AMIGA, Auger Muons and Infill for the Ground Array
aa r X i v : . [ a s t r o - ph ] O c t AMIGA, Auger Muons and Infill for the Ground Array
A. Etchegoyen , for the Pierre Auger Collaboration . Departamento de F´ısica (Tandar), Centro At´omico Constituyentes, Comisi´on Nacional de En-erg´ıa At´omica and UTN-FRBA, Observatorio Pierre Auger, Av. San Mart´ın Norte 304 (5613) Malarg¨ue, Prov. Mendoza, Ar-gentina [email protected]
Abstract:
The Pierre Auger Observatory is planned to be upgraded so that the energyspectrum of cosmic rays can be studied down to 0.1 EeV and the muon component of showerscan be determined. The former will lead to a spectrum measured by one technique from 0.1EeV to beyond 100 EeV while the latter will aid identification of the primary particles. Theseenhancements consist of three high elevation telescopes (HEAT) and an infilled area havingboth surface detectors and underground muon counters (AMIGA). The surface array of theAuger Observatory will be enhanced over a 23.5 km area by 85 detector pairs laid out asa graded array of water-Cherenkov detectors and 30 m buried muon scintillator counters.The spacings in the array will be 433 and 750 m. The muon detectors will comprise highlysegmented scintillators with optical fibres ending on multi-anode phototubes. The AMIGAcomplex will be centred 6.0 km away from the fluorescence detector installation at Coihuecoand will be overlooked by the HEAT telescopes. We describe the design features of the AMIGAenhancement. The cosmic ray spectrum shows three featuresat higher energies: the second knee, the ankle,and the GZK-cut off, and in order to seamlessstudy this region [1] Auger will be upgradedwith HEAT (High Elevation Auger Telescopes,[2]) and AMIGA (Auger Muons and Infill forthe Ground Array). These two enhancementswill encompass the second knee - ankle re-gion where the transition from galactic to ex-tra galactic cosmic rays is assumed to occur.The two main experimental requirements aregood energy resolution in order to obtain thespectrum and primary type identification sincethe galactic (heavy primaries) to extra galactic(light primaries) source transition is directlylinked to primary composition.In this note we concentrate on AMIGA. Itwill consist of 85 pair of water Cherenkov sur-face detectors (SD) and 30 m plastic scintilla-tors buried ∼ .In regards to the mentioned spectrum measure-ment with good energy resolution, Auger hy-brid detecting system was conceived in order toperform careful systematic uncertainty cross-checks which are currently under way. Theywill eventually permit to consolidate an energyspectrum of unprecedented precision. Mainuncertainties in the energy calibration are theabsolute calibration, the atmospheric light at-tenuation, and the fluorescence yield for theFD system, and the simulated airshower muoncomponent for the SD system. There areclear indications that simulations under pre-dict the shower muon contents [4] and as suchthe Auger SD energy estimator will be biasedand this bias would increase with zenith an-gle [5]. Large muon counters will aid towardssolving this problem by directly measuring the MIGA
Figure 1: Layout of Auger enhancements.White and black lines show the six originaland three enhanced telescopes FOVs, respec-tively. Grey, white and black dots indicate SDsplus buried muon counters placed 433, 750, and1500 m apart, respectively. In this area a fur-ther enhancement of radio detection of exten-sive air showers will start its R&D phase [3].number of muons with reduced poisson fluctu-ations.In regards to composition analyses, the tworelevant shower parameters are the atmo-spheric depth at shower maximum, X max , andthe shower muon contents. Other composi-tion sensitive parameters dependent on them.Gamma-hadron discrimination is easier to per-form than hadron-hadron discrimination sinceat E ≥ X max values for gamma in-duced showers are already well above thosefrom hadron primary showers [6]. Also gammashowers are essentially electromagnetic with avanishing muon component. No photon detec-tion has been reported so far and a direct de-tection at E ≥ ≥ X max (the elongation rate) or muon contentsas a function