1. Measurement of the group refractive index in the ANTARES site with the Optical Beacons at 3 wavelengths ● Introduction ● Method ● Measurement with Monte Carlo ● Data results (Public plot) Velocity of light in water

2. Introduction (ANTRS-Phys/2001-005) ● Cerenkov angle given by phase velocity, but light propagates with group velocity ● Group to phase refractive index related by: ● Parametric formulation of phase refractive index has dependence of wavelength, temperature, salinity and pressure

3. Group and phase refractive index as function of lambda Measure velocity of light emitted by Optical Beacons in Antares water Fix: T=13.2ºC S=3.844%

4. Expected effects 1. Scattering: larger distance=>smaller velocity=> larger n 2. Early photon: Gaussian time distribution at source arrival time of first photon at nearby PMT earlier than expected larger time=>smaller velocity=> larger n OB PMT

5. Method (Led run) Led run=Flash light from a given position Measure time when light reaches PMT Time : T_1 Distance: D_1 Time : T_2 Distance: D_2

6. Method (Plot MC) Fit linear function => v_measured n=v_vacuum/v_measured

7. Method ● Take Led run ● Time = peak given by highest bin ● Position = nominal OM position ● Eliminate not significant peak (Noise>Peak-5*sqrt(Peak))=>defines max. fit value ● Charge per hit defines min. fit value ● Fit linear function (time vs. distance)=>v_measured ● Refraction index: n=v_vacuum/v_measured

8. MC test with two different refraction index Measurement n=1.379 +-0.001 (fit error) MC Input n=1.3786 (at 472nm=Led run) (absorption=60m and scattering=infinite) Measurement n=1.370+-0.003 (fit error) MC Input n=1.3681 (at 532nm=Laser run) (with absorption=28m and scattering=55m) Conclusion: Method gives reliable results Ask Ciro for MC

9. MC Measured refractive index n Absorption Scattering n 60 Inf 1.379+-0.001 60 55 1.382+-0.001 60 30 1.386+-0.002 120 55 1.382+-0.001 30 55 1.386+-0.002 MC Input n=1.3786 (at 472nm) Conclusion: Small model dependence (about 0.5%) more scattering=>larger n

10. Early photon effect (MC) Introduce cut to minimize early photon effect: Charge per hit minimal distance

11. Stability against cuts (Noise>Peak-5*sqrt(Peak))=>defines maximum fit value Charge per hit defines minimum fit value Change charge per hit cut to 5.0 => 0.2% Change charge per hit cut to 1.2 => 0.2% Change Peak-10*sqrt(Peak) cut => 0.5% Conclusion: Small cut dependence (about 0.5%)

12. Data (run 33987) 3 OM per position For fit: error in time 1 ns error in position 1 m

13. Data at 472 nm (22 runs) n=1.390+-0.007/sqrt(22) (+-0.2%)

14. Put errors together 1.Method stable against cuts +-0.5% 2.Method stable against different assumption for absorption and scattering +-0.5% 3.Measurement at lambda=470 nm (Led) n=1.390+- 0.2%(stat.)+-0.5%(exp.)+-0.5%(theo.) =>n=1.390+-0.010 ● Do same for Laser (3 runs) and UV (1 run)

15. How well do we know the wavelength of OB?

16. Optical Beacon Wavelength Measurement The three wavelength sources (LED @ 470 nm, and 400 nm and Laser) have been measured with a high res calibrated spectrometer from Ocean Optics HR4000CG-UV-NIR Measurements:  A sample of 20 cleaved blue LEDs (Agilent HLMP-CB15)  Usual standard LED of LOB Beacons  Nominal wavelength: 472 nm (35 nm FWHM)  A sample of 15 cleaved HUVL 400-520 B  LED used in L12F2 and L11F2  Nominal wavelength: 400 nm (20 nm FWHM) Spectrometer checked with the Green Nd-YAG Laser (532 nm) (D. Real, F Urbano, A Sánchez, J Ruiz-Rivas)

17. Optical Beacon Wavelength Measurement Blue LEDs @ 472 nm 3414.4468.5 FWHMSigmaMean value Mean Value distribution

18. Optical Beacon Wavelength Measurement Blue LEDs @ 400 nm 16.26.9403.1 FWHMSigmaMean value Mean value distribution

19. Conclusion