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

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

Spherical mirror

PMT camera

Diaphragm

UV Filter, corrector ring Shutter

The Schmidt optics

C

Spherical aberration Coma aberration

Diaphragm Coma

suppressed

C

C

C

spot

F

Spherical focal surface

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

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

The FD telescope at Los Leones

PMT camera

mirror

Front end / read-outelectronics HV + LV

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

“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

90 cm

440 PMTs

PMT active dividerBetter gain stability

passive

active

Dark sky background

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

FD data acquisition system

data

GPS time (hybrid operation)

Relative calibrationXe lamp + optical fibers

• Equalization of PMT gain• Stability of gain

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

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.

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

43

2

2,3,4

Fraction of stereo FD

18 19 20

Log Energy (eV)

0

20

40

60

80

100

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

First hybrid eventFD on line display

FD - SDmatching

FD shower candidate

100 ns time binTriggered

pixelsFADC traces

Background event Cosmic passing through PMTs

FD shower

crossing telescopes boundary

Laser shots reconstruction

degrees

Laser shot axis

ψ

Laser

Laser shots reconstruction

degrees

RP (Km)

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 kmΘ ~ 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 !