ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 614 (2010) 206–212

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Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

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journal homepage: www.elsevier.com/locate/nima

Procedures and results of the measurements on large area photomultipliersfor the NEMO project

S. Aiello d, E. Leonora m,�, A. Aloisio o, F. Ameli i, I. Amore a,j, M. Anghinolfi e, A. Anzalone a, G. Barbarino o,E. Barbarito k, M. Battaglieri e, M. Bazzotti l, R. Bellotti k, A. Bersani n, N. Beverini p, S. Biagi l, M. Bonori q,B. Bouhdaef p, G. Cacopardo a, C. Calı a, A. Capone q, L. Caponetto m, G. Carminati l, B. Cassano b, A. Ceres b,T. Chiarusi c, M. Circella b, R. Cocimano a, R. Coniglione a, M. Cordelli f, M. Costa a, A. D’Amico a,G. DeBonis p, G. DeRosa g, G. DeRuvo b, R. DeVita e, C. Distefano a, V. Flaminio p, K. Fratini e, A. Gabrielli l,S. Galeotti h, E. Gandolfi l, G. Giacomelli l, F. Giorgi l, G. Giovanetti a, A. Grimaldi d, A. Grmek a, R. Habel f,M. Imbesi a, A. Lonardo i, D. LoPresti m, F. Lucarelli q, A. Margiotta l, A. Marinelli p, A. Martini f,R. Masullo q, F. Maugeri a, E. Migneco a, S. Minutoli e, M. Mongelli b, M. Morganti p, P. Musico e,M. Musumeci a, A. Orlando a, M. Osipenko e, R. Papaleo a, V. Pappalardo a, P. Piattelli a, D. Piombo e,F. Raffaelli p, G. Raia a, N. Randazzo d, S. Reito d, G. Ricco n, G. Riccobene a, M. Ripani e, A. Rovelli a,M. Ruppi k, G.V. Russo m, S. Russo o, P. Sapienza a, M. Sedita a, E. Shirokov e, F. Simeone q, D. Sciliberto d,V. Sipala m, C. Sollima p, M. Spurio l, F. Stefani h, M. Taiuti n, G. Terreni h, L. Trasatti f, S. Urso d, M. Vecchi q,P. Vicini i, R. Wischnewski i

a Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Sud, Via S. Sofia 62, 95123 Catania, Italyb INFN Sezione di Bari, Via E. Orabona 4, 70126 Bari, Italyc INFN Sezione di Bologna, V.le Berti Pichat 6/2, 40127 Bologna, Italyd INFN Sezione di Catania, Via S. Sofia 64, 95123 Catania, Italye INFN Sezione di Genova, Via Dodecaneso 33, 16146 Genova, Italyf Laboratori Nazionali di Frascati INFN, Via Enrico Fermi 40, 00044 Frascati (RM), Italyg INFN Sezione di Napoli, Via Cintia, 80126 Napoli, Italyh INFN Sezione di Pisa, Polo Fibonacci, Largo B. Pontecorvo 3, 56127 Pisa, Italyi INFN Sezione di Roma1, P.le A. Moro 2, 00185 Roma, Italyj Dipartimento di Fisica e Astronomia, Universita‘ di Catania, Via S. Sofia 64, 95123 Catania, Italyk INFN Sezione di Bari and Dipartimento Interateneo di Fisica Universita‘ di Bari, Via E. Orabona 4, 70126 Bari, Italyl INFN Sezione di Bologna and Dipartimento di Fisica Universita‘ di Bologna, V.le Berti Pichat 6/2, 40127 Bologna, Italym INFN Sezione di Catania and Dipartimento di Fisica e Astronomia Universita‘ di Catania, Via S. Sofia 64, 95123 Catania, Italyn INFN Sezione di Genova and Dipartimento di Fisica Universita‘ di Genova, Via Dodecaneso 33, 16146 Genova, Italyo INFN Sezione di Napoli and Dipartimento di Scienze Fisiche Universita‘ di Napoli, Via Cintia, 80126 Napoli, Italyp INFN Sezione di Pisa and Dipartimento di Fisica Universita‘ di Pisa, Polo Fibonacci, Largo B. Pontecorvo 3, 56127 Pisa, Italyq INFN Sezione di Roma1 and Dipartimento di Fisica Universita‘ di Roma ‘‘LaSapienza’’, P.le A. Moro 2, 00185 Roma, Italy

a r t i c l e i n f o

Article history:

