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APPENDIX D. OVERVIEW OF BASIC RADIATION PHYSICS, CHEMISTRY,
AND BIOLOGY
Understanding the basic concepts in radiation physics, chemistry, and biology is important to the
evaluation and interpretation of radiation-induced adverse health effects and to the derivation of radiation
protection principles. This appendix presents a brief overview of the areas of radiation physics,
chemistry, and biology and is based to a large extent on the reviews of Mettler and Moseley (1985),
Hobbs and McClellan (1986), Eichholz (1982), Hendee (1973), Cember (1996), and Early et al. (1979).
D.1 RADIONUCLIDES AND RADIOACTIVITY
The substances we call elements are composed of atoms. Atoms in turn are made up of neutrons, protons
and electrons: neutrons and protons in the nucleus and electrons in a cloud of orbits around the nucleus.
Nuclide is the general term referring to any nucleus along with its orbital electrons. The nuclide is
characterized by the composition of its nucleus and hence by the number of protons and neutrons in the
nucleus. All atoms of an element have the same number of protons (this is given by the atomic number)
but may have different numbers of neutrons (this is reflected by the atomic mass numbers or atomic
weight of the element). Atoms with different atomic mass but the same atomic
numbers are referred to as
isotopes of an element.
The numerical combination of protons and neutrons in most nuclides is such that the nucleus is quantum
mechanically stable and the atom is said to be stable, i.e., not radioactive; however, if there are too few or
too many neutrons, the nucleus is unstable and the atom is said to be radioactive. Unstable nuclides
undergo radioactive transformation, a process in which a neutron or proton converts into the other and a
beta particle is emitted, or else an alpha particle is emitted. Each type of decay is typically accompanied
by the emission of gamma rays. These unstable
atoms are called radionuclides; their emissions are called
ionizing radiation; and the whole property is called radioactivity. Transformation or decay results in the
formation of new nuclides some of which may themselves be radionuclides, while others are stable
nuclides. This series of transformations is called the decay chain of the radionuclide. The first
radionuclide in the chain is called the parent; the subsequent products of the transformation are called
progeny,
daughters, or decay products.
In general there are two classifications of radioactivity and radionuclides: natural and artificial (man-
made). Naturally-occurring radioactive material (NORM) exists in nature and no additional energy is
necessary to place them in an unstable state. Natural radioactivity is the property of some naturally
occurring, usually heavy elements, that are heavier than lead. Radionuclides, such as radium and
uranium, primarily emit alpha particles. Some lighter elements such as carbon-14 and tritium (hydrogen-
3) primarily emit beta particles as they transform to a more stable atom. Natural radioactive atoms
heavier than lead cannot attain a stable nucleus heavier than lead. Everyone is exposed to background
radiation from naturally-occurring radionuclides throughout life. This background
radiation is the major
source of radiation exposure to man and arises from several sources. The natural background exposures
are frequently used as a standard of comparison for exposures to various artificial sources of ionizing
radiation.
Artificial radioactive atoms are produced either as a by-product of fission of uranium or plutonium atoms
in a nuclear reactor or by bombarding stable atoms with particles, such as neutrons or protons, directed at
the stable atoms with high velocity. These artificially produced radioactive elements usually decay by
emission of particles, such as positive or negative beta particles and one or more high energy photons
(gamma rays). Unstable (radioactive) atoms of any element can be produced.
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APPENDIX D
Both naturally occurring and artificial radioisotopes find application in medicine, industrial products, and
consumer products.
Some specific radioisotopes, called fall-out, are still found in the environment as a
result of nuclear weapons use or testing.
D.2 RADIOACTIVE DECAY
D.2.1 Principles of Radioactive Decay
The stability of an atom is the result of the balance of the forces of the various components of the nucleus.
An atom that is unstable (radionuclide) will release energy (decay) in various ways and transform to
stable atoms or to other radioactive species called daughters, often with the release of ionizing radiation.
If there are either too many or too few neutrons for a given number of protons, the resulting nucleus may
undergo transformation. For some elements, a chain of daughter decay products may be produced until
stable atoms are formed. Radionuclides can be characterized by the type and
energy of the radiation
emitted, the rate of decay, and the mode of decay. The mode of decay indicates how a parent compound
undergoes transformation. Radiations considered here are primarily of nuclear origin, i.e., they arise from
nuclear excitation, usually caused by the capture of charged or uncharged nucleons by a nucleus, or by the
radioactive decay or transformation of an unstable nuclide. The type of radiation may be categorized as
charged or uncharged particles, protons, and fission products) or electromagnetic radiation (gamma rays
and x rays). Table D-1 summarizes the basic characteristics of the more common types of radiation
encountered.
D.2.2 Half-Life and Activity
For any given
radionuclide, the rate of decay is a first-order process that is constant, regardless of the
radioactive atoms present and is characteristic for each radionuclide. The process of decay is a series of
random events; temperature, pressure, or chemical combinations do not effect the rate of decay. While it
may not be possible to predict exactly which atom is going to undergo transformation at any given time, it
is possible to predict, on average, the fraction of the radioactive atoms that will transform during any
interval of time.
The
activity is a measure of the quantity of radioactive material. For these radioactive
materials it is
customary to describe the activity as the number of disintegrations (transformations) per unit time. The
unit of activity is the curie (Ci), which was originally related to the activity of one gram of radium, but is
now defined as that quantity of radioactive material in which there are:
1 curie (Ci) = 3.7x10
10
disintegrations (transformations)/second (dps) or 2.22x10
12
disintegrations
(transformations)/minute (dpm).
The SI unit of activity is the becquerel (Bq); 1 Bq = that quantity of radioactive material in which there is
1 transformation/second. Since activity is proportional to the number of atoms of the radioactive
material, the quantity of any radioactive material is usually expressed in curies, regardless of its purity or
concentration. The transformation of radioactive nuclei is a random process, and the number of
transformations is directly proportional to the number of radioactive atoms present. For any pure
radioactive substance, the rate of decay is usually described by its
radiological half-life, T
R
, i.e., the time
it takes for a specified source material to decay to half its initial activity. The specific activity is an
indirect measure of the rate of decay, and is defined as the activity per unit mass or per unit volume. The
higher the specific activity of a radioisotope, the faster it is decaying.