Time Calibration with Optical Beacons Pylos, 16-19 Apr 2007 C.Bigongiari IFIC Pylos, 16-19 Apr 2007...

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


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


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


ANTARES Optical BeaconsANTARES Optical Beacons Optical Beacon Well controlled pulsed light source,

(LEDs or LASERs)

LED BeaconLASER Beacon

LASER

LEDs


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


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


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


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€ )


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


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


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


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%


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


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


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


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


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


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


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.


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


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


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


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


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 €


Overdrive ModeOverdrive Mode

Everything worked as expected

~33Hz

~330Hz

The LED OB flashing frequency has been recently increased by a factor ten


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


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


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

Time Calibration with Optical Beacons Pylos, 16-19 Apr 2007 C.Bigongiari IFIC Pylos, 16-19 Apr 2007...

Documents

Transcript of Time Calibration with Optical Beacons Pylos, 16-19 Apr 2007 C.Bigongiari IFIC Pylos, 16-19 Apr 2007...