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There are many types of ionizing radiation (Figure 13.1), but the most familiar
are alpha, beta, and gamma/x-ray radiation. Neutrons, when expelled from atomic
nuclei and traveling as a form of radiation, can also be a significant health concern.

 Alpha particles are clusters of two neutrons and two protons each. They
are identical to the nuclei of atoms of helium, the second lightest and second most
common element in the universe, after hydrogen. Compared with other forms of
radiation, though, these are very heavy particles--about 7,300 times the mass of an
electron. As they travel along, these large and heavy particles frequently interact with
the electrons of atoms, rapidly losing their energy. They cannot even penetrate a
piece of paper or the layer of dead cells at the surface of our skin. But if released
within the body from a radioactive atom inside or near a cell, alpha particles can do
great damage as they ionize atoms, disrupting living cells. Radium and plutonium are
two examples of alpha emitters.
Thus, α-radiation is characterized by low (lowest) penetrating power and high
specific ionization
 Beta particles are electrons or positrons traveling at very high
energies. If alpha particles can be thought of as large and slow bowling balls, beta particles can
be visualized as golf balls on the driving range. They travel farther than alpha particles and,
depending on their energy, may do as much damage. For example, beta particles in fallout can
cause severe burns to the skin, known as beta burns. Radioisotopes that emit beta particles are
present in fission products produced in nuclear reactors and nuclear explosions. Some beta-
emitting radioisotopes, such as iodine 131, are administered internally to patients to diagnose and
treat disease.

 Gamma rays and x-rays radiation consist of packets of energy known
as photons. Photons have no mass or charge, and they travel in straight lines. The
visible light seen by our eyes is also made up of photons but at lower energies.
The energy of gamma rays is typically greater than 100 kiloelectron volts (keV--"k" is the
abbreviation for kilo, a prefix that multiplies a basic unit by 1,000) per photon, more than 200,000
times the energy of visible light (0.5 eV).
If alpha particles are visualized as bowling balls and beta particles as golf balls, photons of
gamma and x-radiation are like weightless bullets moving at the speed of light.

Photons are classified according to their origin.
Gamma rays originate from events within an atomic nucleus; their energy and
rate of production depend on the radioactive decay process of the radionuclide that is
their source.
х-rays are photons that usually originate from energy transitions of the
electrons of an atom. These can be artificially generated by bombarding appropriate
atoms with high-energy electrons, as in the classic x-ray tube. Because x rays are
produced artificially by a stream of electrons, their rate of output and energy can be
controlled by adjusting the energy and amount of the electrons themselves.
Both x rays and gamma rays can penetrate deeply into the human body. How
deeply they penetrate depends on their energy; higher energy results in deeper
penetration into the body.

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