1. Test Results on the Photomultiplier Tubes for the ANTARES Neutrino Telescope Juan-de-Dios Zornoza IFIC (CSIC–Valencia University, Spain) On behalf of the ANTARES collaboration International Workshop on UHE Neutrino Telescopes Chiba, July 29-30, 2003

2. J. D. Zornoza - IFIC Contents  ANTARES experiment  PMTs specifications  Comparison of PMT candidates –Experimental setup at IFIC –Results –Conclusions  Tests on the Hamamatsu R7081-20 –Experimental setup at DAPNIA –Results –Conclusions

3. J. D. Zornoza - IFIC ANTARES Detector  The 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. 350mactive SubmarineElectro-optical Cable ~40km Cable ~40km Junction Box Readout cables Shore station anchor buoy Electronics container ~60m Compass,tiltmeter Optical Modules Acoustic Beacon ~100m 12 strings

4. J. D. Zornoza - IFIC The Optical Module Pressure-resistant sphere Material: Vitrovex 8330 Resistant to 700 bars Inner diameter: 425 mm Transm.: >95% (350 nm) Good optical matching Material: μ metal (nickel-iron alloy) Magnetic permeability: 5x10 4 -1.5x10 5 Magnetic field at ANTARES site: 44 μT Magnetic Shield Optical glue Material: Silicone rubber gel Optical matching glass sphere - PMT Fixation of the mechanical position of the different elements PMT base Modified version of PHQ5912 from iSeg Compact design Low voltage supply (±5V) Voltage stability: 10 -4 Three last dynodes active Internal LED Blue LED to monitor PMT transit time Pulse rise time: ~2 ns Light output: 0 to 40 pJ Stability:±5%. The main element of the Optical Module is the PMT. The sphere also houses additional elements for operation and calibration.

5. J. D. Zornoza - IFIC 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 5x10 7. In order to have a safety margin, the PMT must be able to reach a gain of 5x10 8.   Nominal voltage: the high voltage at which the gain is 5x10 7 is called nominal voltage (HV nom ) 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 40 K decays, the dark noise limit is not very stringent in the ANTARES experiment. This limit has been established at 25% the rate due to 40 K, i.e. 15 kHz for a 10'' PMT.

6. J. D. Zornoza - IFIC 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. TypeTime window Selection Limit Pre-pulses[-100 ns, -10 ns]<1% Delayed pulses[10 ns, 100 ns]<5% After-pulses-1*[10 ns, 100 ns]<1% After-pulses-2[100 ns, 16 μs]<10% *After-pulses-1 are correlated in time with a main pulse.

7. J. D. Zornoza - IFIC Candidates  Several models were studied in the pre-selection stage.  Three models were considered candidates for the ANTARES experiment (*). Photomultiplier Photocathode diameter Number of stages Dynode Structure Hamamatsu R201815”13Venetian blind Hamamatsu R5912-028”14Box line Photonis XP18029”11Linear focused ETL 93538”12Linear focused ETL 9360KB11”12Linear focused *Hamamatsu R7081-2010”14Box line *Photonis XP1804/D210.6”10Linear focused *Hamamatsu R805513”11Box line Hamamatsu R7081-20 Photonis XP1804/D2 Hamamatsu R8055

8. J. D. Zornoza - IFIC Experimental Setup at IFIC (I)   The PMT is inside a black box.   A μ-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. Laser NIM Modules Generador de pulsos Digital Oscilloscope CAMAC Modules PC with Labview Scheme of the experimental setup used at IFIC, Valencia.

