age in the body.

Common to all three types of nuclear decay radiation is their ability
to ionize (i.e., unbalance electrically) the neutral atoms through
which they pass, that is, give them a net electrical charge. The alpha
particle, carrying a positive electrical charge, pulls electrons from
the atoms through which it passes, while negatively charged beta
particles can push electrons out of neutral atoms. If energetic betas
pass sufficiently close to atomic nuclei, they can produce X-rays which
themselves can ionize additional neutral atoms. Massless but energetic
gamma rays can knock electrons out of neutral atoms in the same fashion
as X-rays, leaving them ionized. A single particle of radiation can
ionize hundreds of neutral atoms in the tissue in multiple collisions
before all its energy is absorbed. This disrupts the chemical bonds
for critically important cell 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.

Most manmade radioactive isotopes have much shorter half-lives, ranging
from seconds or days up to thousands of years. Plutonium-239 (a
manmade isotope) has a half-life of 24,000 years.

For the most common uranium isotope, U-238, the half-life is 4.5