CENTRO DI CULTURA SCIENTIFICA A.VOLTA

1. CENTRO DI CULTURA SCIENTIFICA A.VOLTA National Technical University of Athens NTUA VILLA OLMO COMO-ITALIA Design and Data Analysis Method of Receivers of HSRL for Atmospheric Monitoring in Ultra High Energy Cosmic Ray Experiments S. Maltezos, E. Fokitis, P. Fetfatzis, A. Georgakopoulou, V. Gika, I. Manthos and G. Koutelieris 11th ICATPP Conference on Astroparticle, Particle, Space Physics, Detectors and Medical Physics Applications Villa Olmo, 5-9 October, 2009 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

2. . Cosmic rays Three main shower components Muonic Hadronic Electromagnetic The detected cosmic ray flux peaks at about 15 km in altitude. The intensity increases with altitude and it changes with latitude. We consider that: 90% of the cosmic rays are protons, 9% are alpha particles and 1%are electrons. 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

3. Cosmic rays • Atmosphericfluorescenceexperiments • ASHRA [All-skySurveyHighResolutionAir-showerdetector] • Auger Project Fluorescence Group • EUSO (Extreme Universe Space Observatory) • HiRes (The High Resolution Fly’s Eye) • OWL (Orbiting Wide-angleLight collectors) Expected SOURCES Supernova Remnant SN1006 (AGN) Active galactic nuclei (with black-holes at their center) Cygnus X-3 The active galaxy NGC-4261 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

4. Extensive Air Showers EAS • Almost 90% of primary radiation is fluorescence radiation and that’ s why this method is useful. • The remaining energy is then distributed over the secondary particles. • Beyond the "shower maximum", the shower particles are gradually absorbed with an attenuation length of ~200 g/cm2. 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

5. Air Fluorescence • The passage of charged particles in an extensive air shower through the atmosphere results in the ionization and excitation of the gas molecules (mostly of nitrogen). The emitted radiation by de-excitation of the nitrogen molecules is extended mainly in the UV region. 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

6. Emission of cherenkov radiation • The aerosols cause exclusively Mie scattering and their presence in the atmosphere plays a significant role in the scattering of the Cherenkov radiation and it has to be monitored since it may be mixed, after scattering, with the air-fluorescence radiation. • To correct the Extensive Air Shower signal of air fluorescence for the air Cherenkov contamination, caused mainly by the aerosols, accurate data for the aerosols are needed. This can be made by atmospheric monitoring. 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

7. ATMOSPHERIC MONITORING • The atmospheric monitoring forms what is known as remote sensing of atmospheric properties with use in Air Fluorescence telescopes. • In the experiments for Ultra High Energy Cosmic Rays the signal detected by the fluorescence telescopes have to be corrected by means of mixing with the scattered Cherenkov radiation mostly by the aerosols in the atmosphere. • The main target of the monitoring is to determine the concentration of the aerosols, which is variable in time. 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

8. LIDAR LIght Detection And Ranging One of the main methods of atmospheric monitoring is the LIDAR. Types of Lidar • Doppler Lidar • Raman Lidar • DIAL Lidar (Differential Absorption Lidar) • High Spectral Resolution Lidar HSRL 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

9. High spectral resolution lidarhsrl • The High Spectral Resolution Lidar (HSRL) isa device based on a narrow-band laser and a pairof high resolution Fabry-Perot etalons to separatethe aerosol (Mie) and molecular (Rayleigh) scattering. • Different geometries have been proposed for LIDARatmospheric monitoring. • One isthat the emitter and the receiver are in backscatter mode. This means that light is collected only at the angle of 1800 as measured from the emitter. The light source is a pulsed Laser (expensive), so that the time interval determines the distance from the system. • The other is in Bi-static mode with acw (continuous wave) laser (low cost) on which we present the development of aprototype. By this technique we combine low cost and high accuracy. Spectral profile of backscattering from a mixture of molecules and aerosolsfor a temperature of 300 K. The spectral width of the narrow aerosol return is normallydetermined by the line width of the transmitting laser. 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

