, ranging from�15% to �80%, as plotted in Fig. 13. Anyway, an increaseof about 100V of the anode voltage was always enough torecover the original gain factors.

4.6. Dark counts and spectrum

We measured the PMT dark count rate at roomtemperature and at cryogenic temperature. To this purposethe output pulses coming from the PMT under test,operating in complete darkness condition, were 20 dBamplified, discriminated and counted during repetitivecycles in which the discrimination threshold was adjustedfrom 1 to 255mV (2mV step), corresponding to about twophotoelectrons (phe) wide range. An example of countrates, as a function of the discrimination threshold and fortwo different operating temperatures are shown in Fig. 14.The curves were acquired fixing the PMT power supply tothe value corresponding to a multiplier gain of about 107 atroom temperature ðV A ¼ 930VÞ.The spectrum structure is characterized by a clear bump

profile centered in the region around one photoelectron,caused mainly by the cathode dark noise. This increase ofthe counting rate at cryogenic temperature was experiencedfor all the tested PMTs. The reason could be ascribed to adecrease of the lattice-energy of the cathode material at low

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0

2

4

6

8

10

12

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Sam

ples

Dark Rate (kHz)

611Min.1782Max

54Points1142Mean

235RMS

2

Fig. 15. Distribution of the dark count rate at 0.2 phe threshold for the 54

tested PMTs at cryogenic temperature.

101

102

103

104

0 0.5 1 1.5 2

Rat

e (H

z)

Threshold (phe)

T = 77 K

T = 300 K

Fig. 14. PMT dark count rate as a function of the discrimination

threshold at room and at cryogenic temperature. The threshold valued is

normalized to fraction of photoelectron (phe).

3It is worth noting that the contribution to the transit time spread due to

the different illumination positions on the PMT window surface was not

taken into account in the analysis because of the difficulty to evaluate the

illuminated area and the spot position on the photocathode with the PMT

immersed in liquid nitrogen.

A. Ankowski et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 146–157 155

temperature resulting in an increase of the electronescaping probability [10].

The measurement distributions over the 54 tested samplesoperating at 107 nominal gain, shown in Fig. 15, gave verylow counting rates at room temperature (mean ¼ 700Hz,RMS ¼ 175Hz at 0.2 phe threshold) with a sensible risewhen operating in liquid nitrogen (mean ¼ 1142Hz,RMS ¼ 235Hz at 0.2 phe threshold, see Fig. 15).

4.7. Transit time spread distribution

The adopted procedure for the transit time spreadstudies required the achievement of the single-photonillumination conditions of the PMT, as previously de-scribed for the SER measurement. For each event, the lasertrigger signal was used to start a time-to-amplitudeconverter (TAC)ORTEC 566; the PMT pulse, properlydiscriminated by means of a constant-fraction discrimina-tor (CFD)ORTEC 934 was used to stop the TAC. The

CFD threshold was fixed to about 0.1 photoelectrons andthe TAC output distribution was recorded by means of amultichannel analyzer. The system allowed the measure-ment of the start–stop delay distribution with 25 ps/chresolution.The transit time spread was measured at room tempera-

ture for a nominal PMT gain of about 107. Themeasurement was then carried out at cryogenic tempera-ture both at the same power supply and then restoring thenominal gain. Typical results are shown, in logarithmicscale, in Fig. 16.Five features of the recorded spectra must be under-

lined:3 (1) in each distribution it is evident the presence of amain narrow peak; (2) a reduction of the transit timespread at cryogenic temperature; (3) the peak profilefollows a Gaussian shape only at room temperature and,at cryogenic temperature, it differs evidently in the rightside from the best fitted curve; (4) there is no presence ofpre-pulses; (5) there is a negligible presence of late-pulses(about 5:2% of the total in each cases) which produce asmall satellite peak displaced about 50 ns from the mainone.A figure of merit for the time spread estimation can be

derived from the full width at half maximum (FWHM) ofeach distribution. The resulting values are: (a) 2.450 ns at300K and 107 gain; (b) 1.450 ns at 77K and 2� 106 gain;(c) 1.200 ns at 77K and 107 gain. These results demonstratethe intrinsic low transit time spread of the PMT in bothtemperature conditions.

4.8. After-pulses

After-pulse events are usually present in the response ofa photomultiplier and can induce systematic errors in lightintensity measurements. These pulses have been studiedand their ionic origin was experimentally determined bymeans of the time-of-flight (TOF) analysis described inRef. [11]. Since their presence is connected to the deviceinternal vacuum level, an investigation about a possibletemperature dependence deserves some interest.The experimental set-up used for the TOF analysis was

the same adopted for the transit time spread study, with theadditional use of a timing veto circuit (200 ns) in order toavoid the untimely TAC stops induced by the PMT promptsignals. In order to work with a high repetition rate, ifcompared to the dark noise, and with a low stochasticcoincidence rate, the discriminator threshold was fixed to1.5 photoelectrons and the illumination rate was set to50 kHz.Examples of TOF distributions measured at room and at

cryogenic temperatures are shown in Fig. 17. The two

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0

0.05

0.1

0.15

0.2

0.25

0 500 1000 1500 2000 2500 3000 3500 4000

Cou

nts

(A.U

.)

