Results from a new search for proton decay

The authors have searched for the decay of protons and neutrons bound in nuclei using Soudan 1, a 31.5-metric-ton iron-loaded concrete tracking calorimeter, instrumented with 3456 proportional tubes. During a live time of 0.38 yr, the authors have observed one candidate event wholly contained within the detector. If this event is attributed to a background source, a lower bound (90% confidence level) can be established on the lifetime for nucleon decay through any of a wide range of modes of 1× 1030 yr.