T.O. 33B-1-16-66Table 6-15. Relative Absorption of MaterialsMaterial Kilovoltage ExposureTimeThicknessLeadCopperSteelTitaniumAluminumMagnesium200 kVp200 kVp200 kVp200 kVp200 kVp200 kVp1 min1 min1 min1 min1 min1 min1/16"1/2"3/4"1"4"5"6.7.3.5.1If the penetrating power of the radiation is great, it can be seen that each increment of thickness in the object willabsorb less of the total than if the penetrating power of the radiation is lower. And, conversely, it follows that if lowkilovoltage is utilized; less of the total radiation will be transmitted through the object. Each small change inabsorption due to thickness of material will then cause a relatively large change in transmission. Thus, the lower thevoltage 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 cognizantengineering approval.6.7.3.6 SubjectContrast.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 ofaluminum to cause a given change in X-ray transmission than with steel. Hence, it follows that aluminum has lessobject contrast than steel. Figure 6-32 shows graphically the change in thickness versus the change in transmittedradiation. In the radiographic process, these differences in object contrast are, however, partially compensated forbecause lower energy radiation (longer wavelength) can be used to examine a given thickness of aluminum than for thesame thickness of steel. In general, a 1- percent change of thickness will produce sufficient density change on film tobe visible when viewed on most metal subjects. But with magnesium and lighter metals, it is difficult to record 2percent thickness change. Thus, object contrast is a somewhat limiting factor in light metals and material with bothlow density and atomic number. See Figure 6-33 for the relations between X-ray absorption of steel, aluminum andmagnesium.
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