Received 16 September 2009

Received in revised form

9 December 2009

Accepted 10 December 2009Available online 22 December 2009

Keywords:

Photomultiplier

Large area photo-detector

Neutrino telescope

02/$ - see front matter & 2009 Elsevier B.V. A

016/j.nima.2009.12.040

esponding author. Tel.: +39 095 378 5285.

ail address: emanuele.leonora@ct.infn.it (E. Le

a b s t r a c t

The selection of the photomultiplier plays a crucial role in the R&D activity related to a large-scale

underwater neutrino telescope. This paper illustrates the main procedures and facilities used to

characterize the performances of 72 large area photomultipliers, Hamamatsu model R7081 sel. The

voltage to achieve a gain of 5�107, dark count rate and single photoelectron time and charge properties

of the overall response were measured with a properly attenuated 410 nm pulsed laser. A dedicated

study of the spurious pulses was also performed. The results prove that the photomultipliers comply

with the general requirements imposed by the project.

& 2009 Elsevier B.V. All rights reserved.

ll rights reserved.

onora).

1. Introduction

The NEMO collaboration [1] has been working for several yearson R&D activities aimed at developing and validating technologies

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Fig. 1. Schematic view of the experimental setup to illuminate the whole PMT.

S. Aiello et al. / Nuclear Instruments and Methods in Physics Research A 614 (2010) 206–212 207

for a cubic-kilometre scale underwater neutrino telescope in theMediterranean Sea. The basic principle, already used in otherlarge-scale detectors such as ANTARES [2] and NESTOR [3] is toreconstruct the direction of high-energy neutrinos by detectingthe Cherenkov light emitted in water by the muons produced inneutrino interactions with the matter surrounding the detector.Moreover, the ICECUBE [4] project has been working for severalyears on the development of a large detector in the ice at theSouth Pole. In order to detect the Cherenkov light, a lattice ofoptical modules (OM) was deployed in deep sea. It consisted in apressure resistant glass sphere which contains a large photo-multiplier (PMT) embedded in silicon gel to ensure mechanicaland optical coupling [5]. A high permittivity metal cagesurrounded the PMT, shielding it from the Earth’s magnetic field[6]. The measurements of the intensity and of the arrival time ofthe light on each optical module combined with the knowledge ofits position allowed a muon track reconstruction with a precisionof a few tenths of a degree. Simulation results have imposedsevere constraints on the characteristics of the photomultiplier, ifthe desired aims of the neutrino telescope are to be achieved. Theselection of the photomultiplier was a crucial point for everyproject and required an intensive phase of tests and develop-ments, performed in close collaboration with the main PMTproducers. This paper describes the procedures and facilities usedto characterize the performance of 72 Hamamatsu photomulti-pliers, model R7081 sel., with 10 in. photocathode and 10 stages.

Fig. 2. A typical single photoelectron charge spectrum.

2. Requirements for the photomultipliers

The reconstruction of the neutrino direction is based on thedetection of the Cherenkov light by each optical module, typicallyof the order of few photons. Measurements and simulationsindicate that, to reach the planned goals of the neutrino telescope,the photomultipliers must meet the following specifications [7].

2.1. Photocathode requirements

The effective detection area of an optical module is animportant parameter which has a great impact on the layoutand the cost of the telescope.

The effective surface of a photomultiplier can be considered asthe product of its geometrical surface, quantum efficiency andcollection efficiency [8]. Therefore the photocathode surface mustbe as large as possible larger than 500 cm2, while the quantumefficiency should peak at more than 20% in the spectral range380–420 nm [9].

2.2. Gain and nominal voltage

The electronic read-out used today to sample the photomul-tiplier response requires a single photoelectron amplitude of atleast of 50 mV on a 50O load. This means that the gain must be ofthe order of 5�107. The nominal voltage to reach this gain shouldbe less than 2000 V, to ensure long-term stability, to avoid agingproblems, and to reduce the power consumption of the wholetelescope.