10. Bi-static lidar • Newtonian telescope of diameter D=250mm and f-number 5.5. • A solid state CW laser (OEM manufacturer) 120 mW at 532 nm, with coherence length exceeding 50m corresponding to δk~0.02 cm-1. • Two different Fabry-Perot etalons (spacer thickness: 50 mm for aerosol channel and 5 mm for molecular channel) with verified overall finesse of 17.5. • Two colored CCD cameras (Nikon D40) with 6 Mpixel with analysis (3040x2014) and pixel size 7.8 μm have been used for recording the fringe images for the two channels respectively. 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

11. PROTOTYPE Bi-static lidar 2 1 3 4 3 5 4 5 1 2 • Focus system • Input collimating lens • Narrow band filter • Diagonal mirror 5) CCD cameras (Nikon D40) 2) Fabry-Perot etalon 3) Output Lens 4) Optical benches 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

12. ANALYSIS METHOD OF THE RECORDED INTERFEROGRAMS • Main idea: exploit both the light intensity and the spectral information. • The total intensity is calculated by integration along the fringe ring system (rotated scanning). • Possible errors caused by the optical defects may partially destroy the circular symmetry of the fringes. • In the next we describe an analysis method which can be used on one fringe pattern system produced from a monochromatic beam. 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

13. The steps of the method • First step : we select the data points close to the peak of the fringes rings. Finally we identify each data point attributed to it the fringe order in which it belongs. These can be achieved by appropriate algorithms. • Second step: we apply a direct two-dimensional non-linear chi-square fit considering a model of a system of ellipses described in a arbitrary orthogonal coordinate system. where α,β,γ,δ,ζ and Φ are free parameters to be determine, p is the order of the fringe and ε is the excess fraction. By introducing vectors : A=[α,ζ,β,γ,δ,Φ]T, X=[x2,χy,y2,x,y,1] the equation above can be rewritten to the vector form FA(χ)= χ·Α=0 This equation corresponds to a linear system with six equations. We expect to have an ellipses. 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

14. Third step: we determine free parameter values in order to calculate the geometrical parameters of the ellipses and the excess fraction of the fringe pattern. The excess fraction ε is a significant parameter because its variation is proportional to the wavelength variation and thus it is used to study the laser beam frequency stability obtaining a number of successive interferograms. x0 and y0 are the coordinates of the center εcc is the eccentricity θ is the rotation angle of the ellipses 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

15. STABILITY STUDY • The valueof ε is related to the etalon spacing and source wavelength by the equation • and forδε=1 we have: where h is the spacer thickness of etalon and m the integer number of λ0/2. If h=50 mm and λ0= 532 nm then δλ0=0.0028 nm. 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

16. The application of the direct fit First step 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

17. Typical results εcc= 0.9989577 θ = 31.77542 ε = 0.455362 Assuming an expected wavelength 532 nm the abοve value of ε lead to an exact wavelength equal to 531.99849 nm. 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

18. Circular scanning - red is the initial, - blue is the final normalized to the initial for comparison reasons. 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

19. FABRY-PEROT CHARACTIRIZATION This work is extension of our instrumentation development for atmospheric monitoring using the High Spectral Resolution LIDAR as described above. We present spectra of natural mercury lines selected by interference filters, and using Fabry-Perot etalon with 0.25cm-1 free spectral range (FSR). In Figure 1, we see the intensity pattern corresponding to the central and the next fringe. We observe, although with low resolution, splitting of spectral lines due to isotopic shift.The interferogram taken with the 2 cm spacer etalon has smaller resolution than the 5 cm spacer etalon available. Figure 1 Figure 2 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS

20. Conclusions AND PROSPECTS • We are developing a prototype of HSRL in bi-static mode using two channels to separate the molecular and the aerosol signal. We also obtain a set of scattered signals in the laboratory in order to characterize the Fabry-Perot receivers. • We further developed and applied an analysis method based on two dimensional direct fit to fringe pattern. • Using this method we are able to evaluate the stability of the laser. • An alternative thermoelectrically cooled CCD sensor from SBIG and a liquid nitrogen cooled CCD sensor will be tested for the bi-static LIDAR. 5-9 October 11th ICATPP COMO-ITALY G. KOUTELIERIS