The Andyrchy-BUST experiment: primary spectrum and composition around the knee

arXiv:0911.5679v2 [astro-ph.HE] 14 Apr 2010 PROCEEDINGS OF THE 31st ICRC, Ł ´OD´Z 2009 1 The Andyrchy-BUST experiment: primary spectrum and composition around the knee V.B. Petkov∗ for the Andyrchy-BUST collaboration ∗Institute for Nuclear Research of RAS, Russia Abstract. The main goal of the Andyrchy-BUST experiment is to study the primary cosmic rays spectrum and composition around the knee. The experimental data on the knee, as observed in the electromagnetic and high energy muon components, are presented. The electromagnetic component in our experiment is measured using the ”Andyrchy” EAS array. High energy muon component (with 230 GeV threshold energy of muons) is measured using the Baksan Underground Scintillation Telescope (BUST). The location of the ”Andyrchy” right above the BUST gives us a possibility for simultaneous mea- surements of both EAS components. Keywords: EAS, primary spectrum, primary com- position I. INTRODUCTION In the range of primary energies of 1014−1015 eV per nucleus, direct methods for studying the energy spectrum and nuclear composition of primary cosmic rays become inefficient because of a decrease in the flux of primary particles with an increase in their energy. Therefore, at these and, of course, higher energies, indirect methods based on simultaneous measurement of the characteris- tics of different components of extensive air showers (EASs), which are initiated by the primary particle in the atmosphere, are used. But the interpretation of these measurements requires their comparison with EAS simulations in the atmosphere. In turn, the calculation results depend on the hadronic interaction models. The main problem is the extrapolation of these models into kinematical and energy regions still unexplored by present-day collider experiments. So, the measurements of different EAS components are now used for both studying the primary composition and testing interaction models [1] - [7]. In this paper three types of experimental data are analyzed: muon number spectrum, EAS size spectrum and correlation between muon number and EAS size simultaneously measured. Integral muon number spectrum has been measured using the Baksan Underground Scintillation Telescope (BUST) [8]. The EAS size spectrum has been measured using the ”Andyrchy” EAS array [9]. The dependence of the mean number of high energy muons on EAS size has been measured by simultaneous operation of both ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ ❍ the tunnel ∼30◦ The relative position of the BUST and EAS array ❄ ✻ 360 m BUST ✲ ✛ 550 m The projection onto horizontal plane 40 m 37 plastic scintillation detectors (1 m × 1 m × 0.05 m) Fig. 1: Andyrchy EAS array. devices [10], [11]. II. FACILITIES The ”Andyrchy” EAS array is located on the slope of the Andyrchy mountain above BUST (43.3◦N, 42.7◦ E) and consists of 37 plastic scintillation detectors. A plastic scintillator of the 5 cm thickness has an area of 1m2 and is viewed with a single PMT. The detectors are designed for both timing measurements (for EAS arrival direction) and evaluation of primary energy (via EAS core localization and determination of total number of particles in shower). The distance between the detectors is about 40 m in projection to the horizontal plane and the overall area of the installation is 5 · 104 m2; the solid angle at which the array is viewed from the BUST is 0.35 sr. The central detectorof the array is above the 2 V.B. PETKOV THE ANDYRCHY-BUST EXPERIMENT: ... 6 7 8 10 14 simulated spectrum: F(N rp ) ~ N rp -2.5 reconstructed spectrum F(N rp )*N rp 2.5 (arbitrary units) log10(N rp ) Fig. 2: Simulated and reconstructed size spectra. BUST’s center at a vertical distance of about 360 m and at 2060 m above the sea level. The difference between the heights of the upper and lower rows of detectors is ∼150 m. The energy deposition measurement is performed in nat- ural units, so called relativistic particles. One relativistic particle (r.p.) is the most probable energy deposition from a single cosmic ray particle. For our detector it is 10.6 Mev [12]. The range of the energy deposition measurements is from 0.5 r.p. (the threshold of the Charge-to-Time Converter) up to more than 1000 r.p. Trigger formation and all measurements are performed in a registration room, which is placed near the center of the array (length of connection cables is up to 280 m). The shower trigger condition requires signals from 4 detectors within 3 microseconds. The trigger’s rate is about 9 s−1. The array and its characteristics are described in more details in [12]. BUST [13] is a large device 16.7 × 16.7 m2 area and 11.1 m height), located in a cave under mountain slope. The four vertical sides and four horizontal planes are completely covered with standard liquid scintillation detectors. The standard detector consists of an alu- minium tank with 0.7 × 0.7 × 0.3 m3 dimensions and is filled with liq

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