There are approximately 1500 different nuclei known to be unstable. In contrast, only 280 stable nuclear isotopes are presently known. The characteristics of these unstable radioactive elements are summarized in Table 10.1. An unstable nucleus suddenly and spontaneously changes and is accompanied by the emission of one or more rapidly moving particles. Such a nucleus is said to be radioactive and its changes are called radioactive decay. Geotechnical problems interacting with a radiation energy field can be grouped into four general areas:

• Control of radioactive and toxic radon gas. • Disposal or management of nuclear wastes. • Utilization of nuclear energy for large construction operations. • Utilization of gamma-ray for use as a nondestructive testing method.