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[en] The measurements of the muon charge ratio representing the ratio of positive to negative atmospheric muons are performed using a small compact device, WILLI, by detecting the life time of the muonic atoms. Avoiding the difficulties of measurements with magnetic spectrometers, this method gives precise results on muon charge ratio especially in the low energy range relevant for the atmospheric neutrino anomaly. The detector, the method and the results on muon charge ratio for five energy ranges below 1 GeV are presented. The results can be used to improve hadronic interaction models. (author)

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[en] The charge ratio of atmospheric muons is rather sensitive to effects of the geomagnetic field and to the hadronic interaction. Experimental information about this quantity is very useful for tuning the ingredients of models used for calculations of atmospheric neutrino fluxes. For experimental observations, and in particular regarding the asymmetry in the charge ratio of muons from the east and West directions (East-West effect) a rotatable device has been installed in NIPNE-HH (National Institute of Physics and Nuclear Engineering -- Horia Hulubei) Bucharest (44 deg. 26'N, 26 deg. 04' E, 85 m above sea-level at a vertical cut-off rigidity of 5.6 GV). The detector is mounted in a rotatable frame, consisting of a stack of 16 modules, 90 x 90 cm², formed by plastic scintillator layers (3 cm thick) and aluminum support (1.2 cm thick), surrounded by 4 lateral veto counters. The measurements are based on observation of the life time of muons stopped in the absorber layers of the detector. At a mean zenith angle of 35 deg. the East West asymmetry in the ratio has been found to decrease from 0.25 to 0.20, in the momentum range 0.35 - 0.50 GeV/c

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[en] A fesibility study to explore the capability of the WILLI detector to observe the solar events/activity by recording the muon intensity at ground level is presented.The WILLI detector, set up in National Institute of Physics and Nuclear Engineering, Bucharest, is a 1 m² incident area sampling calorimeter. It can measure simultaneously muon events with the muon energy ≥0.4 GeV and, if the muons are stopped in the detector, and muon energy between 0.4< E_μ<0.6 GeV, if the muons pass the minimum of 2 plates of the detector stack.Taking into account muon events with energy ≥0.4 GeV, a modulation of the muon intensity as a diurnal variation is observed. Muon events for a smaller energy range (0.4-0.6 GeV) seem to exhibit an aperiodic variation of the muon intensity, which could be correlated with magnetic activity indicated by the planetary K_p index.

[en] The charge ratio of the atmospheric muons is a quantity sensitive to hadronic interactions of cosmic rays and to the influence of the geomagnetic field. Experimental information is of current interest for tuning models used for the calculation of atmospheric neutrino fluxes. We have performed measurements of the charge ratio based on the observation of the lifetime of the muons stopped in the absorber layers (aluminum support) of the detector WILLI, mounted in a rotatable frame and installed at IFIN-HH (Horia Hulubei National Institute for Physics and Nuclear Engineering) Bucharest (vertical geomagnetic cut-off rigidity of 5.6 GV). Our method to determine the muon charge ratio by measuring the lifetime of muons stopped in the matter, overcomes the uncertainties appearing in measurements based on magnetic spectrometers, which are affected by systematic errors at low muon energies due to difficulties in particle and trajectory identification. The results obtained with the rotatable WILLI detector, inclined at 45 angle (i.e., a mean zenith angle of detected muons of 35 angle) show a pronounced east-west effect in good agreement with simulation results of CORSIKA. The values of the asymmetry of the charge ratio decreases from 0.25 to 0.20 in the muon momentum range of 0.35-0.50 GeV/c, relevant to the atmospheric neutrino anomaly. (authors)

[en] The interpretation of the atmospheric neutrino anomaly in terms of neutrino oscillations is based on extensive Monte Carlo calculations of the neutrino fluxes originating from the interaction of the primary cosmic radiation with the Earth's atmosphere. A possible verification of the calculation results is given by the muons which are produced correlated to the neutrinos. In addition to the absolute muon flux, the ratio of positive to negative muons provides valuable information. The muon charge ratio is directly related to the ratio of neutrinos to antineutrinos used as a correction for the different efficiencies in the experiment, and it is the best quantity to probe the influence of the Earth's magnetic field. The WILLI experiment delivers precise results for the muon charge ratio in the relevant energy range. The recent results are discussed in the context of Monte Carlo simulations of the muon charge ratio with CORSIKA. (author)

[en] The WILLI detector, installed in the National Institute of Physics and Nuclear Engineering - Horia Hulubei Bucharest has been devised for studies of atmospheric muons, in particular of the muon charge ratio. We report on the results of studies for various muon energies (pμ < 1 GeV/c). The results are compared with Monte-Carlo simulations performed with the CORSIKA and GEANT codes. We are exploring the potential of a small detector array to be set up nearby the actual WILLI detector for triggering the muon charge ratio measurements by Extended Air Shower (EAS) events