Coincidences between extensive air showers and the Soudan 1 underground muon detector

We have operated the Soudan 1 underground muon detector in coincidence with a 36-m2 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 3 × 1015 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 1015-1016 eV, but there are significant systematic uncertainties.