-9produced, At this rate the decrease in activity from one hour after to 343 hours after (approximately two weeks) is a thousandfold. The half life of some isotopes will be greater than, and of other isotopes, therefore as time elapses, less than the average; the rate of decline of the radio- activity of the mixture of fission products decreases. Radioisotopes cannot be destroyed by any means. Only by the natural process of decay can isotopes change from the radioactive to the stable state. Radioactivity cannot be detected by the normal senses of sight, sound, touch or odor, either. However, nuclear radiations are capable of ionizing matter through which they pass, and as a result of this process radioac- tivity can be detected and measured by means of electronic devices. In the case of a nuclear radiation passing through a gas (or matter), the electrical field accompanying the passage of the alpha or beta particle or gamma photon dislodges an electron from the gas atom or molecule and thereby changes an electrically balanced atom or molecule into an ion pair -- a positive ion, electron. the gas, and a negative ion, This process is known as ionization and is the most significant characteristic of nuclear radiations. the