In the atmosphere, the radioactive isotopes, including natural and artificial ones, are present as gases or bounded to aerosols. As mentioned previously, the artificial radioactivity originating from nuclear explosions is decreasing, while that from nuclear energy production is increasing. The net effect of these tendencies is the decrease of radioactivity in the atmosphere because the change of natural radioac­tivity can be disregarded.

Of the natural radioactive isotopes, radon, 3H, and 14C are important. The differ­ent isotopes of radon are the members of the natural decay series. 3H, and 14C con­tinuously form from nitrogen under the effects of cosmic radiation. Both elements form many volatile and gaseous compounds, which are present in the atmosphere.

The cloud chamber photograph (see Section 14.5.1) of radon gas (222Rn) is shown in Figure 13.3. The tracks of the alpha particles can be seen well. A solid — state detector picture (see Section 14.5.3) of radon gas is shown in Figure 13.4. The tracks of alpha particles are shown.

Among the artificial radioactive isotopes, the gaseous fission products (iodine and noble gases) and the compounds of 3H and 14C are the important radioactive isotopes of the air.

All other radioactive isotopes in the air are bounded to aerosols. Practically, all natural and artificial radioisotopes can be bounded to aerosols. The long-life fission products and natural radioactive isotopes, thus, can have an impact on the environment.

The radioactive isotopes of the atmosphere are falling onto the surface of the Earth. Depending on the amount of water that accompanies this fallout, the processes are called “dry out,” “rain out,” and “wash out.” The half-life of the fallout of radio­active isotopes from the stratosphere after a nuclear explosion is 7 years.

Figure 13.3 A cloud chamber photograph of radon gas (222Rn). (Thanks to Dr. Peter Raics, Department of Experimental Physics, University of Debrecen, Hungary, for the photograph.)

Figure 13.4 A solid-state detector picture of radon gas. (Thanks to Dr. Istvan Csige, Department of Environmental Physics, Institute of Nuclear Research, University of Debrecen, Hungary, for the photograph.)

The radioactivity of the atmosphere has seasonal changes because of the changes in the weather. The mean atmospheric radioactivity in the Northern Hemisphere in January and in June is about 10 and 2 Bq/m3, respectively.