We describe and discuss the operation, calibration and stability of the EAS
-TOP calorimeter (Campo Imperatore, National Gran Sasso Laboratories), a la
rge area hadron and muon detector devoted to cosmic-ray physics. It consist
s of iron slabs (for a total thickness of 818 g cm(-2)) and Iarocci tubes a
s sensitive layers, operating in the streamer mode and the "quasi proportio
nal" regime. Using a model describing the operation of the "quasi proportio
nal" chambers, we derive a calibration curve in the energy range 50-5000 Ge
V, whose reliability has been indirectly checked through on-site measuremen
ts, by means of an accelerator beam run (up to similar or equal to 600-700
GeV) and by comparing the model predictions on hadron shower transition cur
ves with the data. (C) 1999 Elsevier Science B.V. All rights reserved.