Francisco Salesa GreusIFIC (CSIC–Universitat de València, Spain)

Representing the KM3NeT Consortium

4th International Workshop on Very Large Volume Neutrino Telescopes for the Mediterranean Sea

(VLVnT09)

Athens, Greece, 13-15 October 2009

• 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.

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c=43°

• 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 E > 100 TeV.

The absolute time resolutionabsolute time resolution (provides a specific time for each neutrino event w.r.t. UT) depends on:

GPS timing. Detector electronic paths.

In order to obtain correlations with the physics phenomena (e.g. GRB) an accuracy of 1 ms is enough.

Relative time resolutionRelative time resolution (among OMs) depends on: OM transit time spread (TTS), typically ~1-1.5 ns. Optical properties of the sea water: light scattering + chromatic dispersion

(~1-1.5 ns) for light coming from a distance of 50 m. Electronics (<0.5 ns).

All these components give an overall unavoidable time spread of 1-2 ns. The determination of the calibration constants with a ≤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:– 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.

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Optical fibres

Signal splitte

r

Laser

START STOPGPS E/

O/E

START STOP

On-shore Station

Time Digital Converter 

In-situ

Echo-based system

Junction Boxoptical splitter

Special calibration setupSpecial calibration setup

• 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.

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Laser

LED

• 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.

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LED model Rise time (ns)

(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

• 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.

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RMS ~ 1 nsRMS ~ 1 ns

Trigger signal w.r.t. OM signalTrigger signal w.r.t. OM signal

PRELIMINARYPRELIMINARY

• 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.

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50m

6m

- KM3Net LED model (uncleaved)

- ANTARES LED model (cleaved)

- KM3Net LED model (uncleaved)

- ANTARES LED model (cleaved)

• 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 ( = 532 nm). There is the possibility of working with a blue laser ( = 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.

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LaserProperties

Blue (473 nm)

ANTARESGreen (532

nm)

NewGreen (532

nm)

Average power

20 mW ~0.8 mW 45 mW

Rate kHz range kHz range kHz range

Pulse energy

5 J 1 J 45 J

Rise time < 1.5 ns ~ 0.6 ns ~0.15 ns

Liquid Crystal Retarder

Polarizing Beam-Splitter

Laser Head Polarizing cube beam-splitter

Liquid Crystal Head

Variable Voltage

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Calibration constants correction

RMS ~ 0.7 nsRMS ~ 0.7 ns

Time difference between a LED OB and an OM

Electronics contribution less

than 0.5 ns

RMS ~ 2 ns

RMS ~ 2 ns

RMS ~ 0.6 ns

RMS ~ 0.6 ns

Only 15% larger than 1 ns

Retuning of feeding HV can be corrected with the

OBs

00

0

PRELIMINARYPRELIMINARY

σ ~ 0.4 nsσ ~ 0.4 ns

• 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.

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40K

40Ca

e- ( decay)

Cherenkov

Gaussian peak on coincidence plot

OM 0

OM 1

OM 2

Taking differences by pairs

• 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).

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RMS=0.71 ns RMS=0.50 ns

Laboratory Calibration constantsLaboratory Calibration constants OB calibration constantsOB calibration constants

• 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 E > 100 TeV.

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