1. van der Heijde, H. B.; Beens, H.; and De Monchy, A. R., 1977, The Occurrence of Radioactive Elements in Natural Gas: Ecotoxicol. Environ. Saf., 1, p. 49?87.
2. Tanner, A. B., 1964, Radon Migration in the Ground: A review,in J. A. S. Adams and W. M. Lowder (Eds.), The Natural Radiation Environment: University of Chicago Press, Chicago, Illinois, p. 161.
3. Tanner, A. B., 1980, Radon Migration in the Ground: A supplementary review,in T. F. Gesell and W. M. Lowder (Eds.), Natural Radiation Environment III; U.S. Department of Energy, p. 5.
4. Trusheim, F., 1961, Über radioaktive Leithorizonte in Buntsandstein Norddeutschlands zwischen Ems und Weser: Erdoel-Z., 14, p. 797.
5. United Nations, 1977, Sources and Effects of Ionizing Radiation: UN No. E. 77, New York, New York.