We have operated the Soudan 1 underground muon detector in coincidence with a 36-${\mathrm{m}}^2$ detector situated at the Earth's surface. Such a combination of detectors can yield information on the composition of the primary cosmic rays at energies above Å 3 × ${10}^{15}$ eV, where there is an abrupt change in the slope of the energy spectrum. The present experiment was meant to test the feasibility of operating such a system, and to obtain a first sample of data before the complete installation of the much larger Soudan 2 detector. These initial data seem to favor a light composition in the energy range ${10}^{15}$-${10}^{16}$ eV, but there are significant systematic uncertainties.

• Received 15 October 1990

DOI:https://doi.org/10.1103/PhysRevD.45.1459