Catania Assembly and Design of the Optical Modules for the NEMO Phase-2 E. Leonora, S. Aiello INFN, sez. Catania

Catania Erlangen October 2 NEMO Phase-2 It consists of new infrastructure at the deep-sea site of Capo Passero, Sicily, at 3500 m depth : km cable, linking the site to the shore - a shore station, inside the harbor of Portopalo of Capo Passero - the underwater infrastructures need to connection - the prototype of the detector : - 8 storeys tower - 2 Optical Modules (OMs) at each end ( Vertical, Horizontal) - 4 OMs per storey

Catania Erlangen October 3 Optical Module in NEMO Phase-2 A glass sphere 13” (Vitrovex): Single large area photomultiplier : Hamamatsu 10” PMT R7081 Optical gel : Waker SilGel 612 μ-metal wire cage PMT base circuit : ISEG PHQ7081-i- 2m modified FEM (Front End Module) electronic board System for timing calibration (TIM- CAL) 13” OM sketch: lateral view TIM-CAL FEM ISEG

Catania Erlangen October 4 13” OM Lateral view 90° turned Pressure gauge 12-pin connector FEM Optical fibre Optical Module in NEMO Phase-2 Pressure gauge 12-pin connector (SEACON)

Catania Erlangen October 5 Glass sphere standard 13 inches deep-sea instrumentation vessels in borosilicate glass, produced by Vitrovex two half spheres: ½ transparent, ½ painted black no vacuum valve unique penetration for the 12-pin connector Refractive index1.48 (>350 nm) Transmission>95% (>350 nm) Density at 20 0 C2.23 g cm -3 Thermal conductivity1.2 W m -1 K -1 Characteristics of 13 inches spheres Depth rating (m)10000 Overall diameter (mm)330 Wall thickness (mm)11 Mass (kg)7.89 Buoyancy (empty) (N)114 Diameter shrinkage per 1000m depth (mm) 0.30

Catania Erlangen October 6 Large area photomultiplier R7081 Hamamatsu: 10 inch. photocathode Standard bialkali photocathode (QE ≈ 400nm) 10 stages A batch of 72 PMTs was bought and characterized Picture of a test box Dimensions of the R7081 (Courtesy of Hamamatsu) Sketch of test apparatus

Catania Erlangen October 7 mean value values range Voltage at Gain 5E7 [V] ÷1775 Dark Count rate [Hz]1388* 674÷3000* P/V ratio ÷4.3 Charge resolution σ % ÷ 41.4 TTS FWHM [ns] ÷3.3 Pre-Pulse % ÷ 0.11 Late Pulse % ÷6.6 Type 1 after pulse % ÷1.9 Type 2 after pulse %4.4** 2.2÷7.3** Procedure and results of measurements were published on NIM A: S.Aiello, E. Leonora et al. Nucl. Instr. Meth. A, 614 (2010) Measurements on 72 R7081 Hamamatsu PMTs * Excluding one PMT with DC rate of 4093 Hz ** Excluding one PMT with type 2 after pulse fraction of 10.4%.

Catania Erlangen October 8 Main features: Active base +5 Volts supply (bipolar voltage supply before modification) Cathode-1^dynode and 1^dynode-anode voltages individually controllable Anode current max : 100 microAmpere Power consumption : 2000 Volts Modified on the ouput on NEMO requiremts ISEG PMT base PHQ7081-i-2m (modified) Picture of the ISEG base soldered Modifications on ISEG base

Catania Erlangen October 9 Effect on the anode signal of the ISEG modification The same signal obtained with the damping resistors - no ringing in the signal - increase in signal rise time and width - saturation starts around at p.e. - limit about 1 nC for laser pulsed (width of 60 ps ) 1 p.e typycal signal from Hamamatsu R7081with the ISEG base

Catania Erlangen October 10 Purposes : Optical link between PMT photocatode and glass sphere Mechanical assembly of the PMT with the glass sphere. Main requirements: High trasparency Refractive index close to that of sphere and PMT window Good rigidity to hold the OM components with a sufficiently elastic properties to absorb shocks Properties should be stable over 10-year period Waker SilGel 612 two components (A e B) (silicone gel) Tests on 4 different mixtures 40B/100A, 50B/100A, 60B/100A, 70B/100A Optical Gel

Catania Erlangen October 11 mixture SilGel 40B/ 100A 50B/ 100A 60B/ 100A 70B/ 100A absorption length at 400 nm 12 cm30 cm33 cm35 cm mixture SilGel 40B/ 100A 50B/ 100A 60B/ 100A 70B/ 100A Transmittance at 400 nm 85%93.8%94.3%94.7% Silgel Optical Properties measurements