2.3. Peak-to-valley ratio and charge resolution

In order to correctly identify a single photoelectron pulse fromthe pedestal noise, a peak-to-valley ratio greater than 2 isrequired. The charge resolution, calculated as the sigma of thepeak in the single photoelectron charge spectrum, must be lessthan 50%.

2.4. Transit time spread (TTS)

The resolution of the Cherenkov light arrival time has a severeimpact on the angular resolution of the detector. The results of aMonte Carlo study of the angular resolution in a referencedetector [7] stated that to reach the goal of a telescope angularresolution better than 0.11, a single photoelectron arrival timeresolution less than 2 ns (RMS) was required, considering thecontributions of the photomultiplier, and the electronic and theposition measurements. To this aim, the NEMO collaboration hasstated that the Transit Time Spread of each photomultiplier has tobe not greater than 3 ns, calculated as the FWHM of the singlephotoelectron transit time spectrum.

2.5. Dark count rate

The decay of radioactive 40K and the presence of biolumines-cence make sea water an environment with optical noise. Thephotomultiplier dark current should not increase this noise. To

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S. Aiello et al. / Nuclear Instruments and Methods in Physics Research A 614 (2010) 206–212208

this end, the photomultiplier dark count rate is required to be lessthan the 20% of the expected 40K rate [7]. In the marine siteselected for the detector deployment, an average rate of about30 KHz of optical noise was measured at a depth of 3000 m and fora 10 in. PMT at 0.5 single photoelectron threshold [10].

2.6. Spurious pulses

Spurious pulses are noise pulses, time-correlated with the PMTmain response, which can appear in the wake or in place of the truepulses. They can deteriorate the time response or be confused withreal physical events, preventing the muon direction from beingcorrectly reconstructed. They can be divided into four differentgroups: pre-pulses, late pulses, and type 1 and type 2 after pulses

�

Pre-pulses appear in the 10–80 ns interval time before the mainpulse and take the place of the main pulse. They arise from adirect photo-effect on the first dynode due to the photons whichpass the photocathode without photoelectron conversion [11]. � Late pulses appear in the 10–80 ns interval time after the main

pulse and take the place of main pulse. They are caused bybackscattered photoelectrons from the first dynode [12].

� Type 1 after pulses appear in the 10–80 ns interval time after

the main pulse, in the wake of the main pulse. They areproduced by a luminous reaction on the electrodes bombardedby the electrons.

Fig. 3. Distribution of the nominal voltage measurements.

Fig. 4. Left (a) Raw data of the dark count rate as function of time

�

Type 2 after pulses arrive in the 80 ns–16 �ıs interval time afterthe main pulse, in the wake of the main pulse. They are causedby the ionization of residual gases left or formed inside thePMT. These gas atoms can be ionized by electrons and, sincethey are of opposite charge, will accelerate back towards thephotocathode where they can produce further photoelectrons[13]. Common ions are H2

+, He+, CH4+, and Cs+ [14].

The relation between spurious and true pulses was quantifiedby the ratio of the number of spurious pulses to the number of themain pulses.

3. The testing facility

A system was designed to measure the time and chargeproperties of the overall response of each PMT illuminating thewhole photocathode surface with a pulsed laser beam, as shownin Fig. 1. The photomultiplier under test was placed in a blackplastic box (1 m�0.5 m�0.5 m) in order to shield theenvironmental light. The laser source was located outside thebox, and its light was split into two optical beams with an 80/20ratio between the outputs. The lower output was guided insidethe test box while the higher output was sent to a monitor PMT tocheck the stability of the laser intensity during all measurements.In order to spread the laser beam to illuminate the wholephotocathode surface, a calibrated optical diffuser has beenapplied at the extremity of the optical fibre inside the dark box,in front of the photomultiplier. The PMT was positioned vertically,with its active area aligned perpendicularly to the optical axis ofthe fibre. The distance between the light source and the PMT headwas 60 cm. In these conditions the whole photocathode surfacewas illuminated. The light source was a 410 nm pulsed laserproduced by PICOQUANT, with a 60 ps width and a frequencywhich can vary up to 40 MHz using an external pulse generator.

