Sensitivity of the ARGO-YBJ strip size spectrum to differ- ent models of the primary cosmic ray composition in the energy range 10 ÷ 500 TeV

with a single layer of Resistive Plate Counters (RPCs), surrounded by a partiallyinstrumented guard ring. Signals from each RPC are picked-up with 80 read outstrips 6 cm wide and 62 cm long. These strips allow one to count the particlenumber of small size air showers. In this paper we discuss the digital responseof the detector for showers with core located in a small fiducial area inside thecarpet. The results enable us to assess the sensitivity of the strip size spectrummeasurement to discriminate between different models of the Primary CosmicRay composition in the energy range 10÷500 TeV.1. IntroductionThe energy spectrum of cosmic rays is well described by a power law overseveral decades of energy, before and after the so called knee region, 10