Transcript Time calibration for the KM3NeT deep sea neutrino telescope

4th International Workshop on Very Large Volume Neutrino
Telescopes for the Mediterranean Sea (VLVnT09)
Athens, Greece, 13-15 October 2009
Francisco Salesa Greus
IFIC (CSIC–Universitat de València, Spain)
Representing the KM3NeT Consortium
• The future KM3NeT detector.
• Time calibration requirements.
• Time calibration systems for KM3NeT:
– Laboratory calibration.
– Internal clock calibration.
– Optical calibration system:
• The Nanobeacon.
• The Laser Beacon.
– Cross-check methods: K40 and atmospheric muons.
• Summary.
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• KM3NeT will be a NT of at least 1 km3 of sea water (Mediterranean
Sea) and a deep-sea infrastructure for earth and sea science.
• Formed by 41 institutions from 10 countries.
• To be deployed after 2013.
qc=43°
n
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•
The angular resolution is critical in a NT. A good angular resolution provides the needed point
spread function to resolve cosmic neutrino sources from the atmospheric background.
The angular resolution relies on a good positioning and time calibration.
Working within the specifications the attainable angular resolution of KM3NeT is better than
0.1° for En > 100 TeV.
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•
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The absolute time resolution (provides a specific time for each neutrino
event w.r.t. UT) depends on:
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
GPS timing.
Detector electronic paths.
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In order to obtain correlations with the physics phenomena (e.g. GRB)
an accuracy of 1 ms is enough.
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Relative time resolution (among OMs) depends on:
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OM transit time spread (TTS), typically s~1-1.5 ns.
Optical properties of the sea water: light scattering + chromatic dispersion
(s~1-1.5 ns) for light coming from a distance of 50 m.
Electronics (s<0.5 ns).
All these components give an overall unavoidable time spread of 1-2 ns.
The determination of the calibration constants with a s≤1 ns fulfils the
requirements for relative time resolution.
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•
•
Before the deployment the time calibration constants are determined in the laboratory.
The calibration consists of two main parts: one to obtain the clock-phase delay, another to obtain
the intrinsic OM time offset:
–
–
•
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The clock-phase delay is given very precisely by the internal clock calibration.
A special setup should be designed for the intrinsic time offset computation.
The special setup can also be used for additional calibrations (e.g. electronics & charge) .
The internal clock calibration is repeated in situ.
On-shore Station
Special calibration setup
In-situ
Time Digital
Converter
START
GPS
START STOP
STOP
Echo-based
system
E/O/
E
Junction Box
optical splitter
Optical
fibres
Laser
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Signal
splitter
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•
The experience from the previous projects shows that a system of external light sources
with a known emission time ensures the time calibration and provides measurements
of the optical water properties (c.f. H. Yepes talk).
•
Decoupling the intra/inter detection unit (DU) calibration seems the best solution:
•
–
The calibration in the same storey and in the same DU will be performed by a group of Nanobeacons.
–
The calibration among DU will be performed by several Laser Beacons.
The calibration constants are obtained putting all the information together.
LED
Laser
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•
One Nanobeacon inside each OM (can be used for TTS monitoring).
•
The OM housing the nanobeacon provides the reference time.
•
Designed to illuminate storeys in the same DU (points upwards).
•
Based on the ANTARES LED Beacon, but with improvements:
•
–
Less expensive.
–
High redundancy.
–
Avoid cumbersome synchronization process, only one (or two) LED.
–
Not triggered by the clock which can induce electronic noise.
Laboratory tests performed: angular distribution, pulse shape, etc.
LED model
Rise time
(ns)
l (nm)
FWHM (º)
Intensity
(pJ)
CB11
2.5
470
14
150
CB30
2.2
472
28
90
NSPB520S
3.2
470
51
170
AB87
2.4
470
14
130
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The circuit works at a nominal 24 V.
The flashing rate depends on the feeding voltage (no external
trigger). It is 25 kHz at 24 V and requires only an on/off
interface.
One single LED is expected to reach 350 m (90 pJ ~2 x 108
photons). In ANTARES one LED reach ~200 m.
First tests performed mounting the Nanobeacon in an OM
from ANTARES in water.
Trigger signal w.r.t. OM signal
RMS ~ 1 ns
PRELIMINARY
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- KM3Net LED model (uncleaved)
- ANTARES LED model (cleaved)
50m
•
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In the peak region (± 10°) the KM3NeT LED emits 1.5 orders of
magnitude more than the ANTARES LED.
An opening angle of 15° is sufficient to illuminate OMs placed
above, even in a perpendicular arrangement (NuONE DU).
The pre-selected model is the Avago HLMP-CB30 LED.
6m
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•
Based on the ANTARES Laser Beacon (diode pumped Q-switched Nd-YAG laser).
•
Some Laser Beacons will be deployed at the bottom of several DU.
•
Alternatively to the previous green ANTARES laser (l = 532 nm). There is the possibility of
working with a blue laser (l = 473 nm).
•
The amount of light emitted can be tuned by means of a voltage controlled optical
attenuator.
•
An internal photodiode reads the signal and provides its timestamp.
Laser
Properties
Blue (473 nm)
ANTARES
Green (532 nm)
New
Green (532 nm)
Average
power
20 mW
~0.8 mW
45 mW
Rate
kHz range
kHz range
kHz range
Pulse
energy
5 mJ
1 mJ
45 mJ
Rise time
< 1.5 ns
~ 0.6 ns
~0.15 ns
Variable
Voltage
Laser Head
Liquid Crystal Polarizing cube
Head
beam-splitter
Liquid Crystal Retarder
Polarizing Beam-Splitter
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Time difference between a LED OB and an OM
Calibration constants correction
0
σ ~ 0.4 ns
RMS ~ 0.7 ns
Only 15% larger
than 1 ns
0
0
Electronics
contribution less
than 0.5 ns
RMS ~ 2 ns
RMS ~ 0.6 ns
Retuning of feeding
HV can be corrected
with the OBs
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PRELIMINARY
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•
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Background signal can be used to cross-check the time calibration.
For configurations with adjacent OMs in the same storey, the K40
present in the salt water can be used for charge and intra-storey time
calibration of the detector.
Atmospheric muons (both up-going and down-going) can be used for
the calibration among and in the same DU.
OM 1
OM 0
g
Taking
differences
by pairs
OM 2
Cherenkov
e- (b decay)
40Ca
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40K
Gaussian peak on
coincidence plot
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• The check of the time offsets measured by the K40 shows the
improvement using the calibration constants computed by the OB system.
• The calibration constants computed with intense light sources (OBs) are
still valid at lower intensities (K40).
Laboratory Calibration constants
RMS=0.71 ns
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OB calibration constants
RMS=0.50 ns
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• The time calibration of KM3NeT based on the previous NT projects
experience.
• An absolute time calibration of 1 ms is enough and a relative time
calibration at the nanosecond level is desirable.
• A first calibration will be performed in the laboratories.
• An optical calibration system will be used for the in situ time
calibration (results from ANTARES encourages that system).
• Decoupling of intra/inter DU calibration.
• Optical and muon background can be used as a cross-check.
• Thanks to the time calibration systems, KM3NeT will be able to
achieve an angular resolution better than 0.1° for En > 100 TeV.
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