Determination of the cascade-curve maximum depth from the muon component on the Yakutsk EAS array
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, PARTICLES, FIELDS, GRAVITATION, AND ASTROPHYSICS
Determination of the CascadeCurve Maximum Depth from the Muon Component on the Yakutsk EAS Array A. V. Glushkov* and A. V. Saburov Shafer Institute of Space Physics and Aeronomy, Yakutsk Research Center, Siberian Branch, Russian Academy of Sciences, Yakutsk, 677891 Russia *email: [email protected] Received April 30, 2014
Abstract—The lateral distribution of muons with a threshold Eµ ≥ 1.0 secθ GeV in extensive air showers with E0 ≥ 1017 eV over the period of observations on the Yakutsk array from November 2011 to June 2013 has been investigated. It follows from this distribution that the cascadecurve maximum depth Xm in the energy range (1–5) × 1017 eV increases relatively rapidly, because the cosmicray composition changes from iron nuclei dominating at E0 ≈ 1017 eV to protons. DOI: 10.1134/S1063776114110065
1. INTRODUCTION
2. RESULTS AND DISCUSSION
Ultrahighenergy (E0 ≥ 1015 eV) cosmic rays have been investigated worldwide on extensive air shower (EAS) arrays for more than 50 years. Their mass com position, without the knowledge of which it is difficult to understand the character of nuclear interactions in this energy range, is not yet known with certainty. The key to solving this problem is the shower cascade curve maximum depth Xm. EAS simulations have shown that Xm is a linear function of the logarithm of the atomic number A. The following relation is valid:
Below, we consider the showers recorded over the period of observations from November 2011 to June 2013. For our analysis, we selected 1317 events with E0 ≥ 1017 eV and zenith angles θ ≤ 45° whose axes fell within the array’s central circle with a radius of 1 km and were found with an accuracy of at least 20 m. The primary particle energy was found from the relations
p
17
E 0 [ eV ] = ( 4.8 ± 1.6 ) × 10 ( ρ s, 600 ( 0° ) )
1.00 ± 0.02
, (1)
–2
ρ s, 600 ( 0° ) [ m ] = ρ s, 600 ( θ ) exp ( ( sec θ – 1 )X 0 /λ ρ ) ,
exp
Xm – Xm ln 56, ln A = p Fe Xm – Xm
(2)
2
λ ρ [ g/cm ] = ( 450 ± 44 ) + ( 32 ± 15 ) log ρ s, 600 ( 0° ), (3)
where Xm were obtained experimentally (exp) or com putationally for primary protons (p) and iron nuclei (Fe). In practice, the optical methods of measuring Xm from the observations of Cherenkov and ionization radiations in EASs are used most commonly. These methods have yielded important results on the EAS development and the cosmicray composition (see, e.g., [1–9]). However, the stringent requirements imposed on the atmospheric transparency and the short calendar time of observations limit significantly their information content. This is particularly topical in the energy range 1017–1018 eV, where the ionization radiation is already inefficient, while the statistics of showers with Cherenkov radiation is poor. It was shown in [10, 11] that the cosmicray composition in this energy range could change noticeably in different periods of time. Under these conditions, measuring Xm from the muon component can complement signifi cantly the
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