A method to measure the integral vertical intensity and angular distribution of atmospheric muons with a stationary plastic scintillator bar detector

Abstract

We present a method to measure the integral vertical intensity and angular distribution of atmospheric muons using a stationary thick plastic scintillator bar detector with PMTs at both of its ends over exposures of a few hours in a laboratory setting. The total flux is obtained from the event rate and the angular distribution is inferred from the distribution of the recorded pulse charges, which are correlated with the track length inside the plastic scintillator. A muon generator algorithm was developed and used together with a simple simulation including the effects of the detector resolution, nonlinear saturation of the PMTs, and laboratory building coverage. As a proof of principle we made a measurement assuming a standard $\cos^n\theta$ omnienergetic angular distribution with azimuthal isotropy and different models for the energy spectrum of the source. Using measurements at 5 different azimuthal orientations, we measure an average exponent $n=(1.90\pm 0.06({\rm stat})\pm 0.10({\rm syst}))$, and an average vertical intensity of $I_0=(101.2\pm 1.8({\rm stat})\pm 5.5({\rm syst}))$ $~{\rm m}^{-2}{\rm s}^{-1}{\rm sr}^{-1}$ at the location with geographic coordinates 19.33$^\circ$N~99.19$^\circ$W, altitude 2,268 m above sea level, and geomagnetic cut-off of 8.2 GV.