Lateral distribution function of high energy muons in EAS around the knee

The lateral distribution function of high energy muons in EAS around the knee ($5.9\le \lg N_e \le 7.1$) has been measured for near vertical showers ($\theta \le 20^{\circ}$, effective muon threshold energy is 230 GeV). The measurements have been performed at the Baksan Underground Scintillation Telescope (BUST). The electromagnetic component is measured by the “Andyrchy” EAS array, located above the BUST. The knee in EAS size spectrum is found to be at $\lg N_e \approx 6.3$. The experimental results are compared with Monte Carlo simulations.

💡 Research Summary

The paper reports a detailed measurement of the lateral distribution function (LDF) of high‑energy muons (threshold ≥ 230 GeV) in extensive air showers (EAS) around the knee region of the cosmic‑ray spectrum. The experiment combines data from the underground Baksan Scintillation Telescope (BUST) and the surface Andyrchy array, which together provide simultaneous information on the muon component (via BUST) and the electromagnetic component (via Andyrchy). Only near‑vertical showers (θ ≤ 20°) with sizes 5.9 ≤ lg Nₑ ≤ 7.1 were selected, and the total live time amounted to 24 460.8 h.

Andyrchy, consisting of 37 plastic scintillators (1 m² each) spread over 4.5 × 10⁴ m², supplies the shower size Nₑ, core position, and arrival direction. BUST, a four‑story building located 360 m below the surface (effective depth 850 hg cm⁻²), contains 3150 liquid‑scintillator counters (0.7 × 0.7 × 0.3 m³) and records the number of muons crossing it (1–200 per event) with a spatial resolution of 0.7 m and angular resolution of 1.5°. The coincidence trigger rate of the two systems is about 0.1 s⁻¹.

For each shower the fraction of muons observed in BUST, Δ(R) = n(R)/N_μ, was evaluated as a function of the distance R between the shower axis and the centre of BUST. Here n(R) is the mean muon count in BUST for a given R, and N_μ is the mean total muon number in the shower. The analysis grouped events in 10 m bins of R, calculated the average muon count per bin, and derived Δ(R). The measured Δ(R) curves for three Nₑ intervals (5.9 ≤ lg Nₑ < 6.3, 6.3 ≤ lg Nₑ < 6.5, 6.5 ≤ lg Nₑ ≤ 7.1) are shown in Fig. 1.

To interpret the data, an empirical LDF form was adopted:

 f(r) ≈