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Time Calibration with Optical Beacons. Pylos, 16-19 Apr 2007 C.Bigongiari IFIC (CSIC – Universitat de València). KM3NeT WP3 – Calibration Session. Outline. Time Calibration Optical Beacons LED Beacons LASER Beacons What we have learnt in ANTARES up to now Time resolution - PowerPoint PPT Presentation
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Time Calibration Time Calibration with with
Optical Beacons Optical Beacons Pylos, 16-19 Apr 2007 C.Bigongiari IFIC Pylos, 16-19 Apr 2007 C.Bigongiari IFIC (CSIC – Universitat de València)
KM3NeT WP3 – Calibration Session
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 22
OutlineOutline
• Time Calibration • Optical Beacons
– LED Beacons – LASER Beacons
• What we have learnt in ANTARES up to now– Time resolution – Optical Beacon illuminating same line OMs– Optical Beacon illuminating other line OMs
• Laser OB development• Conclusions
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 33
Time CalibrationTime Calibration
The reconstruction of muons trajectories in a neutrino telescope heavily relies on the measurements of Cherenkov photons arrival times.
A precise relative time calibration of the detector is of utmost importance to achieve a good angular resolution and a high reconstruction efficiency
ν
μ
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 44
Time CalibrationTime Calibration
Different systems are needed to measure time delays and time jitters in different parts of the electronic chain, from PMTs to DAQ.
A redundant time calibration system is very useful to disentangle different effects
In-situ calibration systems are mandatory to monitor the time calibration after the deployment
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 55
ANTARES Optical BeaconsANTARES Optical Beacons Optical Beacon Well controlled pulsed light source,
(LEDs or LASERs)
LED BeaconLASER Beacon
LASER
LEDs
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 66
ANTARES LED Beacon Features ANTARES LED Beacon Features
LED Beacon without container
6 faces with 6 LED each = 36 LED.
3 groups (top, centre, four)
Internal PMT Hamamatsu H6780-03 (RT=0.7 ns) to know the actual time emission of the light.
Wave length emission = 472 nm (blue)
Flux per shot @ max INT = 4 x 108 photons per LED.
Intensity: variable
Light emission: isotropic in 50º < θ < 120º range
Location along the line (storeys: 2, 9, 15, 21).
TOP
CENTRE
FOUR
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 77
ANTARES LASER Beacon FeaturesANTARES LASER Beacon Features
Internal fast photodiode (jitter ~50 ps) to measure the actual time emission of the light.
Wave length emission = 532 nm (green).
Flux per shot = 1012 photons.
Intensity: fixed. Adjustable in new design
Light emission: Lambertian
Location at string bottom
Glass rod to avoid biofouling
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 88
LED Optical BeaconsLED Optical Beacons
• Advantages: – There are Blue LEDs
(472 ± 15) nm
Absorption length ~ 60 m
Effective scattering length > 200 m – LED light yield is tunable– LEDs are cheap
• Disadvantages: – Long rise time 2 ns– Expensive containers– Low yield => Many LEDs
• Cumbersome mounting
• Source spatially spread
• Hazy proper T0
• Dull and tricky synchronization procedure needed
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 99
LASER Optical BeaconsLASER Optical Beacons
• Advantages: – Extremely coherent light
source– High light yield
• No synchronization needed• Well defined proper T0
– Very good rise time (<0.5 ns)– Light yield is tunable
(see following slides)
• Disadvantages: – Green light 532 nm
Absorption length ~ 28 m
– The emitted light is not isotropic
– Expensive (~ 12K€ )
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 1010
Photons emitted by Optical Beacons propagate through sea water and, if they reach OMs, can produce electronic signals in pretty the same way as Cherenkov photons do.
