Varian Linatron High-Energy X-ray Applications 2007

Comparing the exposure and density for the particular
application with standard exposures and densities from an
exposure curve or other reference is another way to
determine blocking effectiveness. If a shorter exposure of
the material corresponds to a higher film density, when
compared with other well-blocked exposures, it is very
likely that scatter has contributed to the exposure.
Lead Intensifying Screens as Filters. The thin metal foil
(usually lead) intensifying screens that are placed in front
and back of the film to intensify the primary beam and
shorten the exposure also function as filters to reduce the
amount of scattered radiation that reaches the film. Much
of the scatter can be stopped by the lead screen since the
average energy of the scattered radiation is lower than the
energy of the primary beam.
Example
At 10 MeV, a front lead foil 0.01 inch (0.25 mm) thick
will reduce scatter reaching the film by 35% when
exposing about 5 HVLs of low-Z material.
It is customary to select the front lead screen thickness so
the screen provides both intensification and filtration.
However, this practice may not always give the optimum
image quality results. The thickness of a front lead screen
should never exceed 0.050 inch (1.3 mm). The
radiographer should consider using a composite filter
placed in front of the film holder if additional filtration is
needed beyond that. The quality of the image should be
the basis for deciding whether filtration is effective. Some
radiographers eliminate backscatter by using external
(back) lead shielding rather than using lead foils.
Filters. Absorbing plates or assemblies placed between the
object and the film holder may be necessary to achieve the
best image quality when:
• The object shape, position, or composition produces
extensive scatter.
• Large object thickness produces scatter greatly in excess
of the primary radiation.
• The object is smaller than the radiation field.
• Large thickness differences exist in the object.
• High-speed photoelectrons produced in the object may
reach the film holder.
page 15
In most cases, the front metal foil intensifying screen will
provide enough filtration to obtain good image quality.
However, in applications (e.g., assembly radiography),
where image sharpness is paramount and a number of the
conditions listed above may exist, an object-film filter of
suitable design is recommended. The filter should be
placed as close behind the object as practicable. The ideal
filter can be layers of high-Z material (such as lead) closer
to the source with subsequent layers of decreasing Z
materials since the filter itself may be a source of scatter. In
some applications a simple filter of lead and brass may be
sufficient.
Screen choices depend on the application, the details of the
radiographic procedure, the energy level, and the degree of
blocking used. For an application in which the
collimation, blocking, and shielding have not minimized
the amount of scatter reaching the film holder, and the
setup geometry must not be modified, the use of external
filters may provide an effective solution. A radiographer
can judge the effectiveness of a filter by comparing image
quality with and without the filter.
Film Holders. The film holder, or cassette, has two
functions. In addition to securing the film from light, it
also maintains firm contact between the film and screens.
Loss of good contact between film and screens results in a
loss of contrast and sharpness in the image.
Three types of film holders or cassettes are frequently used:
semirigid, rigid, and vacuum. The best choice of holder
depends on the requirements of the application.
The Semirigid cardboard or plastic holder is the most
common type. It offers low cost, ease of handling, and
flexibility. Soft plastic, semirigid film holders should be
used when the film must be curved to fit the shape of the
object being inspected.
Rigid holders with mounted screens and spring-loaded
backs are also common. The rigid holder provides more
positive film-screen contact than the semi-rigid holder,
especially when the film must be held in a vertical
orientation.
Varian Linatron applications