Spectroscopy of cosmic ray muons with the electromagnetic calorimeter WILLI

Prompted by a number of recent large scale experiments approaching astrophysical questions there is a current interest in the spectroscopy of high energy muons originating in the interactions of cosmic ray particles with the earth atmosphere. In order to study the feasibility of small and cheaper calorimeter setups, a prototype of an electromagnetic calorimeter 'WILLI' has been built in HH-INPE. The detector is made of 20 absorber plates (10 mm thickness) alternating with scintillator plates (30 mm thickness), the latter being viewed by opposite photomultipliers, maximizing the light collection and reducing and at the same time the position dependence of the signal. High energy muons can generate by electromagnetic interactions secondary particles which initiate electromagnetic cascades. The longitudinal profile of these showers is sampled by the number and energies of secondaries. Using the Maximum Likelihood Method the energy of the muon is estimated. (authors)