Time calibration with Optical Beacons is therefore a very comprehensive system:
– Many different aspects are involved• Sea-water properties
• Detector positioning (Rotation included)
• Other timing systems
• OM response
• Readout system
Time Calibration with Optical BeaconsTime Calibration with Optical BeaconsS
hore
Sta
tion
Junction box
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 1111
What we have learnt from ANTARESWhat we have learnt from ANTARES
We studied the distribution of T1- T0– T0 = Light emission time by OB – T1 = Arrival time of light on OM
• In the following results about:
• Electronic chain time resolution
• OB flashing same line OMs
• OB flashing other line OMs
• LASER beacon flashing
Optical Beacon OBOptical Module OM
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 1212
ANTARES ANTARES Optical Beacons Optical Beacons
LayoutLayout
Flo
or 2
Flo
or 9
Flo
or 1
5F
loor
21
Sec
tor
5S
ecto
r 3
Sec
tor
2S
ecto
r 1
Line 5
MILOM
~101 m
~87 m
~87 m
~94 m
~114 mLine 1
Laser
7+1 Lines deployed so far
5+1 Lines connected
20+9 LED Optical Beacons.
1 Laser Optical Beacon.
LED
Line 2
Line 4 Line 3JUST R
ECOVERED
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 1313
TTOMOM – T – TOBOB (ANTARES Data) (ANTARES Data)
The distribution is clearly asymmetric due to the photon scattering. We consider only the rising part, which is less affected by scattering, early photon effect, and fit it with a Gaussian. Better results can be obtained with a two-steps fit:
Second step: Gaussian from 10% of the mean bin content to the 90% over passed bin.
90%
10%
100%
20%
First step: Gaussian fit from 20% of maximum to the 100 %
T100 = Gaussian mean Sigma = Gaussian sigma T50 = Time @ 50%
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 1414
Time Time ResolutionResolution
Flashing OMs in the storey above the OB at high intensity we can measure the electronic chain contribution to the overall time resolution.
The σ of the distribution is well below the requested resolution, 0.5 ns, for all the OMs
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 1616
OB illuminating same line OMs OB illuminating same line OMs
1. A LED OB is able to illuminate its own storey OMs and even the ones in the storey below2. The statistic is enough to perform the fit up to 8 storeys above (about 116m far away)
OB
OB OB OB
OBOB
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 1717
OB illuminating same line OMs OB illuminating same line OMs
The T100 grows as function of the distance/storey (up to 10 ns) more or less linearly with slope ~1.7 ns/storey
The observed slope is due to the ‘early photons’ effect
The RISE TIME of the light source must be smaller that the requested time resolution
Or
Illuminate OMs at 1 phe level
low intensity
t [ns]
Nu
mb
er
of e
vent
s [a
.u.]
~ 9 ns
high intensity
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 1818
Sigma as function of OB-OM distanceSigma as function of OB-OM distance
The sigma of the fit is a measure of the direct photons peak width. This is the result of the convolution of source time spread (~1.7 ns~1.7 ns) and phototube TTS (~1.3 ns~1.3 ns). TTS depend on the number of photoelectrons NNpe, pe, and therefore on the OB-OM distance. and therefore on the OB-OM distance.
At large distance we reach the phe level -> Sigma =At large distance we reach the phe level -> Sigma =√(1.7ˆ√(1.7ˆ22 + 1.3ˆ + 1.3ˆ22) = 2.1 ns) = 2.1 ns
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 1919
OB illuminating another line OMs OB illuminating another line OMs
OMs in storeys 1 to 13 of line 3 illuminated by the beacon of storey 2 lines 5.
There is the same linear dependency on the OB-OM distance of the previous case due to early photon effect.
There is an anti-correlation between OMs in the same storey due to storey rotation
We can notice some wrong T0s
We need an independent measurement of storey position/rotation
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 2020
Laser OB – Correcting for positionLaser OB – Correcting for position
σ = 2.3 ns
σ = 0.6 ns
Co
rre
cte
dN
ot
Co
rre
cte
d
60 m
30 0 m
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 2121
LASER Beacon DevelopmentLASER Beacon Development
The light emitted by the LASER can be varied using a Voltage Controlled Optical Attenuator, a linear polarizer followed by a liquid-crystal retarder and another linear polarizer.
Varying the voltage applied to the retarder the polarization of outgoing light changes.
In this way the transmission of the attenuator can be varied.
