Thermal neutrons of the terrestrial origin in the Brazilian region

Dynamics of near ground concentration of ambient thermal neutrons (TNs) was studied using data of neutron detectors of the COsmic-ray Soil Moisture Observing System (COSMOS) network and those of the Laboratory of environmental radiation of the Instituto Tecnologico de Aeronautica (Brazil). A significant excess of TN count rate over that of rapid ones was found in two COSMOS probes located in Brazil and other sites in North America and Europe. The effect was explained in terms of neutron production in nuclear ( α , n ) reactions initiated by decay of radon gas comprised in the ground rather than in the atmosphere. A revealed strongly marked diurnal variation of the TN concentration proved to be in antiphase with air temperature that is characteristic for particles in thermal equilibrium with air. A previously unknown effect of saturation of nighttime TN concentration was observed. It is explained in terms of a dynamic equilibrium state, which sets in the absence of vertical convection between the neutrons escaping from the ground and their losses in nuclear reaction in the air. The results described can be used for the determination of ground concentration of ambient radon and its dynamics in boundary layer of atmosphere as well as in other geophysical studies.