Giroletti, E (reprint author), Univ Pavia, Dipartimento Fis, Via Bassi 6, I-27100 Pavia, Italy.
The ARGO-YBJ experiment is an air shower detector for gamma ray astronomy and cosmic ray studies with an energy threshold of similar to 500 GeV. Working in "single particle mode", i.e. counting the single particles hitting the detector at fixed time intervals, ARGO-YBJ can monitor cosmic ray and gamma ray transients at energies of a few GeV. The single particle counting rate is modulated by the atmospheric pressure and temperature, and is affected by the local radioactivity from soil and air. Among the radioactive elements, radon gas is of particular importance since its concentration in air can vary significantly, according to environmental conditions. In this paper we evaluate the contribution of the radon daughter gamma ray emitters to the single particle counting rate measured by ARGO-YBJ. According to our analysis, the radon gas contribution is roughly 1-2%, producing a counting rate modulation of the same order of magnitude of the atmospheric effects. (C) 2014 Elsevier Ltd. All rights reserved.