Measurements of the charge ration of atmospheric muons in the range 0.2 - 1.0 GeV/c

With the propagation of cosmic rays in the atmosphere, pions and kaons are produced by the interactions with air nuclei. Muons and neutrinos are resulting from their decay. The ratio of the numbers of positive to negative muons, called muon charge ratio, provides important information on the interactions of the primary cosmic particles with the atmosphere nuclei since it reveals detailed features of the multiplicity distributions and production cross sections of the parent particles and is a sensitive test quantity of hadronic interaction models. Hence, at low energies (GeV range) studies of muon charge ratio are related to predictions of the atmospheric neutrino fluxes and with the so-called atmospheric neutrino anomaly. This anomaly is the observation that the measured ratio of the neutrino fluxes (νμ + νμ bar)/(νe + νe bar) appears to be significantly lower than the expected value. The WILLI instrument, a small sampling calorimeter (90 x 90 cm2 area), has been built up in IFIN-HH Bucharest (44 angle 26' N latitude, 26 angle 04' E longitude and sea level) for some prototype studies of cosmic muon interactions with the matter, in context with the air shower experiment KASCADE. In a subsequent configuration the detector has been optimised for muon charge ratio measurements. The principle of the measurement is based on the different behaviour of positive and negative muons stopped in matter (forming of muonic atoms), the results reaching accuracies in the order of few percent and excluding systematic errors of magnetic spectrometers. (authors)