In order to set the signals and acquire the data, NIMelectronics, time to amplitude converters (TAC), and chargeamplitude converter (QDC) were used. A LeCroy 6100A 1 GHzdigital oscilloscope was also used to measure the pulse shape,charge, and amplitude of the PMT response.

4. Measurement procedures and results

This section describes the measurements made to characterizeeach photomultiplier and exhibits the results. In dark conditionthe dark count rate was measured. The voltage to achieve a gain of

for two PMTs in test. Right (b) Dark count rate measurements.

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Fig. 5. Left: A typical spectrum of the s.p.e. relative transit time. Right: Distribution of the Transit Time Spread measurements.

Fig. 6. Left: Distribution of the P/V measurements. Right: Distribution of the sigma charge resolution measurements.

Fig. 7. Typical time distribution of pre- and late pulses.

S. Aiello et al. / Nuclear Instruments and Methods in Physics Research A 614 (2010) 206–212 209

5�107, the time and the charge properties at the nominalvoltages were measured for each PMT illuminating the wholephotocathode with a pulsed laser beam in single photoelectroncondition. All measurements were carried out on the PMTspowered by the ISEG-PHQ7081 active base el. based on NEMOrequirements, at room temperature and atmospheric pressurewaiting at least 6 h after the dark adaptation in the box.

4.1. Calibration of the laser source in single photoelectron (s.p.e.)

condition

In order to measure the PMT response to a single photoelec-tron, the pulsed laser was attenuated so that the probability ofeach light pulse producing only one photoelectron is muchgreater than the probability of its gives rise more than one.

Assuming that fluctuations in the number of photons per laserpulse follows the Poisson distribution [14], the s.p.e. conditionwas obtained by attenuating the laser intensity so that less thanone anode pulse occurs per hundred incident light pulses. In thiscondition, the ratio between the probability of more than onephotoelectron (p.e.) being emitted and the probability that onlyone p.e. being emitted is less than 5�10�3 [14]. The followingpicture shows a typical single photoelectron charge spectrum,acquired by a calibrated charge to amplitude converter, by usingfor this acquisition a fast amplifier (LeCroy 612A) for the anoderesponse, as described in Section 4.5.

The typical PMT response amplitude was in the 45 and 55 mVrange, with a rise time of 3–4 ns and a fall time of 10–13 ns(Fig. 2).

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Fig. 8. Left: Pre-pulse measurements distribution. Right: Late pulse measurements distribution.

Table 1Effects of the multiphotoelectrons on the late pulse fraction.

P(n41)/P(1) (%) Late pulse fraction (%)

1.1 6.9

2.2 6.5

5.6 6.1

17.6 4.9

36.6 3.5

P(n) indicates the probability that each laser pulse gives rise ‘‘n’’ photoelectrons,

assuming Poisson distribution for the fluctuations of the photons number per laser

pulse [15].

Fig. 9. The method of measuring the after pulse.

S. Aiello et al. / Nuclear Instruments and Methods in Physics Research A 614 (2010) 206–212210

4.2. Nominal voltage

Using the calibrated charge amplitude converter (mod. 7422produced by SILENA), the voltage to obtain a gain of 5�107 wasmeasured studying the peak position of the single photoelectronpulse charge distribution as a function of high voltage. Fig. 3shows the distribution of nominal voltage values measured foreach PMT.

For all the measured PMTs, the nominal voltage rangedbetween 1600 and 1770 V, with a mean value of around 1650 V,and a standard deviation of 2.4%.

4.3. Dark count rate

The dark count rate of each PMT at nominal voltage wasmeasured with a CFD threshold of 1/3 of an s.p.e. signal using thecalibrated charge ADC. Fig. 4a shows the behaviour of the darkcount rate, measured on two test PMTs, as a function of the timestarting from the power ON at 1 h of adaptation to the dark.Fig. 4b exhibits the distribution of the dark count rates measuredfor all the PMTs after at least 6 h of adaptation to the dark.

The majority of the dark count rate measurements weresmaller than 2 kHz. Excluding one PMT with a value of 4000 Hz,the mean value was about 1400 Hz, with a standard deviationof 43.8%.

