structures like the cytoplasm, which carries the cell's genetic blueprints,
and also produces chemical constituents which can cause as much damage as
the original ionizing radiation.

For convenience, a unit of radiation dose called the "rad" has been
adopted. It measures the amount of ionization produced per unit volume by
the particles from radioactive decay.



Note 4: Nuclear Half-Life


The concept of "half-life" is basic to an understanding of radioactive
decay of unstable nuclei.

Unlike physical "systems"--bacteria, animals, men and stars--unstable
isotopes do not individually have a predictable life span. There is no way
of forecasting when a single unstable nucleus will decay.

Nevertheless, it is possible to get around the random behavior of an
individual nucleus by dealing statistically with large numbers of nuclei of
a particular radioactive isotope. In the case of thorium-232, for example,
radioactive decay proceeds so slowly that 14 billion years must elapse
before one-half of an initial quantity decayed to a more stable
configuration. Thus the half-life of this isotope is 14 billion years.
After the elapse of second half-life (another 14 billion years), only
one-fourth of the original quantity of thorium-232 would remain, one eighth
after the third half-life, and so on.