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ARTICLE IN PRESS
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/nima
Nuclear Instruments and Methods in
Physics Research A
0168-9002/$ - see front matter & 2010 Elsevier B.V All rights reserved.
doi:10.1016/j.nima.2010.04.023
Nuclear Instruments and Methods in Physics Research A 620 (2010) 227–251
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M. Ziolkowski
am
, The Pierre Auger Collaboration
a
Centro Ato
´
mico Bariloche and Instituto Balseiro (CNEA-UNCuyo-CONICET), San Carlos de Bariloche, Argentina
b
Centro Ato
´
mico Constituyentes (Comisio
´
n Nacional de Energı
´
a Ato
´
mica/CONICET/UTN-FRBA), Buenos Aires, Argentina
c
Centro de Investigaciones en La
´
seres y Aplicaciones, CITEFA and CONICET, Argentina
d
Departamento de Fı
´
sica, FCEyN, Universidad de Buenos Aires y CONICET, Argentina
e
IFLP, Universidad Nacional de La Plata and CONICET, La Plata, Argentina
f
Instituto de Astronomı
´
ayFı
´
sica del Espacio (CONICET), Buenos Aires, Argentina
g
Universidad Tecnolo
´
gica Nacional, Facultad Regional Mendoza, (UTN-FRM), Mendoza, Argentina
h
Pierre Auger Southern Observatory, Malarg
¨
ue, Argentina
i
Pierre Auger Southern Observatory and Comisio
´
n Nacional de Energı
´
a Ato
´
mica, Malarg
¨
ue, Argentina
j
University of Adelaide, Adelaide, S.A., Australia
k
Universidad Catolica de Bolivia, La Paz, Bolivia
l
Universidad Mayor de San Andre
´
s, Bolivia
m
Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro, RJ, Brazil
n
Pontifı
´
cia Universidade Cato
´
lica, Rio de Janeiro, RJ, Brazil
o
Universidade de S
~
ao Paulo, Instituto de Fı
´
sica, S
~
ao Carlos, SP, Brazil
p
Universidade de S
~
ao Paulo, Instituto de Fı
´
sica, S
~
ao Paulo, SP, Brazil
q
Universidade Estadual de Campinas, IFGW, Campinas, SP, Brazil
r
Universidade Estadual de Feira de Santana, Brazil
s
Universidade Estadual do Sudoeste da Bahia, Vitoria da Conquista, BA, Brazil
t
Universidade Federal da Bahia, Salvador, BA, Brazil
u
Universidade Federal do ABC, Santo Andre
´
, SP, Brazil
v
Universidade Federal do Rio de Janeiro, Instituto de Fı
´
sica, Rio de Janeiro, RJ, Brazil
w
Universidade Federal Fluminense, Instituto de Fisica, Nitero
´
i, RJ, Brazil
x
Charles University, Faculty of Mathematics and Physics, Institute of Particle and Nuclear Physics, Prague, Czech Republic
y
Institute of Physics of the Academy of Sciences of the Czech Republic, Prague, Czech Republic
J. Abraham et al. / Nuclear Instruments and Methods in Physics Research A 620 (2010) 227–251228
ARTICLE IN PRESS
z
Palacky
´
University, Olomouc, Czech Republic
aa
Institut de Physique Nucle
´
aire d’Orsay (IPNO), Universite
´
Paris 11, CNRS-IN2P3, Orsay, France
ab
Laboratoire AstroParticule et Cosmologie (APC), Universite
´
Paris 7, CNRS-IN2P3, Paris, France
ac
Laboratoire de l’Acce
´
le
´
rateur Line
´
aire (LAL), Universite
´
Paris 11, CNRS-IN2P3, Orsay, France
ad
Laboratoire de Physique Nucle
´
aire et de Hautes Energies (LPNHE), Universite
´
s Paris 6 et Paris 7, CNRS-IN2P3, Paris Cedex 05, France
ae
Laboratoire de Physique Subatomique et de Cosmologie (LPSC), Universite
´
Joseph Fourier, INPG, CNRS-IN2P3, Grenoble, France
af
SUBATECH, CNRS-IN2P3, Nantes, France
ag
Bergische Universit
¨
at Wuppertal, Wuppertal, Germany
ah
Forschungszentrum Karlsruhe, Institut f
¨
ur Kernphysik, Karlsruhe, Germany
ai
Forschungszentrum Karlsruhe, Institut f
¨
ur Prozessdatenverarbeitung und Elektronik, Germany
aj
Max-Planck-Institut f
¨
ur Radioastronomie, Bonn, Germany
ak
RWTH Aachen University, III. Physikalisches Institut A, Aachen, Germany
