Slam fast electrons into a metal target and a few of them throw out X-ray photons. Send the beam through the body and the parts that absorb most, like bone, leave the brightest shadows on the image.
X-rays are made by accelerating electrons through a high p.d. and stopping them in a metal target; most of their energy becomes heat, but a little becomes X-ray photons of maximum energy hfₘₐₓ = eV. In the body the beam is attenuated, I = I₀e⁻μᵛ, and the image forms from the contrast between tissues with different μ. A CT scan combines many such views into a 3D image.
Choose a material and a thickness and watch the transmitted intensity fall along I = I₀e⁻μᵛ. Switch between soft tissue and bone at the same thickness: bone's much larger attenuation coefficient lets far less through, and that difference is the contrast that makes the bone visible.
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Only a small fraction of the electrons' energy becomes X-rays, the rest is heat, which is why the target must be cooled (often a rotating anode). The maximum photon energy hfₘₐₓ = eV comes from an electron giving up all its kinetic energy in one go. Attenuation is exponential, so quoting a half-value thickness (ln2/μ) is often the quickest route in a calculation.
Four quick checks on X-rays. Each correct answer earns XP and lights this skill on your star map.
X-rays are produced in an X-ray tube when:
If electrons are accelerated through a p.d. V, the maximum energy of an X-ray photon produced is:
As an X-ray beam passes through a thickness x of material, its intensity:
Bone shows up clearly on an X-ray image because, compared with soft tissue, it has a:
A CT scan gives a 3D image with excellent soft-tissue contrast by combining many 2D projections, but it uses a much higher radiation dose than a single X-ray, so the benefit must outweigh the risk. Convert the photon energy to joules (eV × 1.6 × 10⁻¹⁹) before using hf to find a frequency or wavelength.
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