time, and the location, size, and orientation of the defect.
Each of these factors has a bearing on the radiograph
Radioactive sources disintegrate with a time scale
rate expressed in terms of half-life. This may be days
for Iridium-192. Any source aboard ship must be
considered as "live" or potentially dangerous to
health, because the source cannot be turned on and off
as can an X-ray machine. NO person is to use
(expose) radioactive sources unless the use and users
are licensed by NAVSEA.
X rays and gamma rays are potentially very
dangerous to health. Therefore, the operator must be
thoroughly familiar with and use all prescribed safety
precautions when using radiation-producing
equipment. When operating radiographic equipment,
all personnel, including the operator, must be at a safe
distance from the source of radiation. A heavy
bulkhead or lead shielding is usually provided between
the radiation source and the operating personnel. The
first step before you begin the operation is to clear all
personnel from the area in which the radiograph is to be
Figure 11-5.--Diagram illustrating inverse square law.
made. The area must then be roped off. Remember that
secondary radiation scattered from the test piece or
nearby and overhead objects is potentially as dangerous
be slight. The intensity at any given distance, however,
as the direct radiation from the beam itself.
depends upon the intensity of the radiation source.
An important factor in minimizing exposure to
The best radiation safety precaution is distance
radiation is to reduce the amount of time material is
between personnel and the radiation source. This is
exposed to X rays. It stands to reason that if the
true because the intensity of radiation varies inversely
exposure time is reduced, the amount of radiation will
with the square of the distance from the radiation
also be reduced. Mathematical calculations are used to
source. X rays, like a beam of light, cover an
determine the amount of exposure required for different
increasingly larger area with lessened intensity as
types and thicknesses of metals.
they travel from the source. This principle, known as
the inverse square law, is illustrated in figure 11-5.
When the distance (D) from the radiating source is
doubled (2D), the area covered by the beam (Cl) is
Shielding also reduces radiation exposure. If a
quadrupled (C2). At the same time, though, the
shield is placed between the operator and the source of
intensity per unit of area is only one-quarter of the
radiation, the amount of radiation received by the
value at the original distance. Thus, with sufficient
operator will be reduced. The denser the shielding
distance, the area covered by the beam may be
material, the better the protection provided. Lead is the
tremendously large, but its intensity per unit area will
best shielding material available due to its dense