Soudan 1

It was a 30-ton tracking calorimeter whose primary purpose was to search for proton decay.

Soudan 1 was installed 590 meters below the surface and brought into routine operation in August 1981 by high-energy physics research groups from the University of Minnesota and Argonne National Laboratory.

The detector was a 3×3×2m3 block of iron oxide-loaded concrete instrumented with 3456 gas proportional tubes.

It was surrounded on five sides by a veto shield of solid scintillator, which was completed in October 1981.

This allowed events which might otherwise have looked like proton decay, but were actually caused by cosmic rays, to be discarded.

Tom Fields of Argonne National Lab (standing, left), Don Perkins of Oxford University (standing, center), and Marvin Marshak of Univ of Minnesota (bending, center) next to the Soudan 1 proton decay experiment in the Soudan Mine , Minnesota .