IL NUOVO CIMENTO Marzo-Aprile 1998 VOL. 21 C, N. 2 An experimental study of Primary Cosmic Rays at the knee energy region by observation of Extensive Air Showers (EAS) G. SAHA, A. BHADRA, C. CHAKRABARTI, S. K. SARKAR and N. CHAUDHURI High Energy and Cosmic Ray Centre, North Bengal University - Darjeeling 734430, India (ricevuto il 22 Settembre 1997; approvato il 2 Ottobre 1997) Summary. — Simultaneous measurements have been made of the radial (lateral) electron density distribution and the radial muon density distribution at various measured muon energies in the range 2.5–100 GeV in vertically incident EAS in the size range 3.15 3 10 4 –1.79 3 10 6 (primary energy range 2.4 3 10 14 –8.3 3 10 15 eV) particles detected near sea level. The characteristics of these radial distributions in terms of the measured shower parameters have been determined and used to draw conclusions about the average nuclear mass of the primaries of these EAS. PACS 96.40.Pq – Extensive air showers. 1. – Introduction It is well established that the Primary Cosmic Ray energy spectrum steepens in the energy range 10 14 –10 16 eV known as the knee energy region. The spectrum steepening is thought to be due to mechanisms related to acceleration and propagation of Primary Cosmic Rays (PCR). An Extensive Air Shower experiment at primary energy in the knee energy region in which the energy spectrum and lateral distribution of muons can be measured accurately over a wide range can indicate the trend of average behaviour of PCR mass at the knee region. In an EAS of size ( N e ) 10 4 –10 6 particles at sea level the total number of muons and muon lateral distribution, if the primary is a heavy nucleus, will be different from those in a primary proton initiated shower and the difference can be detected from the data of a proper set-up experiment. In the present paper the results of an air shower experiment covering the knee energy region will be described. This experiment with the provision for differential measurement of both low- and high-energy muons simultaneously by two shielded magnet spectrographs has yielded both muon energy spectrum and radial density distributions at various muon energies. The properties of these distributions have been analysed to infer the trend of average PCR mass composition at the knee region. G So cietà Italiana di Fisica 215