Catania Erlangen October 12 mixture SilGel 40B/ 100A 50B/ 100A 60B/ 100A 70B/ 100A Refraction index at 400 nm Good matching with the glass sphere (n=1,48 at 350÷450 nm) for ratios 50B/100A and 60B/100A The selected silgel mixture Considering optical tests and mechanical tests  mixture chosen was 50B/100A

Catania Erlangen October 13 A cage of mu-metal wire was chosen as magnetic shield (ITEP, Moscow): a hemispherical part ( 30 cm diameter, 14 cm height) a flat part (30 cm diameter ) with a hole in its centre ( 12 cm diam.) wire of 1 mm of diameter pitch of 68 x 68 mm shadow on the photocathode ≈ 5% average shielding factor measured ≈ 4 Magnetic shield Picture of the parts of the cage The cage around the 10” PMT

Catania Erlangen October 14 The influence of the Earth’s magnetic field, and the effect of the mu-metal cage, was studied on the 10” PMT for three different inclination: vertical downwards (0°), horizontal (90°), and tilted of 50° Effects of the mu-metal cage on PMT : facilities A dark box able to rotate with respect to vertical axes (1° step) and to change its inclination (10 °step) was realized. No magnetic materials were used in its constructions

Catania Erlangen October 15 The magnetic shielding reduced strongly the variations in the PMT and even improved performance. Further information on E. Leonora, “Terrestrial Magnetic Field Effects on Large Photomultipliers” in this Workshop Effects of the mu-metal cage on PMT : results

Catania Erlangen October 16 2 plexiglass vacuum boxes : 1x1x1 m, 300 mbar in less than 2 minutes Purposes: degassing of the optical gel closing the two hemispheres of the OM place where gluing PMT+ metal cage on the glass sphere Picture of the vacuum boxes Facility to assemble optical module

Catania Erlangen October 17 Soldering of the ISEG base on the PMT Mechanical Frame Base positioning Base soldering Wires cut off The end OM assembly procedure: base soldering

Catania Erlangen October 18 OM assembly procedure: cleaning cleaning of each element: optical paper and methyl alcohol - inner surface of the hemi-spheres - mu-metal cage mu-metal cage positioned into the glass hemisphere 1 cycle of outgassing : - 250mbar (15 mim) - air reentry

Catania Erlangen October 19 mixture gel preparation 1.5 litre x OM: 1 lltre A + 0,5 litre B at 120 giri/min. pouring the gel into the glass hemisphere 3 cycles of outgassing - 250mbar (3 mim) - air reentry OM assembly procedure: optical gel mixturing

Catania Erlangen October 20 Picture of outgassing of the gel into the sphere 3 cycles of outgassing remove the air-bubble inside the gel.

Catania Erlangen October 21 PMT mounted on the centering cross by means of a properly support positioning into the sphere by means of the centering cross 3 cycles of outgassing Polimerization of the atmosferic pressure and room temperature (12 h) OM assembly procedure: PMT positioning Mechanical support for PMT base and centering cross PMT mounted on the centering cross PMT positioned in the glass sphere

Catania Erlangen October 22 Assembled OM Picture of an assembled hemisphere: glass, PMT, Gel, mu-metal cage, ISEG

Catania Erlangen October 23 Mechanical support for FEM electronic board and TIM-CAL The mounted FEMThe mounted TIM-CAL

Catania Erlangen October 24 The 13” OM assembled with the 10” R7081 PMT Picture of the optical fibre for calibration Picture of the OM with FEM and TIM-CAL

Catania Erlangen October 25 Closure of the OM Sealing of the OM : hemisperes were aligned and joined closed under-pressure at 250 mbar external adhesive (Terostat) and tape

Catania Erlangen October 26 container with weights to keep OM in the bottom of the chamber The watertight and mechanical resistance of the OM assembled was tested in the hyperbaric chamber of NEMO test site (Catania harbour) up to 350 atm Test in Hyperbaric Chamber Results: No lack of vacuum inside OM No water inside OM No detachment of the gel

Catania Erlangen October 27 Conclusion 13” Optical Modules with single large 10” PMT was designed Each single component was chosen after intense phase of test: - test on PMTs - test on the base for Voltage Supply - test on optical gel - test on a mu-metal cage as magnetic shield A definitive procedure of assembly was defined 32 OM were assembled Tests in Hyperbaric chamber were done experimental OMs ( Acoustic OM, LED-Beacon, PORFIDO system) The OMs will be deployed soon in NEMO Phase-2

Catania Erlangen October 28