This light is so faint that the atmospheric Cerenkov detector is usable only on clear nights when the moon is not visible. It also operates in conjunction with the Soudan 2 detector in the deep underground laboratory. Although the original purpose of the Soudan detector was the search for nucleon decay, researchers have also used data from the Soudan 2 detector to study the properties of neutrinos. The neutrino is an elusive particle whose existence was first hypothesized during the 1930's in order to explain some radioactive decays. Since neutrinos almost never interact with matter, they were not actually discovered for more than 20 years.
Neutrinos come in three varieties, called the electron, muon and tau-type neutrinos.
If neutrinos have mass, they can change from one type to another. Such changes or oscillations may account for the surprising results on neutrinos from the sun first observed at the Homestake Gold Mine in South Dakota. The Soudan 2 detector is currently measuring the number of electron and muon-type neutrinos produced by the cosmic rays. Comparison of observed and expected numbers may therefore provide information about neutrino mass. Preliminary data taken at Soudan confirm results from two other detectors indicating a shortage of muon-type neutrinos, supporting the idea that neutrinos do have mass.