al
Universit
¨
at Karlsruhe (TH), Institut f
¨
ur Experimentelle Kernphysik (IEKP), Karlsruhe, Germany
am
Universit
¨
at Siegen, Siegen, Germany
an
Dipartimento di Fisica dell’Universit
a and INFN, Genova, Italy
ao
Universit
a dell’Aquila and INFN, L’Aquila, Italy
ap
Universit
a di Milano and Sezione INFN, Milan, Italy
aq
Dipartimento di Fisica dell’Universit
a del Salento and Sezione INFN, Lecce, Italy
ar
Universit
a di Napoli ‘‘Federico II’’ and Sezione INFN, Napoli, Italy
as
Universit
a di Roma II ‘‘Tor Vergata’’ and Sezione INFN, Roma, Italy
at
Universit
a di Catania and Sezione INFN, Catania, Italy
au
Universit
a di Torino and Sezione INFN, Torino, Italy
av
Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo (INAF), Palermo, Italy
aw
Sezione INFN, Catania, Italy
ax
Istituto di Fisica dello Spazio Interplanetario (INAF), Universit
a di Torino and Sezione INFN, Torino, Italy
ay
INFN, Laboratori Nazionali del Gran Sasso, Assergi (L’Aquila), Italy
az
Dipartimento di Ingegneria dell’Innovazione dell’Universit
a del Salento and Sezione INFN, Lecce, Italy
ba
INFN, Laboratori Nazionali del Sud, Catania, Italy
bb
Beneme
´
rita Universidad Auto
´
noma de Puebla, Puebla, Mexico
bc
Centro de Investigacio
´
n y de Estudios Avanzados del IPN (CINVESTAV), Me
´
xico, D.F., Mexico
bd
Universidad Michoacana de San Nicolas de Hidalgo, Morelia, Michoacan, Mexico
be
Universidad Nacional Autonoma de Mexico, Mexico, D.F., Mexico
bf
IMAPP, Radboud University, Nijmegen, Netherlands
bg
Kernfysisch Versneller Instituut, University of Groningen, Groningen, Netherlands
bh
NIKHEF, Amsterdam, Netherlands
bi
ASTRON, Dwingeloo, Netherlands
bj
Institute of Nuclear Physics PAN, Krakow, Poland
bk
University of Ło
´
dz
´
, Ło
´
dz
´
, Poland
bl
LIP and Instituto Superior Te
´
cnico, Lisboa, Portugal
bm
J. Stefan Institute, Ljubljana, Slovenia
bn
Laboratory for Astroparticle Physics, University of Nova Gorica, Slovenia
bo
Instituto de Fı
´
sica Corpuscular, CSIC-Universitat de Val
encia, Valencia, Spain
bp
Universidad Complutense de Madrid, Madrid, Spain
bq
Universidad de Alcala
´
, Alcala
´
de Henares (Madrid), Spain
br
Universidad de Granada & C.A.F.P.E., Granada, Spain
bs
Universidad de Santiago de Compostela, Spain
bt
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford, UK
bu
School of Physics and Astronomy, University of Leeds, UK
bv
Argonne National Laboratory, Argonne, IL, USA
bw
Case Western Reserve University, Cleveland, OH, USA
bx
Colorado School of Mines, Golden, CO, USA
by
Colorado State University, Fort Collins, CO, USA
bz
Colorado State University, Pueblo, CO, USA
ca
Fermilab, Batavia, IL, USA
cb
Louisiana State University, Baton Rouge, LA, USA
cc
Michigan Technological University, Houghton, MI, USA
cd
New York University, New York, NY, USA
ce
Northeastern University, Boston, MA, USA
cf
Ohio State University, Columbus, OH, USA
cg
Pennsylvania State University, University Park, PA, USA
ch
Southern University, Baton Rouge, LA, USA
ci
University of California, Los Angeles, CA, USA
cj
University of Chicago, Enrico Fermi Institute, Chicago, IL, USA
ck
University of Hawaii, Honolulu, HI, USA
cl
University of Nebraska, Lincoln, NE, USA
cm
University of New Mexico, Albuquerque, NM, USA
cn
University of Pennsylvania, Philadelphia, PA, USA
co
University of Wisconsin, Madison, WI, USA
cp
University of Wisconsin, Milwaukee, WI, USA
cq
Institute for Nuclear Science and Technology (INST), Hanoi, Vietnam
Corresponding author.
E-mail address: prouza@fzu.cz (M. Prouza).
1
At Caltech, Pasadena, USA.