of energy [9]. A simultaneouschange detected by both FD and muon coun-ters will be the most compelling evidence of acomposition change casting light on the tran-sition of cosmic ray sources from galactic toextra galactic origins [10].AMIGA reconstruction performances are quiteencouraging, they have been outlined in [11,10] for tank infilled areas and muon coun-ters, respectively. Suffice to say that the sur-face detector reconstruction is currently wellunderstood by the Auger collaboration andthat we have developed [12] a detailed muonreconstruction system which is based on theparameterized muon lateral distribution func-tion [13] currently used by KASCADE-Grande.The scintillator modules are simulated andthe reconstruction procedure includes satu-rated (more than 90 muons) and silent (0,1, or2 muons) counters. The shower reconstructedparameter is N µ (600), the estimated numberof muons 600 m away from the shower axis, anexcellent primary type indicator.In this note we are concentrating in the muondetector hardware. These counters will com-prise highly segmented scintillators (to avoidunder counting) with optical fibres ending on64-pixel multi-anode photo multiplier tubes(PMT). The design adopts similar scintillatorstrips as for the MINOS experiment [14]. Thecurrent baseline design calls for 400 cm long × × reflecting coating with a groove inwhere a wavelength shifter fibre is glued (seeFig. 2) and covered with reflective foil. Eachmodule will consist of 64 strips with the fibresending on an optical connector matched to a64 multianode Hamamatsu H7546B PMT of2mm × Figure 2: Muon counter assembly at ArgonneNational Laboratory of a 64 200 cm long proto-type displaying the 64 pixel optical connector.The 4.1 cm wide strips and the green fibers arealso shown.ing. Each muon counter will be composed ofthree of these modules buried alongside a wa-ter Cherenkov tank, i.e. 192 independent chan-nels.The response of each scintillator strip will becharacterized using a 5 mCu
Cs radioactivesource mounted on a scanner designed for thispurpose. The scanner is an X-Y positioningsystem with four tooth belt activated linearguides moved by two step-by-step motors of 8.7Nm torque. The whole positioning system hasup to 1 mm precision in any of the two axis andan effective total displacement of 5 m × µ Cu Cs source mounted on top ofthe scintillators.AMIGA electronics will have both an under-ground and a surface section powered by so-lar panels. Each of the three undergroundmodules per counter will have attached a PCBwith a data handling FPGA and a communi-cation and monitoring system with a micro-controller. Each electronic channel will havean amplifier and a discriminator, set at ∼ ◦ sector antenna which collects the signalfrom the stations directional antennas. Thesestations use three 802.11 independent channels MIGA
Figure 3: AMIGA two level telecommunicationstar topology. Concentrators are labelled from1 to 4. In this layout concentrator-subscribersystems 2 and 3 use the same 802.11 indepen-dent channel. Bottom and upper arrows showchannelling of data to the two access points. E o [EeV] Area [km ] No. events year − E and zenith anglebelow θ max = 60 ◦ was obtained by assuminga cosmic ray flux following a power law withspectral index -2.84 as quoted by Auger ([11]and references within), is displayed in Table 1.AMIGA will start by deploying a prototype, af-ter full commissioning of the 1500 m grid arraywhich is planned to occur early 2008. This pro- totype will permit to gain experience on muoncounters and experimentally estimate possiblepunch-throughs. It is designed to have three 4m X-Y parallel plates buried at three differ-ent depth, near the surface, at ∼ ∼ References [1] G. Medina Tanco [Pierre Auger Collabora-tion], these proceedings , 242 (2006).[8] L. Anchordoqui et al. hep-ph/0407020.[9] T. Abuy-Zayyad et al., Phys. Rev. Letts. , 4276-4279 (2000).[10] A. Etchegoyen et al., Proc. VI SILAFAE,American Institute of Physics, (2007)210-219.[11] M. C. Medina et al, Nucl. Inst. and Meth.A , 302-311 (2006).[12] A.D. Supanitsky et al, to be submitted topublication.[13] J. Buren, T. Antoni, W. Apel, et al. Proc.26 th ICRC, (2005),6