Abstract   The Pierre Auger Cosmic Ray Observatory is designed to elucidate the origin and nature of Ultra High Energy Cosmic Rays (UHECR). With an unprecedented aperture and sensitivity, and with the use of a hybrid technique, it will be able to collect and characterize a large number of Extensive Air Showers generated by primaries with energies greater than 1019 eV. In this paper we describe the Fluorescence Detector and its operation during the first phase of the experiment, called Engineering Array

Stefano Argiro

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1The Pierre Auger Observatory and its Fluorescence DetectorS. Argirò for the Auger Collaboration Università degli Studi di Torino and INFNThe Pierre Auger Cosmic Ray Observatory is designed to elucidate the origin and nature of Ultra High Energy Cosmic Rays(UHECR). With an unprecedented aperture and sensitivity, and with the use of a hybrid technique, it will be able to collect andcharacterize a large number of Extensive Air Showers generated by primaries with energies greater thaneV. In this paperwe describe the Fluorescence Detector and its operation during the first phase of the experiment, called Engineering Array.1. IntroductionExperimental evidence of air showers generated byprimaries above 	 eV is reported by various ex-periments using different techniques [1], [2]. This ev-idence raises several questions. First, it is not clearwhat kind of mechanism is able to accelerate the pri-maries to such energy. Furthermore, the problemof propagation exists: simple considerations, in fact,predict that a proton of 	 eV cannot propa-gate for more than 100 Mpc, due to interaction withthe Cosmic Microwave Background (the GZK cutoff[3], [4]). Finally, the problem of the arrival directionand its isotropy or anisotropy deals with the distribu-tion of sources and the pattern of intergalactic mag-netic fields.The experimental situation is at the moment un-clear, with some experiments claiming detection ofanisotropy and observation of GZK - violating fea-tures of the flux [1], some experiments claiming theopposite [5]. Only a dramatic increase of the statisticsand a robust experimental technique can disentanglethe problem.2. The need for a hybrid detectorMost UHECR experiments use either the groundarray technique, in which the secondary particles aredetected at ground level, or the air fluorescence tech-nique, in which the shower longitudinal profile istracked. The use of both techniques in combinationcould help understanding the incompatibilities, andwould offer an instrument of unprecedented preci-sion. The fluorescence detector has a duty cycle of10-15% (in fact it runs on clear nights when the moonis below 50%). The corresponding fraction of eventscan be used to calibrate the Surface Detector, whichdetermines the energy in a way that has some depen-dence on the hadronic interaction modeling.The hybrid reconstruction method improves uponwhat can be achieved by either the FD or the SD onits own [6]. Core locations have been determined us-ing the engineering hybrid data with an uncertainty ofapproximately 100 meters. Arrival directions in theengineering data are determined to a fraction of a de-gree. The geometric uncertainties are already smallenough to have little effect on the determination ofshower energy and depth of maximum, and are ex-pected to improve with an upgrade of the timing hard-ware. Calibration uncertainties are also small. Statis-tical uncertainties will be appropriately dominated byPoisson limitations in the collected signal and back-ground, except at very high energies where the signal-to-noise will be especially large. New measurementsof the atmospheric fluorescence yield and new meth-ods of atmospheric aerosol monitoring are being pur-sued to reduce the systematic errors that may dom-inate the uncertainties in energy and depth of maxi-mum.3. The Pierre Auger ObservatoryThe Pierre Auger Observatory will consist of twosites, one in the northern, one in the southern hemi-sphere, each supporting a detection area of about3000 km  . This has to be compared with an ex-pected flux of about 1/year/km  /sr above 10  eV and0.01 /year/km  /sr above 10  eV. The instrument willuse a hybrid technique. At each site, 1600 waterCerenkov detectors, placed in a hexagonal grid with2CameraSegmented MirrorApertureShower