2
On leave of absence at the Instituto Nacional de Astrofisica, Optica y Electronica.
3
Deceased.
4
At Konan University, Kobe, Japan.
J. Abraham et al. / Nuclear Instruments and Methods in Physics Research A 620 (2010) 227–251 229
ARTICLE IN PRESS
article info
Article history:
Received 27 May 2009
Received in revised form
16 March 2010
Accepted 7 April 2010
Available online 18 April 2010
Keywords:
Cosmic rays
Fluorescence detector
abstract
The Pierre Auger Observatory is a hybrid detector for ultra-high energy cosmic rays. It combines a
surface array to measure secondary particles at ground level together with a fluorescence detector to
measure the development of air showers in the atmosphere above the array. The fluorescence detector
comprises 24 large telescopes specialized for measuring the nitrogen fluorescence caused by charged
particles of cosmic ray air showers. In this paper we describe the components of the fluorescence
detector including its optical system, the design of the camera, the electronics, and the systems for
relative and absolute calibration. We also discuss the operation and the monitoring of the detector.
Finally, we evaluate the detector performance and precision of shower reconstructions.
& 2010 Elsevier B.V All rights reserved.
1. Introduction
The hybrid detector of the Pierre Auger Observatory [1]
consists of 1600 surface stations—water Cherenkov tanks and
their associated electronics—and 24 air fluorescence telescopes.
The Observatory is located outside the city of Malarg
¨
ue, Argentina
(691W, 351S, 1400 m a.s.l.) and the detector layout is shown in
Fig. 1. Details of the construction, deployment and maintenance of
the array of surface detectors are described elsewhere [2]. In this
paper we will concentrate on details of the fluorescence detector
and its performance.
The detection of ultra-high energy
5
cosmic rays using nitrogen
fluorescence emission induced by extensive air showers is a well
established technique, used previously by the Fly’s Eye [4] and
HiRes [5] experiments. It is used also for the Telescope Array [6]
project that is currently under construction, and it has been
proposed for the satellite-based EUSO (Extreme Universe Space
Observatory) and OWL (Orbiting Wide-angle Light-collectors)
projects, which have recently evolved into the proposals of the
JEM-EUSO mission [7] (EUSO onboard the Japanese Experiment
Module at the International Space Station), and of the S-EUSO
free-flying satellite mission [8].
Charged particles generated during the development of
extensive air showers excite atmospheric nitrogen molecules,
and these molecules then emit fluorescence light in the
3002430 nm range (see Fig. 2). The number of emitted
fluorescence photons is proportional to the energy deposited in
the atmosphere due to electromagnetic energy losses by the
charged particles. By measuring the rate of fluorescence emission
as a function of atmospheric slant depth X, an air fluorescence
detector measures the longitudinal development profile dE=dXðXÞ of
the air shower. The integral of this profile gives the total energy
dissipated electromagnetically, which is approximately 90% of the
total energy of the primary cosmic ray.
For any waveband, the fluorescence yield is defined as the
number of photons emitted in that band per unit of energy loss by
charged particles. The absolute fluorescence yield in air at 293 K
and 1013 hPa from the 337 nm fluorescence band is
5:057 0:71 photons=MeV of energy deposited, as measured in
Ref. [9]. For the reconstruction of the cosmic ray showers at the
Pierre Auger Observatory, this absolute measurement is combined
with the relative yields at other fluorescence bands as measured in
Ref. [10].
6
Since a typical cosmic ray shower spans over 10 km in
altitude, it is important to stress that due to collisional quenching
effects the fluorescence yield is also dependent on pressure,
temperature and humidity of the air [10,12]. More detail about
recent relevant fluorescence yield measurements and a compila-
tion of experimental results is available in a recent review [13].
The fluorescence detector (FD) comprises four observation
sites—Los Leones, Los Morados, Loma Amarilla, and Coihueco—
located atop small elevations on the perimeter of the SD array. Six
independent telescopes, each with field of view of 301 301 in
azimuth and elevation, are located in each FD site. The telescopes
face towards the interior of the array so that the combination of the
six telescopes provides 1801 coverage in azimuth. Fig. 3 shows the
arrangement of the telescopes inside an observation site (Fig. 4).
