X-ray Tube Energies: How High Is Too High?
January 12, 2016
Enjoy this excerpt from our latest paper, How to Select the Newest X-ray Technology: From Nano to Macro. For the full paper, click here.
When choosing an X-ray tube, one of the most important criteria is energy bandwidth. Presently within the industry, the trend is to choose a higher energy tube to obtain better penetration of high absorbing materials and larger wall thicknesses. Increasing the acceleration voltage can increase X-ray photons more than when simply increasing the current of the tube. For example, doubling the current of the tube increases the number of photons by a factor of two, but doubling the voltage can increase the number of photons by a factor of four. This is tempting to do because the larger number of photons results in a shorter measurement time, but keep in mind you are in danger of losing image quality by using this method. Here's why.
There are a few X-ray photon phenomena that are relevant to a discussion about higher energy tubes: photo absorption, Compton scattering and pair production. Depending on material properties, different effects are observed. It all depends on the materials you are inspecting.
With low energies (<40 keV), photo absorption is the dominant effect for most materials. With low energies, the tube is able to produce good contrast—even between two materials that are physically proximate to one another on the same sample. This is because absorption depends on the atomic number (Z) of the material, and at low energies, the influence of Z is very high. The bad news is that materials with very different atomic numbers will produce enormous contrast, which may lead to an image that cannot be inspected.
With higher energies (>200 keV), Compton scattering is the dominant effect. In this case, the atomic number, Z, is only a linear factor in the absorption equation—it has less of an influence. Materials with very different Z values will produce less contrast and can be seen within one shot. Of course, materials with very similar atomic numbers may show up as one grey value in the image.
At energies beyond 2-3 MeV, pair production is, more or less, the only effect that takes place. Inspection at these energies is less optimal for most nondestructive test applications because there is very little absorption, and thus very little contrast.
If your test objective requires a large contrast of materials that have minimum differences in atomic numbers (like carbon fiber–reinforced polymer or CFRP, where almost everything is carbon), make sure the amount of X-ray energy used stays in a range where photo absorption is the dominating effect. On the other hand, if you have a combination of materials with very large differences in atomic number, such as CFRP and steel, make sure the Compton effect is dominant by using higher energies. When inspected at higher energies, those materials will be visible. See Figure 6.
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