T.O. 33B-1-1
6-66
Table 6-15. Relative Absorption of Materials
Material
Kilovoltage
Exposure
Time
Thickness
Lead
Copper
Steel
Titanium
Aluminum
Magnesium
200 kVp
200 kVp
200 kVp
200 kVp
200 kVp
200 kVp
1 min
1 min
1 min
1 min
1 min
1 min
1/16"
1/2"
3/4"
1"
4"
5"
6.7.3.5.1
If the penetrating power of the radiation is great, it can be seen that each increment of thickness in the object will
absorb less of the total than if the penetrating power of the radiation is lower. And, conversely, it follows that if low
kilovoltage is utilized; less of the total radiation will be transmitted through the object. Each small change in
absorption due to thickness of material will then cause a relatively large change in transmission. Thus, the lower the
voltage used, the greater the radiographic contrast. Therefore, kilovoltage may be lowered to perform an inspection,
but SHALL NOT be increased above the level prescribed in the specific inspection instructions without cognizant
engineering approval.
6.7.3.6
Subject Contrast.
Subject or object contrast must also be considered by the radiographer. At X-ray voltages from 30 kVp to 5 MeV,
aluminum has a lower absorption per unit thickness than steel. Therefore, it takes a greater thickness change of
aluminum to cause a given change in X-ray transmission than with steel. Hence, it follows that aluminum has less
object contrast than steel. Figure 6-32 shows graphically the change in thickness versus the change in transmitted
radiation. In the radiographic process, these differences in object contrast are, however, partially compensated for
because lower energy radiation (longer wavelength) can be used to examine a given thickness of aluminum than for the
same thickness of steel. In general, a 1- percent change of thickness will produce sufficient density change on film to
be visible when viewed on most metal subjects. But with magnesium and lighter metals, it is difficult to record 2
percent thickness change. Thus, object contrast is a somewhat limiting factor in light metals and material with both
low density and atomic number. See Figure 6-33 for the relations between X-ray absorption of steel, aluminum and
magnesium.
