Radioactive materials are widely scattered in
all the rocks of the earth. Pick up any volcanic rock from the ground and it is likely to
contain atoms of radioactive elements.
Radioactive elements send out radiation all
the time, although gradually the level of radiatin decreases. However, whatever the
level of radiation, all radioactive materials lose half of their remaining surplus energy
at a fixed rate. Thus,the time taken for a piece of an element to lose half of its
remaining energy is always the same, whether the radioactivity is strong or weak.
The rate of change is described by the term half-life, meaning the time it takes for the
radiation of an element to decrease by half.
The decay of some substances, such as
Uranium-238 and Thorium-232, appears to continue almost without us detecting any
difference in the size or quantity of these substances. Thorium-232, for example, has a
half-life of 14 billion years.
Other radioactive substances show a marked
decrease in their size according to their respective decay rates. Among these is the
isotope Thorium-234 which, after isolation from uranium, decays to half its original
radioactive intensity within 25 days. Another substance, such as Polonium, has a half-life
of less than 10-6 years (0,00001 years).
The half-life can be explained by taking a
practical example. Let's take Carbon-11, which has a decay rate of 3,5% per minute. When
this atom decays, we will find that only 96,5% of the original amount is left after one
minute. After 2 minutes, 93,12% will be left and after 10 minutes, 70% of the original
amount is left. After 20 minutes ± 50% of the original amount is left; therefore we can
state that the half-life of Carbon-11 is 20 minutes.