1. Directional Optical Module Integration

2. Outline
Several solutions were proposed to increase the number of active elements in the km3 detector
Genova group proposed a directional OM based on a 4-anode photomultiplier
Simulations shows that the detector efficiency increases at low energies, if full instrumented with directional OMs
A 4-anode, 10” PMT was built by Hamamatsu
The light guides to achieve directional sensitivity were designed and developed in Genova
The front-end electronics was realized to be compliant with the standard NEMO readout chain
The first directional OM is being instrumented in these weeks

3. Genova Directional OM PMT support structure 4-anode PMT
High reflectivity mirrors
Light guide ~10 cm thick
Pressure resistant glass sphere

4. Detector Efficiency Increase
The detector efficiency change was evaluated if directional OM be used
A net effect on the effective area is predicted
The effect is large up to 10TeV
Further reconstruction code optimization is possible
A further improvement of the efficiency is expected
Results obtained with NEMO km3 geometry

5. Hamamatsu 4-anode PMT
Hamamatsu produced (on our request) a 10” 4-anode PMT based on model R7081
Two prototypes were supplied
The response (summed on the four anodes) was measured at the factory and resulted slightly better (more uniform) than the single anode model

6. PMT Characterization
Several measurements were performed on the directional PMTs both in Genova and in Catania LNS
ANODS A1 A2 A3 A4
TT [ns]
105
106
TTS [ns] (FWHM)
4.62
3.95
3.54
4.16
P/V
3.03
3.09
2.95
2.75
GAIN
5.0·107
5.4·107
3.9·107
RSE % (sigma)
33.62
30.62
37.34
35.15

7. 4-Anode Performances Single Photoelectron Distribution
Transit Time Distribution

8. Cross-Talk
Cross-Talk signal contamination is roughly proportional to PE number
Cross-Talk between dynode cascades due to capacitive coupling
Can be filtered
Cross talk due to internally reflected photons
Small w.r.t. signal A1

9. Cross-Talk Analysis
Channel 3: avg -15mV
Quadrant 1 lighted with 16 PE equivalent laser pulses
Cross talk signals are few % of direct ones
Cross talk signals are significantly smaller than direct
Channel 1: avg -250mV

10. High Reflectivity Mirrors
Several different materials were tested to build the mirrors
Aluminum coated film
Polypropylene multilayer
3M polypropylene multilayer (Vikuiti® film) gave the best performances
A simple mirror structure made of plexiglas and Vikuiti film has been realized
LASER
PHOTODIODE
WATTMETER
LENSES
TARGET
BEAM
STRIP OF REFLECTING MATERIAL
ROTATING AND TRANSLATING SUPPORT   

11. Light Guides
To achieve directionality, a thick (~10 cm) light guide is needed
Plexiglas and optical gel solutions were studied
Plexiglas is more transparent
Optical gel is simpler to handle
Optical gel is elastic ➙ no problem at high pressures
The first directional OM has been realized with optical gel

12. Front-End Electronics (i)
Custom power supply was developed and built
It is based on Cockroft-Walton voltage multiplier
Voltage up to 2000V
Independent setting of K to D1 and D1 to A voltages
Single 5V
Low current absorption: 500 mW
Remote switching, regulation and monitoring
Onboard thermometer
4 independent outputs

13. Front-End Electronics (ii)
The 4 outputs must be conditioned match the NEMO DAQ
A big effort was made to reduce the dynode cascades cross-talk
A solution based on passive filters, a summer and 4 comparators was developed and built
This board is actually under test in Genova

14. Prototype Integration (i)
Mirror positioning
Optical gel preparation

15. Prototype Integration (ii)
PMT positioning in the optical gel
Optical gel degassing

16. Prototype Integration (iii)
OM front view
OM side view