  Measuring radon-due neutron flux at various altitude (100, 1000, 1700, 4300 m
above sea level) we found an evidence of significant increase of radon
concentration with altitude. It was also conirmed by direct radon measurements
at high altitude. This allowed us to assume cosmic rays could take part in
process of soil activation: they transform long-lived nuclei of uranium and
thorium to nuclei with shorter life-time through specific nuclear reactions. If
the resulting nuclei belong to the U-238 radioactive chain they can lead to
producion of Ra-226 and then to Rn-222, thus significantly increasing its
production at high altitudes where cosmic ray flux is high.