Time (ns)

T = 300 K VK = 600 V

VA = 930 V

Th = 1.5 phe

Low mass

Heavy mass

0

0.05

0.1

0.15

0.2

0.25

0 500 1000 1500 2000 2500 3000 3500 4000

Cou

nts

(A.U

.)

Time (ns)

T = 77 KVK = 600 V

VA = 1070 V

Th = 1.5 phe

Low mass

Heavy mass

(a) (b)

Fig. 17. Time distribution (TOF) of the PMT after-pulse events for two different operating temperatures: (a) 300K; (b) 77K.

1

10

100

1000

104

50 60 70 80 90 100 110 120

Cou

nts

Time (ns)

T = 300 KVK = 600 V

VA = 930 V

G = 107

T = 77 KVK = 600 V

VA = 930 V

G = 2 x 106

T = 77 KVK = 600 V

VA = 1070 V

G = 107

1

10

100

1000

104

50 60 70 80 90 100 110 120

Cou

nts

Time (ns)

1

10

100

1000

104

50 60 70 80 90 100 110 120

Cou

nts

Time (ns)

(a)

(b) (c)

Fig. 16. Log plots of the PMT transit time distribution for different operating temperatures and gains: (a) T ¼ 300K, G ¼ 107; (b) T ¼ 77K,

G ¼ 2� 106; (c) T ¼ 77K, G ¼ 107.

A. Ankowski et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 146–157156

plots, expressed in arbitrary units (A.U.), are normalized tothe total number of photoelectrons seen by the PMT.

In each distribution several bumps can be noted that canbe ascribed to residual gases with different ionization state(Hþ, Hþ2 , Heþ, Oþ, Oþþ . . . , see Ref. [11]). An evaluation

of the after-pulse probability is reported in Table 4. Thedistributions were divided in three consecutive timeintervals: 20021000 ns mainly ascribed to low masscontaminators; 100024000 ns due to heavier ions;400028000 ns where the presence of random coincidence

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Table 4

Measurement of the after-pulse event probability

Time interval

(ns)

After-pulse probability

(300K)

After-pulse probability

(77K)

20021000 ð2:5� 0:5Þ � 10�4 ð2:0� 0:5Þ � 10�4

100024000 ð2:5� 0:5Þ � 10�3 ð1:5� 0:4Þ � 10�3

400028000 ð1:5� 0:4Þ � 10�3 ð1:5� 0:4Þ � 10�3

20028000 ð4:2� 0:6Þ � 10�3 ð3:2� 0:6Þ � 10�3

A. Ankowski et al. / Nuclear Instruments and Methods in Physics Research A 556 (2006) 146–157 157

dominates. Counts were then divided by the total numberof photoelectrons and the corresponding statistical errorswere evaluated. The result shows that the studied PMT islittle affected by this process. It is worth noting the generaldecrease, at low temperature, of the total after-pulse rateand the disappearing of some bumps, especially in the TOFrange between 1000 and 4000 ns. This could mean animprovement of the vacuum state inside the PMT at lowtemperature, probably due to a sticking effect on someresidual elements with boiling point greater than 77K(oxygen, for example).

5. Conclusions

The major characteristics of the Electron Tubes Ltd(ETL) 9357 FLA photomultiplier when operating atcryogenic temperature ðT ¼ 77KÞ, can be summerized:

�

good capability to withstand the thermal shock fromroom to cryogenic temperature with a loss rate of abouttwo devices over 50; � good suitability of the bialkali photocathode on Pt

under-layer to work and detect the light both at roomand at cryogenic temperature, with a sensitivity decreaseof about 20% ðl ¼ 470 nmÞ;

� rise time ð3:9� 1:1 nsÞ and FWHM ð7:3� 1:4 nsÞ in

good agreement with the nominal values;

� SER charge resolution characterized by a P/V ratio of

1:86� 0:54, a worsening of about 22% with respect toroom temperature;

�

good linearity up to about 300 photoelectrons both atroom and at cryogenic temperature; � wide variation of gain losses, ranging from �15% to�80%, between room and cryogenic temperature; � dark noise rate of 1142� 235Hz, an increase of about

70% relative to room temperature;

� transit jitter of about 1.2 ns, a factor two better than

that measured at room temperature;

� a slight decrease of after-pulse rate between room and

cryogenic temperature.

These characteristics demonstrate the PMT suitability forlight detection in such unconventional operating condi-tions, certifying this device for the direct measurement ofscintillation light coming from noble-gas liquids in large-scale detectors.

Acknowledgements

The authors are grateful to Electron Tubes Ltd for thefruitful discussions, the technical advices and the usefulinformation received during the entire period of the testactivity. The Polish groups acknowledge the support of theState Committee for Scientific Research in Poland, 105,160, 620, 621/E-344, E-340, E-77, E-78/SPS/ICARUS/P-03/DZ211-214/2003-2005; the INFN, FAI program; theEU Commission, TA-DUSL-P2004-08-LNGS; the PolishAcademy of Sciences.

References

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[10] A.H. Sommer, Photoemissive Materials: Preparation, Properties, and

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