d. The best weapon against electrical shock is knowledge of electricity and
common sense. Ignorance when handling electricity can be a passport to death
Section II. RADIATION--INTERACTION OF PHOTONS WITH MATTER
4-7.
INTRODUCTION
The definition of the term interaction is quite simple: it is one force or body
having a measurable effect on another force or body. One can see daily evidence of
interaction: in a bowling alley, at the lake watching a sailboat, or on the job in the many
uses of electrical transformers. The interaction that the radiologic specialist must fully
understand is that which takes place when a beam of x-ray photons passes through
anything having mass and occupying space, or more simply, matter. An x-ray beam,
consisting of photons of pure energy, transfers its energy to the matter through which it
is passing, whether it be air, an x-ray film, or the living tissues of the radiologic specialist
or the patient. This transfer of energy is not as simple as that seen in bowling, sailing,
or electric transformers: it involves the x-ray photons, which cannot be seen, heard, felt,
or detected with the normal senses; and, in many cases. the interaction itself is not
immediately evident without complicated devices to detect these events. Some, if not
all, of the x-ray energy seems to disappear in certain materials. The term which best
describes this phenomenon is absorption. Absorption is the process by which an x-ray
photon transfers its inherent energy to the medium through which it is passing. Some
results of this absorption are: (1) chemical changes in film emulsion, (2) electrical
4-8.
IONIZATION
The changes mentioned above are all brought about by a process known as
ionization. Ionization can be defined as any process that results in the removal or
addition of an orbital electron from or to an atom or molecule, thereby leaving the atom
or molecule with an overall positive or negative charge.
a. Ionization can occur when an electron is struck by a photon, at which time an
energy transfer will take place. Although it is technically possible for this energy transfer
to take place in the nucleus, the chances of a photon reaching that vicinity are
extremely remote. After an ionizing event occurs, the remaining particles are called a
pair of ions (in the case of electron removal). The parent atom (minus an electron) has
an overall positive charge and is known as a positive ion. The ejected electron has a
negative charge and is known as a negative ion. This process of ionization is illustrated
in figure 4-3.
b. Radiation is measured by the number of ion pairs it causes. The amount of
radiation that causes approximately two billion pairs of ions to be formed in one cubic
centimeter of air (at normal pressure/temperature) is known as one roentgen (R).
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