Measurement of the zenith angle distribution of the cosmic muon flux in Hanoi

The zenith angle distribution of the cosmic muon flux has been measured in Hanoi, where the geomagnetic rigidity cutoff reaches the very high value of 17 GV, using an orientable scintillator telescope. The measurement is of interest for the understanding of the development of cosmic ray showers in the earth atmosphere. The dependence of the cosmic muon flux (integrated over charges and momenta) on zenith angle θ is measured to be of the approximate form (Φ0−asin2θ)×cos2θ with Φ0=72.0±1.6 m−2sr−1s−1 and a=7.8±0.8 m−2sr−1s−1. This result is consistent with an earlier Hanoi measurement and is in excellent agreement with the prediction of a model widely used in the analysis of atmospheric neutrino data.