1. ANTARES time calibration
International Workshop on a Very Large Volume Neutrino Telescope for the Mediterranean Sea (VLVnT08)
Toulon, Var, France, April 2008
ANTARES time calibration
Francisco Salesa Greus
IFIC (CSIC–Universitat de València, Spain)
On behalf of the ANTARES collaboration

2. Outline The ANTARES detector Why time calibration?
Calibration Systems of ANTARES:
Laser by optical fibre
Echo-based clock system
Internal LED
Optical Beacons
K40
Summary
F. Salesa - 23/04/2008

3. The ANTARES detector (see M.Circella’s talk in the plenary session)
Horizontal layout
Storey
Buoy
Electronics container OM
40 km electro-optical cable to shore OM
350 m
12 lines
25 storeys/line
3 PMTs/storey
900 PMTs in total OM
Junction box
Submersible
100 m
40 km off Toulon coast (France)
2500 m depth
42º50’N 6º10’E
10 lines (+1 instrumentation) taking data !
12 lines
3x25 PMT/line
60-75 m
Interlink cables
Anchor
F. Salesa - 23/04/2008

4. Importance of time calibration
The ability to point back to the detected sources is critical in a neutrino telescope. In order to get the best possible angular resolution a good positioning and time calibration are required.
In ANTARES the attainable angular resolution is better than 0.3º for En greater than 10 TeV.
The absolute time resolution (fix a specific time for each neutrino event w.r.t. UT) depends on:
Time offsets.
Electronic paths (Electro-optical cable between the junction box and the shore station).
In order to obtain correlations with the physics phenomena (e.g. GRB) an accuracy of ~1 ms is required in the absolute timing.
Relative timing resolution (among OMs) depends on:
Transit time spread (TTS) of the PMTs (s~1.3 ns).
Optical properties of the sea water: light scattering, chromatic dispersion (s~1.5 ns).
Electronics (s<0.5 ns).
In order to get the best angular accuracy (i.e. only limited by reconstruction) the relative time uncertainty coming from the electronics must be < 0.5 ns.
F. Salesa - 23/04/2008

5. Offset = TREF - TOM/LOB - clock_ph – fibre_path
Laser by optical fibre
The computation of the relative time-offsets between the OMs of the detector are performed in the integration sites in dedicated black boxes or a dark room.
A laser provides a common light signal sent through optical fibre to every OM and LED Optical Beacon.
The time difference between both signals corrected by the fibre path gives the relative time offset. All the offsets are referred to a particular OM.
In addition, calibrations of the read-out electronic cards are also done using the data obtained with the laser system.
LED beacon
Example of LED OB time offset calculation
Flower pot
Offset = TREF - TOM/LOB - clock_ph – fibre_path
Optical fibres
F. Salesa - 23/04/2008
Laser
Signal splitter

6. Echo-based clock system
On-shore Station
In-situ
EMC Cable
1550 nm1310 nm
Time Digital Converter 
Dt from anchor to a particular OM
Inter Link Cables
m fibre
START
STOP
GPS
E/O/E TX RX
TDC
Main Electro-optical cables (42 km) (1549 nm1532 nm )
s = 12 ps
Dt MEOC
Junction Box
optical splitter
The system has shown a stability of 2ns/208μs during one year of operation
The 20 MHz clock signal (synchronized with the GPS) is generated on shore and distributed throughout the detector. Start and stop signals are generated and sent to the TDC which measures the round-trip delay
Local Control Modules
(LCM) clock boards
Very good absolute calibration!
F. Salesa - 23/04/2008

7. The transit time of each OM stable within 0.5 ns
Internal LED
Every OM has an internal blue LED glued to the back of the PMT which can illuminate the photocathode from inside.
The aim of this system is to monitor the transit time (TT) of the PMTs measuring the difference between the OM signal arrival time and the time of the LED flash.
Blue LED Agilent HLMP-CB15
The transit time of each OM stable within 0.5 ns
F. Salesa - 23/04/2008

8. Optical Beacon system 60 m
The Optical Beacons are controlled sources of light with a well-known time emission. There are two kinds of OB: Laser and LED.
Four LED Beacons are placed along each line.
Two Laser Beacons are placed at the bottom of two central lines.
60 m
300 m
F. Salesa - 23/04/2008

9. Optical Beacon system LED Laser
The time of the Optical Beacon is known very precisely by means of a small internal PMT (LED OB) and by a built-in photodiode (Laser OB).
Both signals are very stable in rise-time.
Flashing a nearby OM the time resolution is checked, because the only uncertainty is the one coming from the electronics.
With the LED OB we can estimate the change of the offsets once in situ w.r.t. the ones computed in the laboratory.
DAQ waveform signal from the small PMT
LED
DR - OB offset difference
σ = 0.4 ns
Lines 1-10
RMS 0.7 ns
DAQ waveform signal from the internal photodiode
Laser
Time difference between the LED OB and an OM
Electronics contribution less than 0.5 ns
Only 15% are larger than 1 ns
F. Salesa - 23/04/2008

10. The attenuation length @ 472nm has been computed with the LED OB
Optical Beacon system
The effects of the orientation and the scattering are seen in the time distribution given by the LED OB runs.
storey 2
storey 9
storey 15
storey 21
Instrumentation Line
Line 1
storey 3
LOB light direction
The attenuation 472nm has been computed with the LED OB
PRELIMINARY
F. Salesa - 23/04/2008

11. Gaussian peak on coincidence plot
K40 calibration
The K40 present in the salt water can be used for charge and time calibration of the detector.
Taking differences by pairs
OM 0
OM 1
OM 2
Gaussian peak on coincidence plot g
RMS 0.7 ns g
Cherenkov
e- (b decay)
40K
40Ca
F. Salesa - 23/04/2008

12. Cross-check between systems
The K40 test is done by pairs because we know neither the position of the source nor the time emission of the light.
With the LED Beacon we can reproduce this test with more precision and with a more intense light source.
Lines 1-5
LED OB
K40
40K
40Ca
e- (b decay)
Cherenkov light g
K40
The offsets calculated with the LED OBs are validated by the K40 (independent calibration procedure)
LED OB – K40
LED OB
F. Salesa - 23/04/2008

13. Cross-check with positioning
Without alignment
With alignment
The laser is fixed on the anchor. Therefore it can be used to check the line movements.
If the line is considered rigid and straight, the time difference distribution has a RMS ~ 2.3 ns.
Taking into account the shape of the line the values are distributed within RMS ~ 0.6 ns.
σ=0.6 ns
F. Salesa - 23/04/2008

14. Summary
The ANTARES calibration system has been successfully tested in the laboratory and once deployed in the sea.
The absolute time resolution required (~1ms) is ensured by the high precision of the clock system.
The relative time resolution required (~0.5 ns) has been confirmed with the OB system.
The OB system provides the time offset corrections. The K40 analysis validates these results in an independent way.
The time calibration system has also shown a good agreement with the positioning.
With the offsets corrected and the relative time resolution checked, the angular accuracy of the detector of < 0.3º for high-energy neutrinos can be reach.
F. Salesa - 23/04/2008