4.4. Time characteristics

The time resolution for each photomultiplier is an importantparameter as regards the performance of the telescope and it wasvery accurately measured. The Transit Time spectrum at nominalvoltage was acquired using a NIM Time Amplitude Converter,mod.7072T produced by FAST, with a 25 ps per channel resolu-tion. The laser source sync out signal was the START signal. Thecorresponding PMT response was discriminated to provide theSTOP signal. The threshold was regulated to 1/3 of the s.p.e.signals. In order to measure the time resolution, we were notinterested in measuring the absolute transit time, but only itsspread. Therefore, the transit time measured was a relative value.On the basis of the Transit Time spectrum acquired for each PMTat the nominal voltage, the transit time spread (TTS) wascalculated as the FWHM of those distributions (see Fig. 5).

The mean value of the TTS was equal to 2.8 ns, with a deviationof 5.51%, and only a few of the tested PMTs had a time resolutionslightly greater than the 3 ns.

4.5. Charge characteristics

The single photoelectron charge spectrum at nominal voltagewas acquired for each PMT detecting the anode output by usingthe calibrated charge ADC, with a resolution of 0.16 pC perchannel. The photomultiplier response was amplified by a LeCroy612A fast amplifier with a measured charge amplification equal to9.7. The acquisition of the PMT pulses was enabled with a gateproduced from the LASER sync out signal, with a gate width of100 ns. Previously, Fig. 2 showed a typical s.p.e. pulse chargespectrum.

The peak-to-valley ratio (P/V) was measured as the ratio of theheight of the peak and the height of the valley in each s.p.e. chargespectrum. Moreover, the charge resolution was calculated as thesigma of the single electron peak divided by the position of thepeak subtracked by the pedestal. Fig. 6 shows the measurementsdistribution.

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Peak-to-valley ratio and charge resolution had mean values 3.5(with a deviation of 10.8%) and 31.6 (with a deviation of 10.8%),respectively.

4.6. Spurious pulses

Spurious pulses were measured at nominal voltage for each PMT,in single photoelectron condition, with a CFD threshold of about 1/3s.p.e. The percentage of the number of spurious pulses with respectto the number of true pulses was measured for each different group.For each group, in order to avoid measuring the dark pulses, thenumber of events measured when the laser was off was subtractedfrom the number of events measured when the laser was on.

4.6.1. Late pulses and pre-pulses

The pre-pulse and late pulse fractions were measured on thebasis of the transit time spectrum acquired for each PMT,counting the number of events falling within the time range 80and 10 ns before the main pulse for the pre-pulses, and 10 and80 ns after the main pulse for the late pulses. Fig. 7 shows a typicaltime distribution for pre- and late pulses. Fig. 8 gives the pre- andlate pulse percentages measured on the 72 PMTs.

As can be seen in Fig. 8, the PMTs exhibit a very low level ofpre-pulses, with a mean value of the fraction equal to 0.02%. As a

Fig. 10. Left: Typical time distribution of type 1 after pulses. Right: Typical time distr

atomic mass of residual gas.

Fig. 11. After pulses measurements, type 1

result of only two measurements over 0.1%, in itself a low value,the deviation is around 96%. The late pulse percentages measuredmainly ranged between 5% and 6%, with a mean value was 5.5%,and deviation 9.8%.

The effects of multiphotoelectrons on the late pulse fractionwas also studied, maintaining the pulsed laser frequency constantand increasing the laser intensity, so that the fraction of multi-photoelectrons events increased with respect to single photoelec-tron events.

From the results shown in Table 1, we observe that the latepulse fraction decreases as the probability of multiphotoelectronsevents increases. Considering that a late pulse is caused by thereflection of photoelectrons on the first dynode, this trend can beexplained considering that the probability that more than oneelectrons which left the photocathode are all backscattered fromthe first dynode is very small. This probability also depends on theinner geometric characteristics of the PMT.

4.6.2. Type 1 and type 2 after pulses

The measurement of the after pulses was made using adedicated electronic setup (see next Fig. 9) in order to acquireonly pulses correlated with the main pulses [16].

The response of the PMT was split into two copies, one delayedby a ‘‘delay time’’, and connected to the START of the Time to

ibution for type 2 after pulses. The time of occurrence is closely correlated to the

(on the left) and type 2 (on the right).