TrackCorrector RingFigure 1. Schematic view of the Fluorescence Telescope1500 m spacing, will be overlooked by 24 air fluores-cence telescopes. While the ground array detects sec-ondary particles, giving a direct measurement of thelateral distribution function, the fluorescence detectorrecords fluorescence photons as a function of time,therefore measuring the longitudinal development ofthe shower.An engineering hybrid prototype consisting of 32ground detectors and two fluorescence telescopes(corresponding to a field of observation of 60  in az-imuth and 30  in elevation) has been successfully op-erated from December 2001 to March 2002.4. The Surface DetectorEach ground station consists of a cylindrical wa-ter tank with 10m  top area and 1.2m height. ThreePMTs detect Cerenkov light produced by muons andelectrons traversing the radiator. The station is pow-ered by means of solar panels and batteries, and com-municates with the Central Data Acquisition via awireless link. The signal of each PMT is sampled at40 MHz by two 12 bit FADC, one reading out theanode and one the last dynode. Two levels of localtrigger are implemented at the station, while the thirdlevel trigger fires when four stations are hit, achievingfull efficiency above 	  eV. The goal of the grounddetector is the determination of the particle density at1000 m from the core [6], which is related to primaryenergy, while the shower geometry can be inferredfrom the arrival time of the shower front at differenttanks. In addition, the time structure of the signal ismeasured, which makes the detector sensitive to pri-mary composition.5. The Fluorescence DetectorIn the following sub-sections the Fluorescence De-tector of the Auger Observatory will be described.5.1. Fluorescence PhysicsWhen the electrons of the shower interact with thenitrogen in the atmosphere, fluorescence photons are3emitted isotropically, with wavelengths in the range300-420 nm. Each electron emits four to five photonsper meter of track length, depending on the tempera-ture and pressure and therefore on altitude [7]. Fromthe number of photons, one can infer the shower sizeat each atmospheric depth (X). The light signal origi-nating in a layer between  and  (where depends on the sampling) and received by the detectoris given by:! #"%$'&)(  +* "-,/.  #"01243 65 798:;8=< 3 " (1)where & (  +* " is the fluorescence yield at the emissionaltitude*,0is the collection area,3the observationdistance, 7 is the time the shower takes to travel from to >? when viewed from the detector ( 7 de-pends solely on the geometry), : is the collection ef-ficiency and <  3 " factorizes the transmission of theatmosphere. The atmosphere in the sensitive volumeof the Observatory is characterized by means of Li-dars, Horizontal Attenuation Monitors, weather sta-tions, cloud monitors and radiosondes.The contribution of direct and scattered Cerenkovlight must be subtracted. The electromagnetic energycan then be calculated as: .-@ $ BADCE, .  #"F (2)where , .  #" is the number of electrons at depthX[ GA @IH ], BA is the critical energy of electrons in airand  C their radiation length.5.2. OpticsThe Auger collaboration has selected a Schmidt op-tics for the fluorescence telescopes, which structureis shown in figure 1. A telescope unit consists of aspherical mirror, composed of 36 to 60 subsegments,with a radius of curvature of 3400 mm. On the focalsurface is a camera of 440 photomultipliers (pixels),each with 1.5 J 1.5 FKLM field of view. The camerasurface is also quasi-spherical, with R N 1700mm . Tofill the gaps between the hexagonal photomultipliers,special light collection devices have been engineered[8]. The circular aperture has a diameter of 2.20 mand features both an optical filter (MUG-6) to selectfluorescence photons in the range 300-400 nm [9]and the Corrector Ring. This is an annular Schmidtlens 25 cm wide, effectively doubling the collectionarea without degrading the image with spherical orcoma aberration. This feature is particularly impor-tant when taking into account that the signal to noiseratio is proportional to the aperture radius.The above geometry results in a field of view of 30  inelevation and 30  in azimuth for each telescope unit.Groups of six telescopes form an eye with 180  viewin azimuth. Four such eyes overlook the 3000 km ground array.5.3. ElectronicsThe