This arrangement of four FD sites was the optimum solution to
the primary design goal of ensuring 100% FD triggering efficiency
above 10
19
eV over the entire area of the surface detector. At the
Malarg
¨
ue site a large flat area, ideal for deployment of the surface
detector, is bordered with a number of small hills suitable for FD
sites. The arrangement of four inward-looking FD sites is a cost-
effective way of ensuring full coverage without wasteful overlaps,
and of minimizing the average distance to detected air showers,
thus reducing uncertainties in atmospheric transmission correc-
tions. Requiring ‘‘stereo’’ observations (showers triggering two FD
sites) was not a strong design criterion because of the excellent
‘‘hybrid’’ geometry reconstruction available when combining
information from the FD and the SD (Section 7), although stereo
events are common at the highest energies.
Fig. 3 depicts an individual FD telescope. The telescope is
housed in a clean climate-controlled building. Nitrogen fluores-
cence light enters through a large UV-passing filter window and a
Schmidt optics corrector ring. The light is focused by a 10 square
meter mirror onto a camera of 440 pixels with photomultiplier
light sensors. Light pulses in the pixels are digitized every 100 ns,
and a hierarchy of trigger levels culminates in the detection and
recording of cosmic ray air showers.
This paper is organized as follows. In Section 2 we describe the
components of the optical system of an individual telescope, and
in Section 3 we focus on the telescope camera. The electronics of a
fluorescence telescope and the data acquisition system (DAQ) of
an FD station are described in Section 4. The details of the
calibration hardware and methods are given in Section 5, and the
performance, operation and monitoring of the fluorescence
detector are explained in Section 6. Finally, in Section 7 we
concentrate on the basics of shower reconstruction using the
measured fluorescence signal, and in Section 8 we summarize.
5
For the purposes of this paper we define the ultra-high energy to be
\10
18
eV.
6
This apparently unusual combination of two distinct experiments is
determined by the fact that although the latter experiment, the AIRFLY, has
already produced very detailed studies of relative band intensities, and of yield
(footnote continued)
dependence on pressure, temperature and humidity, only the preliminary results
for the absolute fluorescence yield were published [11].
J. Abraham et al. / Nuclear Instruments and Methods in Physics Research A 620 (2010) 227–251230
ARTICLE IN PRESS
2. Optical system
The basic elements of the optical system in each FD telescope
are a filter at the entrance window, a circular aperture, a corrector
ring, a mirror and a camera with photomultipliers.
7
The
geometrical layout of the components is depicted in Fig. 6 .
The window is an optical filter made of Schott MUG-6 glass
[15]. This absorbs visible light while transmitting UV photons
from 290 nm up to 410 nm wavelength, which includes
almost all of the nitrogen fluorescence spectrum (see Fig. 5).
Without the filter window, the fluorescence signals would be lost
in the noise of visible photons (Fig. 6).
The aperture, the corrector ring, the mirror, and the PMT
camera constitute a modified Schmidt camera design that
partially corrects spherical aberration and eliminates coma
aberration. The size of the aperture is optimized to keep the spot
size
8
due to spherical aberration within a diameter of 15 mm, i.e.
90% of the light from a distant point source located anywhere
within the 30 1 301 FOV of a camera falls into a circle of this
diameter. This corresponds to an angular spread of 0.51.In
comparison, the FOV of a single camera pixel is 1.51. The light
distribution within the spot is described by the point spread
function (PSF) shown in Fig. 7.
The schematic view of the spot size diagrams over the whole
FOV is shown in Fig. 8, where the rows correspond to viewing
Fig. 1. Status of the Pierre Auger Observatory as of March 2009. Gray dots show the
positions of surface detector stations, lighter gray shades indicate deployed detectors,
while dark gray defines empty positions. Light gray segments indicate the fields of view
of 24 fluorescence telescopes which are located in four buildings on the perimeter of
the surface array. Also shown is a partially completed infill array near the Coihueco
station and the position of the Central Laser Facility (CLF, indicated by a white square).
The description of the CLF and also the description of all other atmospheric monitoring
instruments of the Pierre Auger Observatory is available in Ref. [3].
Fig. 2. Measured fluorescence spectrum in dry air at 800 hPa and 293 K. From Ref. [10].
Fig. 3. Schematic layout of the building with six fluorescence telescopes.
Fig. 4. Schematic view of a fluorescence telescope of the Pierre Auger Observatory.
7
We remind the reader that the system is housed in a clean climate-
controlled building, where an air-conditioning system is set to stabilize the
temperature at 21 1C. This helps to minimize thermal dilation of the system and to
maintain the alignment of mechanical and optical components.
8
The image of the point source at infinity on the focal surface of the optical
system is commonly called the ‘‘spot’’ in optics, but it may be better known as a
‘‘point spread function’’. The size of the spot characterizes the quality of the optical
system.
J. Abraham et al. / Nuclear Instruments and Methods in Physics Research A 620 (2010) 227–251 231
