T.O. 33B-1-1
6-20
a. Focal Spot Size. Usually isotope sources have a larger focal spot size than X-ray tubes in order to have
a sufficient quantity of radiation to prevent very long exposure times.
b. Fixed Radiation Energy. Isotopes emit radiation with an energy characteristic of that particular
radioactive material. Therefore, the operator has no choice of radiation energy, and it is not always
possible to select the radiation energy compatible with the absorption characteristics of the part being
inspected.
c. Exposure Techniques. Exposure times are important and often isotopes are weak in radiation output,
and source-to-film distances must be decreased to reduce exposure times. This leads to poor definition.
6.3.6
Isotope Cameras.
Isotopes emit their radiation continuously and cannot be shut off or stopped like an X-ray generator. These isotopes are
stored in a radiation shielded container to reduce the radiation to a level safe for unprotected personnel when not
making radiographic exposures. The shielded container must be designed so that the radioactive isotope can be
remotely positioned for the radiographic exposure. Many schemes have been devised for remote handing of isotopes.
Source holders, commonly called isotope cameras, generally are of two typical designs. The simplest cameras are
designed for direct beam radiography, and the source is only allowed to produce a restricted conical direct beam. The
container itself is used to absorb radiation that in not emanating from the window or port. Some units are designed so
that opening and closing can be accomplished at a remote distance. The other types of isotope cameras are normally
used for circumferential radiography. These are devised to move the source from its shielded container to a point some
distance away and then, upon completion of exposure, return the source to the container. In the latter types of cameras
the radiation is being emitted in all directions.
6.3.7
Maintenance.
NOTE
In the event of any malfunction, the appropriate equipment service manual shall be
consulted.
All man-made radioactive isotopes are under the jurisdiction of the Nuclear Regulatory Commission (NRC). A license
is required to purchase and use these isotopes. The Air Force possesses a Master Materials License from the NRC. In
order to obtain sources at base level, contact the base Radiation Safety Officer (RSO). Normally the base RSO is the
Bioenvironmental Engineer (BEE). The base RSO will help obtain a permit from the USAF Isotope Committee, the
regulatory body within the Air Force. Each permit will give specific requirements for any radioactive isotope used for
radiography.
SECTION IV
RADIOGRAPHIC FILM
6.4
FILMS, FILM HOLDERS AND SCREENS.
6.4.1
Films.
6.4.1.1
Function.
Films can be used as a recording medium because their emulsions are sensitive to the quantity and the energy of
electromagnetic radiation over a wide spectral range. In the photographic process, the electromagnetic radiation of the
visible spectrum is focused by a lens upon the film surface to record the variations of light intensities and form an
image. In radiographic applications, the radiation is of such high energies they cannot be focused by a lens. In
radiography, a shadowgraph of the test object is formed by recording the variations in radiation quantities caused by
absorption and scattering by the test specimen. After final processing, film that has been exposed with X or gamma
