Radiographic equivalence factors
Exposure charts are almost always drawn for carbon steel. To shoot aluminium, titanium, copper or a nickel alloy without redrawing the chart, the actual thickness is converted into the thickness of steel that would attenuate the beam by the same amount. That conversion is the radiographic equivalence factor: steel-equivalent thickness = actual thickness x factor.
The factor depends on both material and photon energy, because the two competing interaction mechanisms scale differently. At low energies the photoelectric effect dominates and its cross-section rises roughly as the fourth or fifth power of atomic number, so lead is fourteen times steel at 100 kV. As energy rises, Compton scattering takes over, and that depends on electron density rather than Z, so material differences collapse – the same lead is only 2.3 times steel at cobalt-60 energies. Titanium moves the other way, from 0.54 at 150 kV to 0.9 at megavolt energies.
This is why the same aluminium casting needs a completely different technique at 100 kV than at 220 kV, and why equivalence factors quoted without an energy are useless. It is also why high-energy sources give poor contrast on multi-material assemblies: at 2 MV, brass, steel and zirconium are all within 10 percent of each other and simply do not separate on the film.
The values here are ASTM E94 Table 1. They are approximate by design – the standard describes them as such – and they assume the beam quality typical of that source. Use them to get the first shot in the right decade, then refine against your own step wedge.
Worked example
| Material | cu |
| Radiation energy | ir192 |
| Actual material thickness | 20 mm |
| Equivalence factor | 1.1 |
| Steel-equivalent thickness | 22 mm |
| Thickness of this material equal to the entered steel thickness | 18.18 mm |
Copper at Ir-192 energies has an equivalence factor of 1.1, so 20 mm of copper attenuates like 20 x 1.1 = 22.00 mm of steel and the exposure chart is read at 22 mm. Conversely 20 mm of steel is matched by 20 / 1.1 = 18.18 mm of copper.
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