Observation of an Excess of Positive Air Ions in Underground Cavities

We present the results of measurements of the air ion densities of both signs in the unventilated underground laboratory of the Moscow State University. We used the stationary station based on a Sapfir-3M modified air ion counter. We solved the problems of accuracy of relative measurements of air ion densities and stability of measurements in conditions of high humidity of underground cavities. We investigated the spatial distribution of air ion densities and their variability over time. An excess of positively charged air ions was found everywhere in the laboratory. The obtained data indicate the presence of a weak electric field directed downward in the underground laboratory. We observed the dependence of the air ion densities of both signs on the atmospheric pressure. We found the correlation of air ion densities with radon activity in the laboratory air and show that the ionization of air by alpha particles from the decay of radon and its daughter elements is the main source of air ions in closed underground cavities. A variant for explaining the presence of an excess of positive charge in underground cavities is proposed.