Auger Fluorescence Detector
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Transcript of Auger Fluorescence Detector
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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
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The Auger Observatory 4 peripheral stations (eyes) 6 fluorescence telescopes / station Azimuthal angle of view 1800
Los Leones, Coihueco, Los Morados, Norte
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Los Leones building
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Coihueco FD building
Installation of the mirror supports
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The FD telescope
Spherical mirror
PMT camera
Diaphragm
UV Filter, corrector ring Shutter
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The Schmidt optics
C
Spherical aberration Coma aberration
Diaphragm Coma
suppressed
C
C
C
spot
F
Spherical focal surface
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Design of the telescopesBasic parameters defined from the requirement of accurate measurement of the shower profileAperture: 1.5 m2 effective areaPixel 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)2 , Δθ = Δs/R ~ (h/R) 3
f/1 optics is a good compromise: R = 3.4 mDiaphragm diameter = 1.7 mSpot 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
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The mirror systemShape 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
• AluminumReflectivity: 88.0% (with Al2O3 coating)• Polished GlassReflectivity: 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
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The FD telescope at Los Leones
PMT camera
mirror
Front end / read-outelectronics HV + LV
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The corrector ring
factor 2 gain in light collection
The ring lenses (aspherical profile) correct the additional spherical aberration, keeping the spot size within the design value of 15 mm diameter
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“Image” of a bright star
• The diameter of the spot is 15 mm as calculated.
• Good check of the alignment of the mirror elements
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Fluorescence spectrum of nitrogen
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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.
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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)
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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.
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The FD camera
90 cm
440 PMTs
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PMT active dividerBetter gain stability
passive
active
Dark sky background
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FD electronics/triggerThe 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 adjacentpixels
Third Level Software Trigger: time – space correlation
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FD data acquisition system
data
GPS time (hybrid operation)
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Relative calibrationXe lamp + optical fibers
• Equalization of PMT gain• Stability of gain
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Absolute calibrationDirect 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
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Absolute calibration
Preliminary result givesabout 5 photons / FADC count as average over all pixels of the camera
The drum mounted at Los Leones
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.
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Surface Hybrid Surface Hybrid
Δθ 2.00 0.40 1.00 0.40
Δ core 80 m 30 m 40 m 30 m
ΔE/E 18 % 4.2 % 7.0% 2.5 %
ΔXmax 17 g/cm2
15 g/cm2
1019 eV 1020 eV
Hybrid vs. Surface Detector
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43
2
2,3,4
Fraction of stereo FD
18 19 20
Log Energy (eV)
0
20
40
60
80
100
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Shower geometry reconstruction
ψ
First step: reconstruct the Shower – Detector Plane (SDP)
χ0χi
RP
Shower
Telescope
ti (χi) = t0 + tg
χ0- χi
2
3 parameter fit : t0, RP and χ0
RP
c
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First hybrid eventFD on line display
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FD - SDmatching
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FD shower candidate
100 ns time binTriggered
pixelsFADC traces
Background event Cosmic passing through PMTs
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FD shower
crossing telescopes boundary
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Laser shots reconstruction
degrees
Laser shot axis
ψ
Laser
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Laser shots reconstruction
degrees
RP (Km)
Ψ (degrees)
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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
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A “low-energy” shower fully contained in the atmosphere
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Longitudinal profile and geometrical reconstruction
RP ~ 13 kmΘ ~ 570
χ0 ~ 820
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Time vs. angle correlation for a laser shot at RP = 25 km
Very useful to understand the analysis of the real cosmic ray events !
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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 !