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 2222
Variable intensity LASER BeaconVariable intensity LASER Beacon Schematic view of the Variable Intensity Laser Beacon.The amount of outgoing light can be changed by varying the voltage V applied to the liquid crystal retarder
Liquid Crystal Retarder
Polarizing Beam-Splitter
Laser Head Polarizing cube beam-splitter
Liquid Crystal Head
Variable Voltage
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 2323
Variable Intensity LASER BeaconVariable Intensity LASER Beacon
We measured the energy per pulse emitted at different pulsing frequencies as function of the applied voltage. The maximum output is above 1 μJ for all frequencies considered.
A variable intensity LASER Beacon has been already installed on line 7
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 2424
Conclusions (1/2) Conclusions (1/2)
• OB allow an in-situ time calibration and monitoring of the detector
• From ANTARES data we have learnt: – The overall time resolution is below 0.5 ns– LED_OB-OM time difference depends on the distance
due to early photons effect• Very short rise time light sources are needed• Otherwise operate at 1 phe level to avoid this effect
• The Optical Beacon system is very comprehensive– Sensitive to Optical Module position (Rotation Sensitive to Optical Module position (Rotation
included)included)– Sensitive to sea-water conditionsSensitive to sea-water conditions
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 2525
Conclusions (2/2) Conclusions (2/2)
• This is an advantage: – We can get information on other aspects of the detector
• Attenuation and scattering lengths of water• Efficiency of bright point reconstruction• Cross-check of position measurements
• But it has also drawbacks: – What you get is the convolution of many different phenomena– It’s hard to get precise results without additional information
• Optical Beacons are not cheap -> Reduce cost. – Containers make a non negligible fraction of the cost. Study
alternative solutions. – Mass production should help Km3Net to reduce the cost
• A variable intensity LASER beacon already realized, tested and installed
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 2626
Optical Beacon Cost Optical Beacon Cost
• LED Beacon Cost – Container 4000 €
– Mounting 600 €
– Faces (6 x 40) 240 €
– Motherboard 300 €
– PMT 600 €
TOTAL 5740 €
Pressure and Climatic internal tests not included
• Laser Beacon Cost – Laser 5000 €
– Container 4500 €
– Optical attenuator 2000 €
– Electronics 500 €
TOTAL 12000 €
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 2727
Overdrive ModeOverdrive Mode
Everything worked as expected
~33Hz
~330Hz
The LED OB flashing frequency has been recently increased by a factor ten
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 2828
Correcting for positionCorrecting for position
• Fixed geometry takes, for the position of the OMs in a storey, the point in the centre of the OMs plane.
• Correction for position takes:• Geometric constants from ANTARES-CALI-2006-002 (G. Lelaizant)• Rotation matrix from ANTARES-SLOW/1999-001 (F. Cassol)• Euler angles (A1,A2,A3) from table “ALIGNMENT_VALUES2”. They are referred to the centre of
the OMs plane (0,0, 0.576)• We took the ALIG_VALUES which are closer in time w.r.t. the start of the OB run.
Y
X
rOM_0=(0.437, 0, 0)
rOM_1=(-0.218, -0.378, 0)
rOM_2=(-0.218, 0.378, 0)
rLOB=(0, 0, 1.003)rcorr = R * ri + r
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 2929
Effect of scattering Effect of scattering From MonteCarlo we know that scattering has a minor effect in the rising edge of time difference distributions, although systematic shift of in T100 peaks of +0.5 ns is expected. However delays larger than ~3 ns~3 ns are unlikely due to scattering.
~1 ns
The T100 delay depends on the increasing fraction of scattered photons as we move away from the light source. Therefore T100 depends on the water properties. However a delay greater than 3 ns is not expected.
AN
TA
RE
S-C
ali/
200
0-00
5A
NT
AR
ES
-Cal
i/2
000-
005
Pylos, 16-19/04/2007Pylos, 16-19/04/2007 C. BigongiariC. Bigongiari 3030
Early Photons EffectEarly Photons Effect• Order statistic:
– Theorem:
• If X1,…,Xn are r.v. following with density f and distribution function F, then the minimum has density function:
Naive Monte-Carlo simulation (No real data):The time measured in the OM is given by the early photonsThere is a backward shift in the arrival time distribution which is function of the Npe
)())(1( 1 xfxFn n
All Monte-Carlo (Calibob) simulations were done at phe level, hence this effect was not considered