T.O. 33B-1-1
6-37
because of the high electron absorption in light materials, the surface must be kept free from dirt and lint which will
produce light densities on the radiograph. Remove grease and lint from the surface of lead foil screens with a solvent
such as isopropyl alcohol. If more thorough cleaning is necessary, rub screens gently with the finest grade of steel
wool. Films may be fogged if left between lead screens longer than is reasonably necessary, particularly under
conditions of high temperature and humidity. When screens have been freshly cleaned with an abrasive, this effect will
be increased. It is best to delay the use of freshly cleaned screens at least 24 hours.
6.4.3.2.5
Precautions.
Lead screens must be used with great care. Common problems are:
a. A fuzzy image results from lack of intimate contact between the screens and film.
b. Dark lines on the image can result from scratches on the screens.
SECTION V
INTERACTION OF RADIATION WITH MATERIAL
6.5
INTERACTION OF RADIATION WITH MATERIAL.
6.5.1
Absorption Mechanisms.
Absorption of gamma or X radiation by materials requires detailed consideration. These radiation photons are
electromagnetic waves of energy, have no mass or electrical charge, and can penetrate the densest materials. These
waves are dimensionally so short that they have wavelengths less than the electron spacing in the atoms and therefore
have the capability of traveling through the atomic structure. The absorption of the photons is a result of the photon
either striking an electron or entering the nuclear field of the atom. The energy lost by a radiation beam as it travels
through matter is due to interactions of the photons with matter. In these interactions the energy of the photon is
transferred principally through three processes. These are photoelectric absorption, Compton effect and pair
production. (See Figure 6-15.) At extremely high photon energies a small amount of absorption is due to the
photodisintegration process, but this is of little consequence in radiographic applications. Most of the radiation
absorption is due to interactions of the photons with electrons in the atoms of the absorbing material. Therefore, an
absorber may be judged somewhat in relationship to the electron density of the absorber, or approximately the number
of electrons in the radiation beam path. The parameters that contribute to this electron density are the atomic number,
the density, and the thickness of the absorber. The atomic number is the number of protons in the nucleus of the
particular atom, and material density (usually expressed as grams per cubic centimeter) is related to the number of
atoms that are compacted in a given material volume. The thickness of the absorber can be mechanically measured.
Atomic number, material density, and absorber thickness combine to present an absorber value to the radiation. The
radiation photons interact with the atoms in the absorber in different manners, depending upon the energy or
wavelength of the photon.
