a. The step-by-step process by which radioactive elements emit a, b and ¡ radiations, change into other elements, and finally reach stability is called a decay series. The steps from U-238 to Th-234 to Pa-234 described above represent two steps in what is called the natural radioactive uranium series, U-238 being the original element in the series. There are several other steps described here in this uranium series before stability is reached and there is no longer any radioactive change. The last step in this process occurs when the element polonium (Po-210) emits an alpha particle and becomes the element lead (Pb-206), which is stable. This is the end of the uranium series, but there are other radioactive series involving other heavy elements.

b. The question arises--how long does it take U-238 to decay into the next series step of Th-234 or for the last decay step, polonium, to decay into stable lead? The question of the time involved in radioactive decay is explained by the use of a term called half-life. Half-life is defined as the length of time it takes for one-half of a given number of atoms of one element to decay into another element. For example, if you have 5,000 atoms of U-238 today, in a period of time designated as one-half life, you would have only 2,500 atoms of U-238 remaining. This would take 4-1/2 billion years. Half-lives may vary from fractions of microseconds to billions of years. Some half-lives of typical elements are given in table 1-2.


Table 1-2. Half-lives of typical elements.
X-ray Schools | X-ray and Radiation Safety
For Informational Purposes Only - Based On US Army Radiation Safety Training