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13 IONIZING RADIATION

13.1 Basic Concepts and Definitions of Ionizing Radiation

From the time that radioactivity was discovered, it was obvious that it caused damage. As
early as 1901, Pierre Curie discovered that a sample of radium placed on his skin produced
wounds that were very slow to heal.
Ionizing radiation is much more dangerous than non-ionizing. A dose of only 300 joules of
x-ray or γ-ray radiation is fatal for the average human, even though this radiation raises the
temperature of the body by only 0.001C. α-particle radiation is even more dangerous; a dose
equivalent to only 15 joules is fatal for the average human.
We live in a sea of radiation. In recent years, people have learned to fear the effects of
radiation. They don't want to live near nuclear reactors. They are frightened by reports of links
between excess exposure to sunlight and skin cancer. They are afraid of the leakage from
microwave ovens, or the radiation produced by their television sets.
Several factors combine to heighten the public's anxiety about both the short-range and
long-range effects of radiation. Perhaps the most important source of fear is the fact that radiation
can't be detected by the average person. Furthermore, the effects of exposure to radiation might not
appear for months or years or even decades.

To understand the biological effects of radiation we must first understand the
difference between ionizing radiation and non-ionizing radiation. In general, two
things can happen when radiation is absorbed by matter: excitation or ionization.
Excitation occurs when the radiation excites the motion of the atoms or
molecules.
Ionization occurs when the radiation carries enough energy to remove an
electron from an atom or molecule.

Because living tissue is 70-90% water by weight, the dividing line between
radiation that excites electrons and radiation that forms ions is often assumed to be
equal to the ionization of water: 1216 kJ/mol. Radiation that carries less energy can
only excite the water molecule. It is therefore called non-ionizing radiation.
Radiation that carries more energy than 1216 kJ/mol can remove an electron from a
water molecule, and is therefore called ionizing radiation.
Table 13.1 contains estimates of the energies of various kinds of radiation.
Radio waves, microwaves, infrared radiation, and visible light are all forms of non-
ionizing radiation. X-rays, -rays, and - and β-particles are forms of ionizing
radiation.
The dividing line between ionizing and non-ionizing radiation in the
electromagnetic spectrum falls in the ultraviolet portion of the spectrum. It is,
therefore, useful to divide the UV spectrum into two categories: UV A and UVB.
Radiation at the high-energy end of the UV spectrum can be as dangerous as x-rays
or -rays.

Ionizing radiation occurs in two forms - waves or particles.

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