Muons (greater than or equal to 1 GeV) in large extensive air showers of energies between 10(exp 16.5) eV and 10(exp 19.5) eV observed at Akeno

Abstract

The properties of muons (greater than or equal to 1 GeV) in giant air showers between 10(exp 16.5) eV and 10(exp 19.5) eV are measured by the Akeno 1 sq km, 20 sq km and 100 sq km air shower arrays. The lateral distribution of muons is well fitted by the formula given by Greisen with parameters beta = 2.52 +/- 0.02 and log (R(sub 0)) = (0.58 +/- 0.04)(sec theta -1) + (2.39 +/- 0.05) between 10(exp 16.5) eV and 10(exp 19.0) eV within 800 m from the air shower axis where theta is the zenith angle of its arrival direction. The relation between total number of muons (N(sub mu)) and total number of electrons (N(sub e)) derived using these parameters is expressed on average as N(sub mu) = (2.6 +/- 1.3) x 10(exp 5.0 + a)(N(sub e)/10(exp 7))(sup b) where a = (1.07 +/- 0.13)(sec theta -1) and b = (0.77 +/- 0.02) - (0.17 +/- 0.02)(sec theta -1). The lateral distribution of muons becomes steeper than that expected from the extrapolation of the above formula for core distances exceeding 800 m and is expressed as rho(sub mu) = N(sub mu)(C'(sub mu)/R(sup 2)(sub 0))r(exp -0.75) (1+r)(exp -beta)(1+(R/800 m)(exp 3))(exp -delta) (r= R/R(sub 0), where R is a core distance). No deviation from this formula has been observed for sec theta being less than 2.0 up to 10(exp 19.0) eV. The relation between the muon density (rho(sub mu)(600)) and the charged particle density on the ground at 600 m from the core (S(600)) is studied between 10(exp 16.5) eV and 10(exp 19.0) eV. A systematic change in the chemical composition of cosmic rays from a predominantly heavy to a predominantly light composition above 10(exp 17.5) eV claimed by the Fly's Eye group has not been detected beyond the present experimental uncertainties. The present experiment suggests much smaller rate of change of composition between 10(exp 17.5) eV and 10(exp 18.5) eV than that from the Fly's Eye experiment.

明野の1平方キロメートル、20平方キロメートルおよび100平方キロメートルの空気シャワーアレイによって、10{16.5}eV-10{19.5}eVの大空気シャワーにおけるミューオン(1GeV以上)の性質を測定した。ミューオンの横方向分布は空気シャワーの軸から800ｍ以内のエネルギー10{16.5}eV-10{19.5}eでGreisenの公式によってパラメータ(ベータ)= 2.52±0.02およびlog(R(0))=(0.58±0.04)(sec(ｼｰﾀ)ｰ1)+(2.39±0.05)に適合した。ここで(シータ)は到達方向の天頂角である。これらのパラメータを用いて得られたミューオンの総数(N(ﾐｭｰ))と電子の総数(N(e))の関係は平均でN(ﾐｭｰ)=(2.6±1.3)×10{5.0+a}(N(e)/10{7}){b}で表される。ここでa=(1.07±0.13)(sec(ｼｰﾀ)-1)およびb=(0.77±0.22)-(0.17±0.02)(sec(ｼｰﾀ)-1)である。ミューオンの横方向分布は800ｍ以上のコア領域で上記の公式の外挿から予想されるより急勾配となる。10{19.0}eVまでの2.0以下のsec(ｼｰﾀ)ではこの公式からの偏差は見られなかった。コアから600mの地上でのミューオン密度と荷電粒子密度の間の関数をエネルギー10{16.5}eV-10{19.0}eVについて調べた。Fly's Eyeグループの主張する10{17.5}eV以上の宇宙線の化学組成における重い組成から軽い組成への系統的変化は今回の実験では検出されなかった。しかし、今回の実験では、Fly's Eye実験の値より小さい組成変化が10{17.5}eV-10{18.5}eVで示唆された。