Transcript Document
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
The Auger Observatory
4 peripheral stations (eyes)
6 fluorescence telescopes / station
Azimuthal angle of view 1800
Los Leones, Coihueco, Los Morados, Norte
Los Leones building
Coihueco FD building
Installation of the mirror supports
The FD telescope
Diaphragm
PMT
camera
Shutter
UV Filter,
corrector ring
Spherical
mirror
The Schmidt optics
Spherical
aberration
C
Coma aberration
C
C
F
spot
Diaphragm
Coma
suppressed
C
Spherical focal
surface
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)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 degrees elevation
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
The FD telescope at
Los Leones
PMT
camera
Front end / read-out
electronics
mirror
HV + LV
The corrector ring
The ring lenses (aspherical profile) correct
the additional spherical aberration, keeping
the spot size within the design value of
15 mm diameter
factor 2 gain in
light collection
“Image” of a bright star
• The diameter of the spot is 15 mm
as calculated.
• Good check of the alignment of the
mirror elements
Fluorescence spectrum of nitrogen
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.
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)
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.
The FD camera
440 PMTs
90
cm
PMT active divider
Better gain stability
passive
active
Dark sky background
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
FD data acquisition
system
GPS time (hybrid operation)
data
Relative calibration
Xe lamp + optical fibers
• Equalization of PMT gain
• Stability of gain
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
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.
Hybrid vs. Surface Detector
1019 eV
1020 eV
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
Fraction of stereo FD
100
2,3,4
80
60
2
40
3
20
4
0
18
19
20
Log Energy (eV)
Shower geometry reconstruction
First step: reconstruct the
Shower – Detector Plane (SDP)
Shower
χi
RP
χ0
ψ
Telescope
RP
ti (χi) = t0 +
tg
c
χ0- χi
2
3 parameter fit : t0, RP and χ0
First hybrid event
FD on line display
FD - SD
matching
FD shower candidate
Triggered
pixels
FADC traces
100 ns time bin
Background event
Cosmic passing through PMTs
FD shower
crossing telescopes boundary
Laser shots reconstruction
Laser shot
axis
ψ
Laser
degrees
Laser shots reconstruction
RP (Km)
degrees
Ψ (degrees)
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
A “low-energy” shower fully
contained in the atmosphere
Longitudinal profile and
geometrical reconstruction
RP ~ 13 km
Θ ~ 570
χ0 ~ 820
Time vs. angle correlation for
a laser shot at RP = 25 km
Very useful to understand the analysis of the
real cosmic ray events !
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 !