Precise knowledge of the mass composition of the primary cosmic ray flux below and above the knee in the primary energy spectrum at $E\sim 3\times {10}^{15}$ eV is essential for understanding the acceleration and propagation of high energy cosmic rays. Simulations have shown that the correlations between the electron and muon components of showers are very sensitive to the mass of the primary nuclei. In particular, different regions of the muon multiplicity distribution observed with a large area muon detector, for well-defined selection on the electron component of showers, have a high sensitivity to different nuclear groups present in the primary flux. The GRAPES-2 experiment at Ooty seeks to exploit this sensitivity using a $200{\mathrm{m}}^2$ area muon detector to study the mass composition of the primary flux in the energy range ${10}^{14}–{10}^{16}$ eV. The details of the experiment are presented and early observations are discussed in relation to the expectations from simulations.

• Received 22 May 2003

DOI:https://doi.org/10.1103/PhysRevD.68.052005