Varian Linatron High-Energy X-ray Applications 2007

Drilled Hole Plaque Penetrameter Sensitivity. The
drilled hole plaque is the standard American penetrameter
for all radiography. It consists of a small rectangular strip
of material with three drilled holes. Thickness of the
penetrameter is specified as 1%, 2%, or more, of the object
thickness. The three hole diameters are equal to 1 x T, 2 x
T, and 4 x T where the thickness of the penetrameter “T”.
Sensitivity is described in terms of the thickness percent (1,
2, 4) and the required hole size that must be discerned.
Example
2-2T is the standard way to describe a penetrameter whose
thickness must be 2% of the object thickness and whose 2
x T hole must be seen on the film.
Drilled hole sensitivity data for each Linatron show that all
sources should demonstrate better than 1% sensitivity
above 2 inches (50.8 mm) of steel, up to 10 HVLs, and
better than 2% for thicknesses down to 1 inch (25.4 mm).
Radiographic Sensitivity. Radiographic sensitivity refers
to the ability to see and recognize a variety of defects and
other internal conditions in the radiograph in this manual.
Standard penetrameters provide an indicator of the level of
image quality that is achieved with a particular
radiographic technique used. Penetrameters do not,
however, give defect size. Penetrameters must be placed on
the source side of an object since the image improves as the
defect occurs closer to the film.
Exposure Curves
The plot of exposure level that corresponds to a specific
film density (2.0 for example) for each thickness of
material is called an exposure curve. Exposure curves
should be made for the Linatron after it is completely
operational for all applications. Such curves represent
operational data that are indicative of the operating
characteristics of the setup. Subsequent exposure curves for
the same setup should provide consistent results.
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Established exposure curves also allow the radiographer to
simplify the exposure calculation process. With these
curves, the radiographer only needs to look up the required
exposure level for that thickness and make the exposure. If
the source-to-film distance must be changed, using the
inverse square law provides the new exposure level. A
change to a different film can lead to an additional
adjustment of exposure level using the relative speed table.
Method for Making Exposure Curves. This process
involves making a series of test exposures of an object with
uniform density but varying thickness. Typically, a wedge
or stair-step shaped block is used in conjunction with
additional slabs of uniform thickness. Because exposure
time can be represented as a straight line on semi-log graph
paper, at least two measurements are needed to establish
this line.
FIGURE 4-8. Wedge-shaped block used for experimentally
obtaining exposure curves. Standardized to a film density of 2.0.
To make an exposure curve for a new material, a step or
wedge-shape block (see Figure 4-8) of the same material
that is being radiographed, and a slab of that material, are
placed together to form the object for the first test exposure.
It is most helpful if the slab thickness and the wedge center
thickness are approximately 1 HVL. Note, the material
used in this work should first be radiographed to assure that
it is uniform and free of defects.
Varian Linatron applications