readout of each PMT is of the current samplingtype. The waveform is sampled at 10 MHz with a 12bit resolution and 15 bit dynamic range, as describedbelow. The structural diagram of the electronic chainis shown in figure 2 Attached to the PMT is the HeadElectronics, which consists of a bias printed circuitboard (pcb) and a driver pcb. The bias to the PMTis designed to achieve low power and high linearityeven in presence of a large DC current, such as thecurrent induced by the night sky background, whichis of the order of 0.5 O A for dark sky, but increases to3.7 O A when a star is in the field of view, or to 2 O A inpresence of scattered moonlight. The driver pcb im-plements a differential line driver with a high rejec-tion of common mode noise. The signals out of thedrivers are collected at the Distribution Boards androuted to the Front End Crate .Each crate serves one telescope, and contains twentyFirst Level Trigger (FLT) / Analog Boards and oneSecond Level Trigger (SLT) board. Each FLT / AnalogBoard serves 22 channels, or a column of pixels in thecamera. The Analog Board processes the signal priorto sampling. It implements a receiver section, an antialiasing Bessel filter and a programmable gain sec-tion. The 15 bit dynamic range with a 12 bit FADC isachieved through the following technique. Each pixelis processed with the normal high gain channel. Thesum of eleven pixels (even or odd in a column) is pro-cessed by a virtual low gain channel. If one pixel inthe group of eleven saturates, the signal can be ob-tained from the virtual channel. Thanks to the pixelarrangement and the time characteristics of the fluo-rescence signal, at most one channel saturates in thegroup [10].The FLT board samples the analog signal of eachchannel at 10 MHz and provides the first level trigger,or pixel trigger. A boxcar running sum of ten FADC4PMTBIAS PCBDRIVER PCB Distribution BoardFront−EndCrateSLTFLTAB11x20x2xCamera20 boards x 22 ch= 440 chEye PC Mirror PCFigure 2. Schematic view of the DAQ chain of a Fluorescence telescopesamples is performed and compared with a thresholdvalue. This threshold changes dynamically to keepthe trigger rate close to 100 Hz. Fired pixels are thenscanned by the second level trigger (SLT), or cameratrigger, which recognizes patterns induced by a fluo-rescence track. The FLT flag for a single pixel persists20 O s, and the camera is scanned for patterns everyother O s. The SLT rate during the EA run was around0.3 Hz.Each Front End Crate is connected to a Mirror PC,which runs the processes that take care of its acqui-sition. On the Mirror PC runs the Third Level Trig-ger. This is a software trigger that is responsible forrejection of background events. These events consistmainly of muons hitting directly the camera and light-nings. The 3 CQP level trigger rate was around RSJT	VUXWHz.The six Mirror PC’s are then connected to the Eye PC,on which event building and run control take place.5.4. Detector CalibrationAbsolute detector calibration is required for the airfluorescence measurement. The strategy envisagedfor the Auger Fluorescence Detector is the follow-ing. Each telescope undergoes an absolute calibra-tion monthly, with the method of the drum calibra-tion. Between absolute calibrations, that is for everynight of observation or even several times within thenight, a relative calibration is performed using a setof optical fibers which carry the light from a xenonlamp.The drum calibration device consists of two layersof diffusely reflective material, holding in the centera calibrated light source. The outer drum surface istranslucent to allow photon emission. It reflects anddiffuses light in every direction, with anisotropies lessthan 2% in the region of interest. The light output ismeasured by means of calibrated PMTs. The drumis placed at the aperture and therefore illuminates allpixel directions uniformly. Knowing the light flux andcomparing with the detector response, absolute cali-bration is achieved.5t0χ0R PχiSDPFDFigure 3. The geometry of the air shower axis withrespect to the fluorescence detector5.5. Reconstruction techniquesThe first step in reconstructing a shower with theFluorescence Detector is finding the Shower DetectorPlane (SDP), which is the plane defined by the detec-tor and the shower axis. This is done selecting the trialplane that