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Test Results on the Photomultiplier Tubes for the ANTARES Neutrino Telescope

International Workshop on UHE Neutrino Telescopes Chiba, July 29-30, 2003. 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. Contents. ANTARES experiment

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Test Results on the Photomultiplier Tubes for the ANTARES Neutrino Telescope

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  1. International Workshop on UHE Neutrino Telescopes Chiba, July 29-30, 2003 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

  2. 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 J. D. Zornoza - IFIC

  3. 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. Submarine Electro-optical Cable ~40km Shore station Optical Modules 12 strings Compass, tiltmeter buoy ~60m 350m active Electronics container Readout cables ~100m Junction Box anchor Acoustic Beacon J. D. Zornoza - IFIC

  4. The Optical Module Pressure-resistant sphere The main element of the Optical Module is the PMT. The sphere also houses additional elements for operation and calibration. Internal LED • Blue LED to monitor PMT transit time • Pulse rise time: ~2 ns • Light output: 0 to 40 pJ • Stability:±5%. • Material: Vitrovex 8330 • Resistant to 700 bars • Inner diameter: 425 mm • Transm.: >95% (350 nm) • Good optical matching PMT base Magnetic Shield Optical glue • Material: Silicone rubber gel • Optical matching glass sphere - PMT • Fixation of the mechanical position of the different elements • Modified version of PHQ5912 from iSeg • Compact design • Low voltage supply (±5V) • Voltage stability: 10-4 • Three last dynodes active • Material: μ metal (nickel-iron alloy) • Magnetic permeability: 5x104-1.5x105 • Magnetic field at ANTARES site: 44 μT J. D. Zornoza - IFIC

  5. 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. J. D. Zornoza - IFIC

  6. 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. J. D. Zornoza - IFIC

  7. Candidates • Several models were studied in the pre-selection stage. • Three models were considered candidates for the ANTARES experiment (*). J. D. Zornoza - IFIC Hamamatsu R7081-20 Hamamatsu R8055 Photonis XP1804/D2

  8. Generador de pulsos Laser NIM Modules Digital Oscilloscope CAMAC Modules PC with Labview 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. Scheme of the experimental setup used at IFIC, Valencia. J. D. Zornoza - IFIC

  9. 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) J. D. Zornoza - IFIC

  10. SPE spectrum • The single photoelectron (SPE) charge spectrum is fitted to a function which takes into account the different contributing terms: notation: G  Gaussian distribution P  Poissonian distribution F  Valley distribution (exp + G) μn= n·μ1 and n2= n·12 ω  fraction of wrongly amplified electrons. β  fraction of the valley events moving to the exponential α  slope of the exponential J. D. Zornoza - IFIC

  11. Gain Nominal voltage (Gain=5x107) • 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 108. J. D. Zornoza - IFIC

  12. Amplitude (SPE peak) Amplitude @ HVnom • 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. J. D. Zornoza - IFIC

  13. Peak to Valley Peak to Valley @ HVnom • 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 HVnom. J. D. Zornoza - IFIC

  14. Energy Resolution Energy resolution @ HVnom *The energy resolution is defined as the ratio of the charge SPE peak width over its position. • The 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%. J. D. Zornoza - IFIC

  15. 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 J. D. Zornoza - IFIC

  16. TTS TTS @ HVnom • The 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. J. D. Zornoza - IFIC

  17. Dark Current Noise Dark Current Noise @ HVnom @½ pe • 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. J. D. Zornoza - IFIC

  18. After-pulses • 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. J. D. Zornoza - IFIC

  19. 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. J. D. Zornoza - IFIC

  20. 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 HVnom and the TTS obtained during the final tests and the results discussed previously. J. D. Zornoza - IFIC

  21. Gain Evolution of HVnom with the batch number • 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 108. J. D. Zornoza - IFIC

  22. 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 40mV, which is within the recommended value. • The amplitude of all the PMTs is larger than 35 mV. J. D. Zornoza - IFIC

  23. Pulse Shape Fall time (ns) Time width (ns) Rise time (ns) 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. J. D. Zornoza - IFIC

  24. 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. J. D. Zornoza - IFIC

  25. 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 cm2. • Only two PMTs are above this limit. Moreover, 85% of the tubes are below 3000 Hz. Threshold=1/4 pe J. D. Zornoza - IFIC

  26. 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%. J. D. Zornoza - IFIC

  27. 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%. J. D. Zornoza - IFIC

  28. 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 kHz • Bursts of 1 MHz lasting 0.1 second every second. J. D. Zornoza - IFIC

  29. Ageing (II) 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. Evolution of gain at HV constant 1.8 Gain (x108) 0.8 0 300 Time (days) J. D. Zornoza - IFIC

  30. 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. J. D. Zornoza - IFIC

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