catania assembly and design of the optical modules for the nemo phase-2 e. leonora, s. aiello infn,...
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Catania
Assembly and Design of the Optical Modules for the NEMO Phase-2
E. Leonora, S. AielloINFN, sez. Catania
Catania
Erlangen 12-14 October 2011 [email protected] 2
NEMO Phase-2
It consists of new infrastructure at the deep-sea site of Capo Passero, Sicily, at 3500 m depth :
- 100 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
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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
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13” OM Lateral view 90° turned
Pressure gauge
12-pin connectorFEM
Optical fibre
Optical Module in NEMO Phase-2
• Pressure gauge
• 12-pin connector (SEACON)
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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 index 1.48 (>350 nm)
Transmission >95% (>350 nm)
Density at 20 0C 2.23 g cm-3
Thermal conductivity 1.2 W m-1K-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
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Large area photomultiplier
R7081 Hamamatsu: • 10 inch. photocathode• Standard bialkali photocathode (QE ≈ 25% @ 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
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mean value
values range
Voltage at Gain 5E7 [V] 1655 1595÷1775
Dark Count rate [Hz] 1388* 674÷3000*
P/V ratio 3.5 2.6÷4.3
Charge resolution σ % 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 % 4.4** 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) 206-212
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%.
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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 : 150mW @ 2000 Volts• Modified on the ouput on NEMO requiremts
ISEG PMT base PHQ7081-i-2m (modified)
Picture of the ISEG base solderedModifications on ISEG base
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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 100-120 p.e.
- limit about 1 nC for laser pulsed (width of 60 ps)
1 p.e typycal signal from Hamamatsu R7081with the ISEG base
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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
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mixtureSilGel
40B/100A
50B/100A
60B/100A
70B/100A
absorption length at 400 nm
12 cm 30 cm 33 cm 35 cm
mixtureSilGel
40B/100A
50B/100A
60B/100A
70B/100A
Transmittanceat 400 nm
85% 93.8% 94.3% 94.7%
Silgel Optical Properties measurements
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mixtureSilGel 40B/
100A50B/100A
60B/100A
70B/100A
Refraction index at 400
nm 1.43 1.50 1.47 1.38
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 testsand mechanical tests
mixture chosen was 50B/100A
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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
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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
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• 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
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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
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Soldering of the ISEG base on the PMT
Mechanical Frame Base positioningBase soldering
Wires cut off
The end
OM assembly procedure: base soldering
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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 :- vacuum @ 250mbar (15 mim)- air reentry
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• 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
- vacuum @ 250mbar (3 mim)- air reentry
OM assembly procedure: optical gel mixturing
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Picture of outgassing of the gel into the sphere
3 cycles of outgassing remove the air-bubble inside the gel .
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• 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 gel @ 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
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Assembled OM
Picture of an assembled hemisphere: glass, PMT, Gel, mu-metal cage, ISEG
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Mechanical support for FEM electronic board and TIM-CAL
The mounted FEM The mounted TIM-CAL
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The 13” OM assembled with the 10” R7081 PMT
Picture of the optical fibre for calibrationPicture of the OM with FEM and TIM-CAL
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Closure of the OM
Sealing of the OM :• hemisperes were aligned and joined• closed under-pressure at 250 mbar • external adhesive (Terostat) and tape
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Test pressure profile
0
50
100
150
200
250
300
350
400
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
Time [ hours ]
Pre
ss
ure
[B
ar]
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
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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