Test Results on the Photomultiplier Tubes for the ANTARES Neutrino Telescope

Test Results on the Photomultiplier Tubes for the ANTARES Neutrino TelescopeJuan-de-Dios ZornozaIFIC (CSICValencia University, Spain)On behalf of the ANTARES collaborationInternational Workshop on UHE Neutrino TelescopesChiba, July 29-30, 2003

ContentsANTARES experimentPMTs specificationsComparison of PMT candidatesExperimental setup at IFICResultsConclusionsTests on the Hamamatsu R7081-20Experimental setup at DAPNIAResultsConclusions

J. D. Zornoza - IFIC

ANTARES DetectorThe detector will consist of a 3D matrix of 900 photomultipliers housed in pressure-resistant glass spheres.These PMTs will detect the Cherenkov light emitted by the muon produced in the CC interaction of the neutrino.The detector will be located in the Mediterranean sea (near Toulon, France) at 2475 m deep.

350mactiveSubmarine Electro-optical Cable ~40kmJunction BoxReadout cablesShore stationanchorbuoyElectronics container~60mCompass,tiltmeterOptical ModulesAcoustic Beacon~100m12 strings


The Optical ModulePressure-resistant sphereMaterial: Vitrovex 8330 Resistant to 700 barsInner diameter: 425 mmTransm.: >95% (350 nm)Good optical matching

Material: metal (nickel-iron alloy)Magnetic permeability: 5x104-1.5x105Magnetic field at ANTARES site: 44 TMagnetic ShieldOptical glueMaterial: Silicone rubber gelOptical matching glass sphere - PMTFixation of the mechanical position of the different elementsPMT baseModified version of PHQ5912 from iSegCompact designLow voltage supply (5V)Voltage stability: 10-4Three last dynodes activeInternal LEDBlue LED to monitor PMT transit timePulse rise time: ~2 nsLight output: 0 to 40 pJStability:5%.The main element of the Optical Module is the PMT. The sphere also houses additional elements for operation and calibration.


Specifications (I)Dimensions: the PMTs for neutrino telescopes should have a large sensitive surface. However, the PMT has to fit in the glass sphere so the photocathode radius of curvature is limited to 19 cm (15'' PMT) and the total PMT height to 35 cm.Gain: the amplitude of the single photoelectron (SPE peak) pulses has to be larger than 40 mV to avoid problems caused by electronic noise. This value corresponds to a gain of 5x107. In order to have a safety margin, the PMT must be able to reach a gain of 5x108.Nominal voltage: the high voltage at which the gain is 5x107 is called nominal voltage (HVnom) and should be lower than 2000 V so as to avoid ageing problems. The specifications explained in this section are required always at the nominal voltage. Dark noise: due to the optical background from 40K decays, the dark noise limit is not very stringent in the ANTARES experiment. This limit has been established at 25% the rate due to 40K, i.e. 15 kHz for a 10'' PMT.


Specifications (II)Peak to valley ratio: in order to isolate properly the single photoelectron signal from the pedestal, a minimum peak to valley ratio of 2 is required.Transit time spread: transit time spread (TTS) is one of the most important parameters since a large value of the TTS could limit the angular resolution of the detector, which depends on time resolution. For this reason, a maximum of 3.6 ns (FWHM) has been established.Pre-pulses and after-pulses: pre-pulses and after-pulses may induce misreconstructed events. The definitions and the maximum rates are in the table below. The time window is defined with respect to the expected time of the main pulse. The third column indicates if the pulse is correlated in time with a main pulse.*After-pulses-1 are correlated in time with a main pulse.


CandidatesSeveral models were studied in the pre-selection stage.Three models were considered candidates for the ANTARES experiment (*).Hamamatsu R7081-20Photonis XP1804/D2Hamamatsu R8055


Experimental Setup at IFIC (I)The PMT is inside a black box.A -metal cage is used in order to shield it from the Earth magnetic field.The laser is pulsed by means of a pulse generator and the light output is guided via an optical fiber cable inside the box.A Lambertian diffuser is used to illuminate uniformly the photocathode.Data acquisition is performed by CAMAC modules and the oscilloscope and sent to a PC via GPIB.

Scheme of the experimental setup used at IFIC, Valencia.


Experimental Setup at IFIC (II)The laser used in the calibration is a Nd-YAG device. It emits intense (~1J) and short (FWHM ~ 0.8 ns) pulses at =532 nm (green).In few minutes, a good stability is reached (

SPE spectrumThe single photoelectron (SPE) charge spectrum is fitted to a function which takes into account the different contributing terms:G Gaussian distributionP Poissonian distributionF Valley distribution (exp + G)n= n1 and n2= n12 fraction of wrongly amplified electrons. fraction of the valley events moving to the exponential slope of the exponentialnotation:


GainThe largest gain and the fastest increase with voltage are attained for the R7081-20 model (it has 14 dynodes).The model with higher nominal voltage is the R8055, since it is optimized for TTS, not for gain. The three models reach a gain of 108. Nominal voltage (Gain=5x107)


Amplitude (SPE peak)The behaviour for amplitude (SPE peak) is similar to the case of gain. The highest amplitude is reached by the R7081-20.The value of the R8055 model at the nominal voltage is slightly low.Amplitude @ HVnom


Peak to ValleyThe peak to valley ratio of the R7081-20 and the XP1804/D2 at nominal voltage is around 2.7. The behaviour of the R8055 is very good: high P/V and quite stable around the HVnom.Peak to Valley @ HVnom

ModelP/VR7081-202.7XP1804/D22.7R80553.7


Energy ResolutionThe behaviour of the R7081 at low voltages is unstable, but around the HVnom is flat (~30%).The XP1804/D2 is also almost flat around the same value. The R8055 curve is a bit worse since it grows with voltage up to 35%.Energy resolution @ HVnom*The energy resolution is defined as the ratio of the charge SPE peak width over its position.