minimizes the distance from the hit pixelsdirections. In second place one must find the posi-tion of the shower axis within the SDP. This can bedone in three ways. If the event is seen in monocularmode, the time fit can be used. The geometrical pa-rameters that describe the shower in the SDP are YSZ(distance shower-detector), 7  (time when the point ofminimum distance is reached) and [  (angle betweenshower and ground) (see fig. 3). These parameters arerelated to the hit times (time when the light reachesthe pixel) by:7  [)\]"%$'7   Y_^57`, [ _a [ \ "b (3)and can be found by minimization of the quantity[9c$edgfihkj U hl j-m Hno j , where 7 \ is the measured hit timefor the p hq pixel. If the event was detected in hybridmode, the point and time of impact of the shower withthe ground adds a useful constraint to the fit. Finally,if the shower was detected by two eyes, the two SDPscan be intersected to find the shower axis.Once the shower geometry is determined, the lightprofile at diaphragm, or the number of photons im-pinging on the aperture versus time, is found. At thispoint, eq. 1 can be used to estimate the number ofparticles at each atmospheric depth. The contributionof direct and scattered Cerenkov light must be sub-tracted. The resulting profile can then be fit using aGaisser-Hillas parametrization, eq. 2 being used toestimate the primary energy. Figure 4 shows an ex-ample of a reconstructed shower profile.slant depth [g/cm^2]400 600 800 1000 1200 1400shower size [particles]02004006008001000120014001600180020006x10Longitudinal ProfileFigure 4. Reconstructed longitudinal profile of ashower landing about 16 km from the detector. Theestimated energy is around 1.5 J!	 r eV. The line isa fit to a Gaisser-Hillas function.5.6. Engineering Array resultsDuring the Engineering Array (EA) run, a specialtrigger was set up to prove the ability of the systemof detecting hybrid events that would fall under the4-tank threshold required for normal operation. Anonline program was setup to reconstruct the showergeometry as seen by the FD in real time and there-fore trigger the SD. 77 events where detected with thistrigger.To test the FD geometrical reconstruction capabilities,laser beams where fired at different positions and di-rections. The results from one of these tests is shown6   [deg]recψ - laserψ0 0.5 1 1.5 202468101214Nent = 54     Mean  = 0.4556RMS   = 0.3769Zenit 35.77 deg - Azimuth -43.1 degLaser BeamFigure 5. Angular difference between true and recon-structed geometry for a set of laser shot at the samedirection.in figure 5, where the true geometry is compared tothe reconstructed geometry of the laser shot. Theseshots were performed aiming at a known star, there-fore the direction is known with high precision. Thelaser position is measured using a GPS system. Stud-ies are underway to determine the behavior of the er-ror with shower inclination and impact parameter.During the EA run, the detector was run for about 250hours, corresponding to a duty cycle around 11%. Op-erations took place with up to 50% of moon illumina-tion. The FD detected around 1000 shower candidatesduring the EA run. Analysis of these events is ongo-ing.6. ConclusionThe Engineering Array phase demonstrated that thedesign requirements of Fluorescence and Surface de-tector are fulfilled and the implementation is satisfac-tory. The Auger Observatory is able to detect hybridevents and reconstruct them with the required preci-sion. Further effort has started to complete the Obser-vatory and answer the questions on UHECRs.REFERENCES1. M. Takeda et al. , Phys. Rev. Letters 81 (1998)1163-11662. D.J. Bird, et al.1995, Ap.J., 441 144.3. K. Greisen, Phys. Rev. Lett. 16 (1966) 7484. G. Zatsepin and V. Kuzmin, JETP Lett. 4, 78(1966)5. D. Bergman, talk given at ICHEP 20026. Pierre Auger Observatory Technical Design Re-view, available http://www.auger.org7. F. Kakimoto et al., Nucl.Instrum.Meth. A 372(1996) 527-5338. M. Ambrosio et al., Nucl. Instrum. Meth. A 478(2002) 125-1299. G. Borreani et al., IEEE Trans. Nucl. Sci. 48 3(2001) 40610. S. Argirò et al., IEEE Trans. Nucl. Sci. 48 3(2001) 444

The Pierre Auger observatory and its fluorescence detector

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The Pierre Auger observatory and its fluorescence detector

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