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.
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Gupta et al. (1992) studied this question.