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Table 2Mean values and range values of all measurements.

Mean value Values range

Voltage at Gain 5E7 (V) 1655 1595–1775

Dark count rate (Hz) 1388a 674–3000a

P/V ratio 3.5 2.6–4.3

Charge resolution s % 31.6 23.5–41.4

TTS FWHM (ns) 2.8 2.5–3.3

Pre-pulse % 0.02 0.001–0.11

Late pulse % 5.5 3.8–6.6

Type 1 after pulse % 1.1 0.8–1.9

Type 2 after pulse % 4.4b 2.2–7.3b

a Excluding one PMT which has a DC rate of 4093 Hz.b Excluding one PMT which has a type 2 after pulse fraction of 10.4%.

Fig. 12. Fractions of the after pulse as a function of the gain for one test PMT.

S. Aiello et al. / Nuclear Instruments and Methods in Physics Research A 614 (2010) 206–212212

Amplitude Converter (TAC), and the other connected to the STOP.In this way, when the main pulse of the PMT response arrived atthe STOP, it did not produce any effect, and the time count wasstarted by the delayed copy of the PMT response, and then wasstopped by the eventual after pulse. In order to ensure that thedetected spurious pulses were correlated to the main pulseproduced by the laser light pulse, the TAC stop was validated bythe coincidence between the laser sync out signal and the PMTresponse. For after pulse type 1 (AP1), the delay time was set to10 ns, and the width of the gate was 100 ns. For type 2 (AP2), thedelay time was set to 100 ns, and the gate width was 20ms. Fig. 10shows the typical time distribution of the measured type 1 andtype 2 after pulses.

Fig. 11 shows the fractions of after pulses measured on all thePMTs.

The mean value of the after pulses type 1 fraction is equal to1.1%, with a deviation of 19.3%. Considering the type 2 afterpulses, excluding one PMT with a value of 10%, the mean valuewas equal to 4.4%, with a deviation of 30%.

A significant decrease in the type 2 after pulse fractions wasdetected as gain decreased [17]. This effect could be caused by thedecrease of the ionization density due to the electrons currentreduction with the HV decrease. Fig. 12 shows the measuredbehaviour:

This measured effect, in conjunction with other effects as the PMTageing, suggests the use of a lower PMT gain, decreasing the HVsupply, compatibly with the requirements of the electronic read-out.

Table 2 summarizes the mean values and the range of valuesfor each parameter measured on the 72 R7081 sel. PMTs.

5. Conclusions

In order to select the photomultipliers for a large-scaleunderwater neutrino telescope, the performance of a sample of72 Hamamatsu photomultipliers, model R7081 sel, with 10 in.photocathode and 10 stages was measured. The measurementswere performed at room temperature and atmospheric pressureand the PMTs were powered by an ISEG active base modelPHQ7081and modified to NEMO requirements.

For all the PMTs, the voltage to achieve the actual requirementfor a gain of 5�107 was less than 1800 V, with a mean value of1655 V. The dark count rate, measured with a threshold of 1/3 s.p.e, presented a mean value of about 1400 Hz. The time andcharge characteristics of the PMT response at nominal voltagewere measured illuminating the whole photocathode with apulsed 410 nm laser calibrated in single photoelectron condition.In spite of its large area, the R7081 sel. had a good time resolution:all the TTS values were less than 3.3 ns as FWHM, with a meanvalue for the entire batch of 2.8 ns. The peak-to-valley ratio andthe charge resolution were good, with mean values of 3.5 and 32%,respectively.

The pre-pulse fraction was very low, less than 0.2%, and thelate pulse fraction ranged in 5–6%. A significant decrease in thelate pulse fraction was detected with an increase in multi-photoelectrons events in the incident light.

The mean value of the fraction of type 1 and type 2 after pulseswere equal to 1.1% and 4.4%, respectively, and an increase of thefractions with the PMT gain was measured.

All the measurements made show that the mean values of thePMT performance comply with the general requirements of aneutrino underwater telescope optical module. However,although possible, the search for particular characteristics inmass industrial production is expensive. So, the performancerequired from photomultipliers which could be used in a large-scale detector must necessarily be compatible with the availableeconomical resources.

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