Timing Calibration of the Optical Sensors for Undersea Neutrino Telescopes

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Timing Calibration of the Optical Sensors for
Undersea Neutrino Telescopes

• M. Circella, C. De Marzo, R. Megna, M. Ruppi
• Istituto Nazionale di Fisica Nucleare, Bari
• on behalf of the NEMO Collaboration

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Time Calibration in Neutrino Telescopes

• Two different problems
• Compare the time measured in the apparatus with
  UTC time (absolute timing calibration)
• Determine the offsets in local time measurements
  (relative timing calibration), i.e. the
  propagation time of the time-reference signal
  from onshore to offshore

Same problem for all neutrino telescopes
Same problem for all neutrino telescopes in which
data are time-stamped offshore
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What precision do we need?
MC for NEMO km3 layout 5832 PMTs
Relative timing calibration accuracy requirement
1 ns! No performance degradation for values
below 1 ns
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Time Calibration in Undersea Neutrino Telescopes
of km3 scale

• We can foresee these constraints
• Large distances
• Light propagation length in the sea water
• Low power consumption budget
• Extended life time

Choices to do Dimensions ? Layout ? Data
acquisition and transmission ? Time stamping ?
Approach lets find out a way to implement a
Time Calibration system which can work in
different apparatus configurations
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NEMO Tower Layout
FCM (Floor Control Module)

• Optical Module,
• Front End Electronics
• and time stamping

Data from OM are collected in FCM and delivered
over an optical fiber in a dedicated DWDM
wavelength to shore (the path is completely
passive between the shore and each FCM)
SDH frames arrive from shore station to each FCM
by passive DWDM devices. In the FCM the clock is
regenerated and used to synchronize the OMs and
the data protocol of OM-FCM
SDH STM-1 (155 Mbps)
Junction Box
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Time Calibration system
II. Optical calibration
A light pulse starts from the FCM at time known
and the arrival time is extracted from the data
flow
I. Echo calibration
The time delay for the commands to reach the
different FCMs is extracted from a go-and-return
time measurement
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Optical timing calibration
To upper floor
An optical pulser illuminates the sensors of more
than one floor by guided light (optical fiber)
This solution allows to perform an alloptical
time calibration as well as redundancy to the
echo measuraments
From lower floor
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Optical beacons are ineffective for km3-scale
detectors!
Why the optical fiber solution ?

• Geometric constraints
• (Difficulties to illuminate from the same source
  different Optical Modules)
• Optical fibers deliver much better the light
  pulse than water
• Solution can be adapted for any geometry
• The uncertainties from the positioning and from
  the scattering in water don't weight on the
  measures

• Low power consumption of light pulsers
• No need High Voltage

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Optical pulser
Among several configurations of the pulser that
we have studied, simulated and implemented, we
have finally selected one, very simple yet very
performing circuit
Analog circuit
Interface LED/fiber
Supply and voltage conditioning
remote digital control to manage the pulse
intensity in coarse and fine sensitivity
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Interfaces and power consumption
Agilent HLMP CB15 InGaN Blue LED Peak Wavelength
472 nm Viewing angle 15
Optical power picked up from the fiber has been
optimized by means of an optical collimator
106.7 mm
To assure stable positioning of collimator with
respect to the LED a very accurate mechanical
support has been designed
40 mm
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Optical fiber and splitters

• An extensive evaluation of the
• performance of different fibers
• (monomode and multimode)
• was done.
• The optimal fiber for this
• application has to be
• Multimode (small intermodal
• dispersion in few tens meters)
• Large NA
• Large core

Selected fiber Thorlabs ASF50/125 Multimode
0.22 NA Core 50 ?m
Commercial splitters for data network were
selected and now available
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The sensors used for the tests
Module PMT Hamamatsu H6780 Rise time 0.78 ns
5.5 cm

• Advantages of this sensor
• Fast
• Low TTS (0.28ns)
• Compact
• High-voltage power supply
• implemented inside the module
• Easy to use

Hamamatsu R7081SEL chosen by the NEMO
collaboration for the prototype activity of
NEMO Phase1
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Tests (1)
Tests with pulser Board collimator fiber
20m Module PMT Hamamatsu H6780
Fall time 2.5 ns
FWHM lt 7 ns
Standard deviation of time delay from trigger lt
100 ps
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Tests (2)
Tests with pulser collimator splitter
1x4 fiber 40m 2 Modules PMT Hamamatsu H6780
Fall time 2.5 ns
FWHM lt 7 ns
Standard deviation of time delay from trigger lt
250 ps
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Tests (3)
Tests with pulser collimator splitter 2x4
fiber 70m PMT Hamamatsu R7081SEL
Fall time 4 ns
FWHM lt 7 ns
Standard deviation of time delay from trigger
1.5 ns (TTS)
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Conclusions

• The timing calibration in NEMO will be performed
  by two redundant systems
• The first system consists of two parts
• I Echo calibration
• II Optical calibration
• The optical calibration system can be extended so
  as to allow redundant measurements to the "echo"
  timing calibration
• The optical calibration system has been developed
  and extensively tested
• All tests made so far give good results
• The solution described in this work is functional
  and scalable to any neutrino telescope of km3
  scale. It will be operated in a prototype tower
  of NEMO Phase 1 next year

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Time calibration in NEMO (the approach)
Goal determine the offsets in local time
measurements (relative timing calibration)
Redundancy
Measurement of the time necessary for time-reset
commands to arrive from the shore to each Optical
Module
Illuminate several optical modules contemporary
and extract the time offsets from the data flow
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Optical beacons are ineffective for km3-scale
detectors!
Optical fibers deliver much better the light
pulse than water
Number of photoelectrons

• Low power consumption of light pulsers
• No need High Voltage

Distance of a PMT from the optical source
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NEMO NEutrino Mediterranean Observatory Collaborat
ion
INFN Bari, Bologna, Cagliari, Catania, Genova,
LNF, LNS, Napoli, Pisa, Roma CNR Istituto di
Oceanografia Fisica, La Spezia Istituto di
Biologia del Mare, Venezia Istituto Sperimentale
Talassografico, Messina Istituto GEOMARE-SUD
(Napoli) Istituto Nazionale di Geofisica e
Vulcanologia Istituto Nazionale di Oceanografia e
Geofisica Sperimentale Centro Interdisciplinare
di Bioacustica e Ricerche Ambientali
(Pavia) UniversitiesBari, Bologna, Cagliari,
Catania, Genova, Napoli, Pisa, RomaLa
Sapienza With the support of Marina Militare
Italiana Saclant NATO Undersea Research Centre
Goal RD toward a km3 apparatus in the
Mediterranean Sea. Optimal site 3500 m depth,
80 km off the Sicily coast
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Neutrino Telescopes
We want to detect neutrinos coming from
astrophysical sources
The apparatus is deployed at large depth undersea
to reduce the background from charged particles.
Neutrinos are searched by detecting particles
arriving from the bottom
neutrino
Cherenkov light
muon
3500m depth
The tracks of muons produced from interacting
neutrinos are reconstructed from the measurements
of Cherenkov light collected by a grid of sensors
neutrino