1-9. RADIOACTIVE DECAY
a. The step-by-step process by which radioactive elements emit α, β and ϒ
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.