Most familiar natural elements like hydrogen, oxygen, gold, and lead
are stable, and enduring unless acted upon by outside forces. But
almost all elements can exist in unstable forms. The nuclei of these
unstable "isotopes," as they are called, are "uncomfortable" with the
particular mixture of nuclear particles comprising them, and they
decrease this internal stress through the process of radioactive decay.
The three basic modes of radioactive decay are the emission of alpha,
beta and gamma radiation:
Alpha--Unstable nuclei frequently emit alpha particles, actually helium
nuclei consisting of two protons and two neutrons. By far the most
massive of the decay particles, it is also the slowest, rarely
exceeding one-tenth the velocity of light. As a result, its
penetrating power is weak, and it can usually be stopped by a piece of
paper. But if alpha emitters like plutonium are incorporated in the
body, they pose a serious cancer threat.
Beta--Another form of radioactive decay is the emission of a beta
particle, or electron. The beta particle has only about one
seven-thousandth the mass of the alpha particle, but its velocity is
very much greater, as much as eight-tenths the velocity of light. As a
result, beta particles can penetrate far more deeply into bodily tissue
and external doses of beta radiation represent a significantly greater
threat than the slower, heavier alpha particles. Beta-emitting
isotopes are as harmful as alpha emitters if taken up by the body.
Gamma--In some decay processes, the emission is a photon having no mass
at all and traveling at the speed of light. Radio waves, visible
light, radiant heat, and X-rays are all photons, differing only in the
energy level each carries. The gamma ray is similar to the X-ray
photon, but far more penetrating (it can traverse several inches of
concrete). It is capable of doing great damage 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.