9. J. D. Zornoza - IFIC Experimental Setup at IFIC (II)  The laser used in the calibration is a Nd-YAG device.  It emits intense (~1μJ) and short (FWHM ~ 0.8 ns) pulses at =532 nm (green).  In few minutes, a good stability is reached (<3%).  An internal photodiode measures the emission time with very good precision (~50 ps)

10. J. D. Zornoza - IFIC SPE spectrum  The single photoelectron (SPE) charge spectrum is fitted to a function which takes into account the different contributing terms: G  Gaussian distribution P  Poissonian distribution F  Valley distribution (exp + G) μ n = n·μ 1 and  n 2 = n· 1 2 ω  fraction of wrongly amplified electrons. β  fraction of the valley events moving to the exponential α  slope of the exponential notation:

11. J. D. Zornoza - IFIC Gain The 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 10 8.Model HV nom (V) R7081-201340 XP1804/D21680 R80551900 Nominal voltage (Gain=5x10 7 )

12. J. D. Zornoza - IFIC 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.Model Amplitude (mV) R7081-2046 XP1804/D260 R805534 Amplitude @ HV nom

13. J. D. Zornoza - IFIC Peak to Valley The 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 HV nom.ModelP/VR7081-202.7 XP1804/D22.7 R80553.7 Peak to Valley @ HV nom

14. J. D. Zornoza - IFIC Energy Resolution The behaviour of the R7081 at low voltages is unstable, but around the HV nom 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%.Model Energy Resolution R7081-2030% XP1804/D231% R805534% Energy resolution @ HV nom *The energy resolution is defined as the ratio of the charge SPE peak width over its position.

15. J. D. Zornoza - IFIC Transit Time Spread The level of illumination is around 0.3. The threshold of discrimination is ½ 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=0.230.01 ns) is used to measure the contribution not due to the PMT

16. J. D. Zornoza - IFIC TTS The slope of the decrease of TTS with voltage is quite similar for the three models. The better value at HV nom is obtained by the XP1804/D2. All of them are within the specifications.Model TTS (ns) R7081-203.0 XP1804/D22.0 R80552.6 TTS @ HV nom

17. J. D. Zornoza - IFIC Dark Current Noise The 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.Model DC (Hz) R7081-20500 XP1804/D24300 R80556900 Dark Current Noise @ HV nom @½ pe

18. J. D. Zornoza - IFIC After-pulses RatePre-pulses [-100 ns, -10 ns] Delayed Pulses [10 ns, 100 ns] After-pulses-1 After-pulses-2 [100 ns, 16 μs] R7081-20<0.01%3.6%1.0%3.8% XP1804/D2<0.01%4.7%3.2%18% R80552.2%5.0%2.4%19% Pre-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.

19. J. D. Zornoza - IFIC Summary  In 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.

20. J. D. Zornoza - IFIC Experimental Setup at DAPNIA  After 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 1.5 m 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 HV nom and the TTS obtained during the final tests and the results discussed previously.

21. J. D. Zornoza - IFIC Gain The 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 10 8. Evolution of HVnom with the batch number

22. J. D. Zornoza - IFIC Amplitude – Peak to Valley There 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 40 mV, which is within the recommended value. The amplitude of all the PMTs is larger than 35 mV.

23. J. D. Zornoza - IFIC Pulse Shape  Mean rise time is slightly higher than 4 ns and fall time is around 12 ns.  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)

24. J. D. Zornoza - IFIC TTS The 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.

25. J. D. Zornoza - IFIC Dark Current Noise Almost 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 cm 2. Only two PMTs are above this limit. Moreover, 85% of the tubes are below 3000 Hz. Threshold=1/4 pe

26. J. D. Zornoza - IFIC Pre-pulses and Delayed Pulses The pre-pulse rate is very low (<0.1% for all the PMTs). Results for delayed pulses are also OK. Only two units exhibit a rate larger than 5%.

27. J. D. Zornoza - IFIC 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%.

28. J. D. Zornoza - IFIC 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 ( 40 K and bioluminiscence), increased by a factor two to speed up the process: –A continuous rate of 100 kHz –Bursts of 1 MHz lasting 0.1 second every second.

29. J. D. Zornoza - IFIC Ageing (II) Time (days) Gain (x10 8 ) 0.8 1.8 0 300 300 Evolution of gain at HV constant  Along 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.

30. J. D. Zornoza - IFIC Conclusions  The 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.