X-rays are a form of high-energy electromagnetic radiation, similar to visible light but possessing much shorter wavelengths. This difference in wavelength gives X-ray photons enough energy to pass through soft materials like human tissue. Whether X-rays can pass through metal depends entirely on the metal’s physical characteristics and the energy level of the X-ray beam being used. The resulting image in medical or security applications is essentially a shadow created by the differential stopping power of the materials within the path of the beam.
How X-Rays Interact with Matter
When an X-ray beam passes through any substance, its intensity decreases in a process known as attenuation, which is the combined result of absorption and scattering of the X-ray photons. The beam’s intensity decreases exponentially based on the material’s thickness and its internal composition. The primary mechanism for X-ray removal at diagnostic energy levels is the photoelectric effect, where a photon is completely absorbed by an atom. The second major interaction is Compton scattering, where the photon collides with an outer-shell electron, losing some energy and being deflected from its original path. Both photoelectric absorption and Compton scattering contribute to the overall loss of X-ray intensity, which creates the contrast seen in an image.
Why Metals Block X-Rays Effectively
Metals are highly effective at attenuating X-rays primarily because of two physical properties: high atomic number (Z number) and high physical density. The Z number indicates the number of protons in an atom’s nucleus, which corresponds to the number of orbiting electrons. Materials with a high Z number, such as lead (Z=82) or tungsten (Z=74), have many electrons tightly bound to the nucleus. This increased electron count significantly raises the probability of a photoelectric interaction occurring when an X-ray photon encounters an atom, dramatically improving blocking capability.
Metals also possess a high physical density, meaning their atoms are packed tightly together within a given volume. This high density ensures that an X-ray photon is likely to encounter many atoms and electrons over a short distance, increasing the chance of an interaction. For instance, lead, commonly used in radiation shielding, combines a high atomic number with a density of 11.34 grams per cubic centimeter. This combination makes lead effectively opaque to the lower-energy X-rays used in medical and security screenings. Other dense metals like iron or copper also appear bright white on standard X-ray images, indicating strong attenuation.
When X-Rays Penetrate Dense Materials
The ability of an X-ray to penetrate a material is relative to the energy of the X-ray photons themselves. While standard diagnostic X-ray machines operate in the kilovoltage (kV) range, specialized applications utilize vastly higher energies. In industrial radiography, also known as Non-Destructive Testing (NDT), high-energy generators produce X-ray beams in the megavoltage (MV) range to inspect thick, dense materials like steel pipes or concrete structures. At these extreme energy levels, the photons are less likely to be stopped by the photoelectric effect and are instead more prone to Compton scattering, which is a less efficient stopping mechanism. These beams can pass through several inches of steel, allowing technicians to detect internal flaws or defects, though the dense metal still attenuates the radiation far more than lower-density materials.