# Underground Muon Counters as a Tool for Composition Analyses - Astrophysics

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Abstract: The transition energy from galactic to extragalactic cosmic ray sources isstill uncertain, but it should be associated either with the region of thespectrum known as the second knee or with the ankle. The baseline design of thePierre Auger Observatory was optimized for the highest energies. The surfacearray is fully efficient above $3 \times 10^{18}$ eV and, even if the hybridmode can extend this range below $10^{18}$ eV, the second knee and aconsiderable portion of the wide ankle structure are left outside its operatingrange. Therefore, in order to encompass these spectral features and gainfurther insight into the cosmic ray composition variation along the transitionregion, enhancements to the surface and fluorescence components of the baselinedesign are being implemented that will lower the full efficiency regime of theObservatory down to $\sim 10^{17}$ eV. The surface enhancements consist of agraded infilled area of standard Auger water Cherenkov detectors deployed intwo triangular grids of 433 m and 750 m of spacing. Each surface station insidethis area will have an associated muon counter detector. The fluorescenceenhancement, on the other hand, consists of three additional fluorescencetelescopes with higher elevation angle $30^\circ-58^\circ$ than the ones inoperation at present. The aim of this paper is threefold. We study the effectof the segmentation of the muon counters and find an analytical expression tocorrect for the under counting due to muon pile-up. We also present a detailedmethod to reconstruct the muon lateral distribution function for the 750 mspacing array. Finally, we study the mass discrimination potential of a newparameter, the number of muons at 600 m from the shower axis, obtained byfitting the muon data with the above mentioned reconstruction method.

Autor: A. D. Supanitsky, A. Etchegoyen, G. Medina-Tanco, I. Allekotte, M. Gómez Berisso, M. C. Medina

Fuente: https://arxiv.org/