WILLI – a scintillator detector setup for studies of the zenith and azimuth variation of charge ratio and flux of atmospheric muons

The primary and secondary components of the cosmic radiation are influenced by the geomagnetic field. This leads - in addition to the variation with the zenith angle - to an azimuth dependence, in particular of the ratio of positive and negative muons, pronounced as East-West effect of muon charge ratio for low energy muons. The compact scintillator detector arrangement, WILLI, able to register incident muons with different azimuth and zenith angles (E<1GeV), has been used to study the azimuth dependence of the muon charge ratio. Positive and negative muons are discriminated by observing the different lifetimes when being stopped in matter, in the WILLI case by Al absorbers. We are considering the development of a mini array, installed close to the WILLI detector, for triggering the muon charge ratio measurements in extended air shower events.