ModelEnergy ResolutionR7081-2030%XP1804/D231%R805534%


Transit Time Spread The level of illumination is around 0.3. The threshold of discrimination is photoelectron.Example of Time Distribution The time distribution measured by the TDC is fitted to the product of a Gaussian and an exponential: A calibrated PMT (TTS=0.230.01 ns) is used to measure the contribution not due to the PMT


TTSThe slope of the decrease of TTS with voltage is quite similar for the three models. The better value at HVnom is obtained by the XP1804/D2. All of them are within the specifications.TTS @ HVnom

ModelTTS (ns)R7081-203.0XP1804/D22.0R80552.6


Dark Current NoiseThe lower dark noise rate of the R7081-20 can be partially attributed to its smaller surface.

When normalizing to the same area, R7081-20 remains the better, but the other models are within the specifications.Dark Current Noise@ HVnom @ pe

ModelDC (Hz)R7081-20500XP1804/D24300R80556900


After-pulsesPre-pulse rate is OK for the R7081-20 and the XP1804, but too high for R8055.All PMTs exhibit acceptable delayed pulse rates.Only R7081-20 have good values of after-pulses 1 and 2.


SummaryIn general, the three models are within the specifications.The nominal amplitude of the R8055 is slightly low.The XP1804/D2 and the R8055 show a high rate of after-pulses.Finally, the R7081-20 was considered the best choice.


Experimental Setup at DAPNIAAfter the choice of the model (R7081-20), the full sample of ANTARES PMTs (912) has been tested by means of an experimental setup capable to provide a fast way to characterize them.This characterization was carried out in the recette test bench, which consists of a light tight plastic cylinder 1.5m long and 0.46 m in diameter.The PMT is illuminated by a blue LED guided by an optical fiber.A Lambertian diffuser is used for uniform illumination of the whole photocathode.All final test have been performed with an optimsed PMT base. This explains the difference between the HVnom and the TTS obtained during the final tests and the results discussed previously.


GainThe mean of the distribution is ~1800 V with an RMS around 50 V.

In all cases, the nominal voltage is below 2000 V, as required. Moreover, all the tubes reach a gain of 108.Evolution of HVnom with the batch number


Amplitude Peak to ValleyThere are 20 PMTs with a low P/V value, but the mean P/V is around 2.8, i.e. within the specifications.

There is a correlation with the batch number, which indicates a gradual improvement in this parameter.The mean amplitude is larger than 40mV, which is within the recommended value.

The amplitude of all the PMTs is larger than 35 mV.


Pulse ShapeMean rise time is slightly higher than 4 ns and fall time is around 12ns.Regarding time width, the value for all the PMTs is lower than 9 ns, i.e. acceptable according to the specifications.Rise time (ns)Time width (ns)Fall time (ns)


TTSThe value of TTS has been improved by a modification in the PMT base design.

The distribution of the values of the transit time spread (FWHM) has a mean value of 2.8 ns and RMS of 0.15 ns.

There is no tube with a TTS value larger than 3.6 ns.


Dark Current NoiseAlmost all the PMTs have a dark current rate lower than 9000 Hz, i.e. within the specifications for a tube whose photocathode surface is 450 cm2.

Only two PMTs are above this limit. Moreover, 85% of the tubes are below 3000 Hz.Threshold=1/4 pe


Pre-pulses and Delayed PulsesThe pre-pulse rate is very low (

After-pulses The distribution of the after-pulses-1 shows two types of PMTs. Those of the first batch are above 2% and the rate for the rest is around 1.5%

The after-pulse-2 rate is within the limits, since all PMTs show a rate below 6%.


Ageing (I)Long term stability has to be checked.Three PMTs were placed in a black box and excited by three independent LEDs.LEDs reproduced the optical background environment (40K and bioluminiscence), increased by a factor two to speed up the process:A continuous rate of 100 kHzBursts of 1 MHz lasting 0.1 second every second.


Ageing (II)Time (days)Gain (x108)0.81.80 300Evolution of gain at HV constantAlong the first 100 days, the three PMTs showed an increase of 50-70% in gain, and then stabilized (running-in phase).

P/V, energy resolution, TTS and dark noise remained stable.

After-pulse-2 rate showed an increase (30%) most likely due to the accumulation of ionized gas atoms.


ConclusionsThe PMTs are a key component of the ANTARES neutrino telescope.A list of specifications has been done in order to choose the optimum PMT.After a pre-selection stage, three models were considered as candidates: R7081-20, XP1804/D2 and R8055.Taking into account these results, the R7081-20 was considered the best option.A large sample of 912 units of the selected model has been tested and the results are satisfactory.


Test Results on the Photomultiplier Tubes for the ANTARES Neutrino Telescope

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