Auger Fluorescence Detector

1. Auger Fluorescence Detector Center for Cosmological Physics Enrico Fermi Institute Mini-Symposium on the Auger Observatory October 4, 2002 Giorgio Matthiae University of Roma II and INFN

2. The Auger Observatory 4 peripheral stations (eyes) 6 fluorescence telescopes / station Azimuthal angle of view 1800 Los Leones, Coihueco, Los Morados, Norte

3. Los Leones building

4. Coihueco FD building Installation of the mirror supports

5. The FD telescope Diaphragm Spherical mirror PMT camera UV Filter, corrector ring Shutter

6. Spherical aberration Coma aberration The Schmidt optics C C C F spot Diaphragm Coma suppressed C Spherical focal surface

7. Basic parameters defined from the requirement of accurate measurement of the shower profile Aperture: 1.5 m2 effective area Pixel size: 1.5 degrees Design of the telescopes Schmidt optics: - coma aberration eliminated, circle of least confusion (spot) independent of the incident direction - aperture defined by the diaphragm - mirror size larger than for classical design Spot size from spherical aberration: Δs ~ h (h/R)2 , Δθ = Δs/R ~ (h/R) 3 f/1 optics is a good compromise:R = 3.4 m Diaphragm diameter = 1.7 m Spot size : 0.5 degree (15 mm diameter) Pixel size: 1.5 degrees (45 mm ) (the spot size is 1/3 of the pixel size) Field of view: 30 degrees azimuth 28.6 degreeselevation

8. The mirror system • Shape nearly square due to square field of view. • Size: 3.5 m x 3.5 m in order to avoid vignetting. • Tesselation: 6 x 6 elements • Aluminum • Reflectivity: 88.0% (with Al2O3 coating) • Polished Glass • Reflectivity: 86.3% (with SiO2 coating) • The mirror elements are mounted on a rigid support structure. • Each element can be accurately aligned independently. • Quality tests: • - reflectivity at 370 nm • - spot size obtained with point light source at the center of curvature

9. PMT camera mirror The FD telescope at Los Leones Front end / read-out electronics HV + LV

10. The ring lenses (aspherical profile) correct the additional spherical aberration, keeping the spot size within the design value of 15 mm diameter The corrector ring factor 2 gain in light collection

11. “Image” of a bright star • The diameter of the spot is 15 mm as calculated. • Good check of the alignment of the mirror elements

12. Fluorescence spectrum of nitrogen

13. The UV filter (M-UG6) matches the fluorescence spectrum of N2. • Transmission: about 85 % at 350 nm, down to 20 % at 300 nm and 400 nm. • Reduction of “dark sky background” by nearly a factor of 8. The UV filter

14. The camera • Array of 440 hexagonal pixels placed on the spherical focal surface. (22 rows x 20 columns) • Pixel: PMT XP3062 with light collectors (45 mm wide)

15. The camera light collectors • Light collectors to recuperate light incident between the PMTs or at the very edge of the photocathode. • Plastic elements covered by aluminized mylar. • Test with light source simulating the spot created by the mirror shows recuperation of light.

16. The FD camera 440 PMTs 90 cm

17. PMT active dividerBetter gain stability passive active Dark sky background

18. FD electronics/trigger The PMT signal is sampled at a rate of 10 MHz by FADC with 12 bits. 100 ns First Level Trigger: Threshold regulated to keep single pixel rate at a given value, around 100 Hz. Second Level Trigger: pattern recognition algorithm 5 adjacent pixels Third Level Software Trigger: time – space correlation

19. FD data acquisition system GPS time (hybrid operation) data

20. Relative calibration • Xe lamp + optical fibers • Equalization of PMT gain • Stability of gain

21. Direct measurement of the response of each channel to a given flux of incident photons. Absolute calibration Wide light beam of uniform intensity provided by a UV LED (375 nm) and a flat cylinder (“drum”) with diffusing walls mounted outside the telescope aperture (ideally a “dome”). The number of photons is obtained from Si photodetector calibrated at NIST

22. Absolute calibration The drum mounted at Los Leones Preliminary result gives about 5 photons / FADC count as average over all pixels of the camera Another method: remote laser of known intensity shot vertically in the atmosphere. Calculation of Rayleigh and aerosol scattering allows predicting flux of photons at the telescope. Similar result.

23. Hybrid vs. Surface Detector 1019 eV 1020 eV

24. Fraction of stereo FD 100 2,3,4 80 60 2 40 3 20 4 0 18 19 20 Log Energy (eV)

25. First step: reconstruct the Shower – Detector Plane (SDP) Shower geometry reconstruction Shower RP χi χ0 ψ Telescope RP χ0- χi ti (χi) = t0 + tg c 2 3 parameter fit : t0, RP and χ0

26. First hybrid eventFD on line display

27. FD - SDmatching

28. Triggered pixels FADC traces 100 ns time bin FD shower candidate Background event Cosmic passing through PMTs

29. crossing telescopes boundary FD shower

30. Laser shots reconstruction Laser shot axis ψ Laser degrees

31. Laser shots reconstruction RP (Km) degrees Ψ (degrees)

32. Preliminary analysis • Pixel calibration • Atmospheric corrections • Fluorescence yield • Estimate of Cherenkov light • Reconstruction of the longitudinal profile • Fit with Gaisser-Hillas form • Estimate of the energy and of the depth of maximum Xmax • Geometrical reconstruction from correlation of time vs. elevation angle χi

33. A “low-energy” shower fully contained in the atmosphere

34. Longitudinal profile and geometrical reconstruction RP~ 13 km Θ ~ 570 χ0 ~ 820

35. Time vs. angle correlation for a laser shot at RP = 25 km Very useful to understand the analysis of the real cosmic ray events !

36. Outlook • All components of the 24 FD telescopes are financed. They are ready or ordered. • Installation and commissioning of the telescopes in the two buildings (Leones and Coihueco) will be completed in 2003. This makes ½ of the overall FD. • Some problem of funding for the construction of the remaining two buildings Morados and Norte but, good reasons for optimism !