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

2. The Auger Observatory 6 fluorescence telescopes / station
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. UV Filter, corrector ring
The FD telescope
Diaphragm
Spherical mirror
PMT camera
UV Filter, corrector ring
Shutter

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

7. Design of the telescopes
Basic parameters defined from the requirement of accurate measurement of the shower profile
Aperture: 1.5 m2 effective area
Pixel size: 1.5 degrees
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) , 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 degrees elevation

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: % (with Al2O3 coating)
Polished Glass
Reflectivity: % (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. The FD telescope at Los Leones
PMT camera
mirror
Front end / read-out
electronics
HV + LV

10. The corrector ring
The ring lenses (aspherical profile) correct the additional spherical aberration, keeping the spot size within the design value of mm diameter
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
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.

14. The camera Pixel: PMT XP3062 with light collectors (45 mm wide)
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 divider Better 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. Absolute calibration Direct measurement of the response
of each channel to a given flux of incident photons.
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. molecular/Rayleigh & aerosol/Mie
Atmospheric absorption
LIDAR
Backscattering
Elastic bcks.
molecular/Rayleigh & aerosol/Mie
(Raman)
DAQ
Laser
Mirror

24. Hybrid vs. Surface Detector
1019 eV eV
Surface
Hybrid Δθ
2.00
0.40
1.00
Δ core
80 m
30 m
40 m
ΔE/E
18 %
4.2 %
7.0%
2.5 %
ΔXmax
17 g/cm2
15 g/cm2

25. Fraction of stereo FD 2,3,4 2 3 4 18 19 20 Log Energy (eV) 100 80 6040 3 20 4
Log Energy (eV)

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

27. First hybrid event FD on line display

28. FD - SD matching

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

30. crossing telescopes boundary
FD shower
crossing telescopes boundary

31. Laser shots reconstruction
Laser shot axis ψ
Laser
degrees

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

33. 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

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

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

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

37. 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 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 !