Bremsstrahlung occurs when a charged particle, such as an electron, accelerates or decelerates due to the influence of another charged particle, typically a nucleus. As the electron changes its velocity (accelerates), it loses energy in the form of electromagnetic radiation. This radiation is called Bremsstrahlung.
When a high-speed electron passes near a nucleus, the electric field of the nucleus causes the electron to slow down or change direction (accelerate). This sudden change in motion results in the emission of X-rays or other electromagnetic radiation.
X-ray photons are high-energy electromagnetic waves. They carry enough energy to ionize atoms and molecules by knocking out electrons. This ionization can damage critical molecules in cells, such as DNA, proteins, and lipids, which may lead to cell death or prevent the cell from dividing and functioning properly.
X-rays are produced when high-energy electrons interact with atoms in the imaging machine's target material (usually made of a metal like tungsten). During these interactions, inner-shell electrons in the target atoms can be ejected, and electrons from higher energy levels transition to fill the vacancy. The energy difference between the inner and higher energy levels is large, resulting in the emission of high-energy X-rays. In a fluorescent lamp, electrons excite mercury atoms, which then emit ultraviolet light as they return to lower energy levels. The energy difference between the excited and ground states of mercury atoms is much smaller than the energy difference in the inner-shell transitions of heavier atoms in the X-ray machine.
X-rays interact with the electrons in an atom. The more electrons an atom has, the greater the likelihood that an X-ray photon will be absorbed or scattered.
X-rays are absorbed when their energy matches or exceeds the energy required to eject tightly bound electrons from atoms (a process called the photoelectric effect). Atoms with more tightly bound electrons—typically those with higher atomic numbers—are better at absorbing X-rays. Bone contains elements like calcium (Ca) and phosphorus (P), which have higher atomic numbers compared to the lighter elements like hydrogen (H), carbon (C), oxygen (O), and nitrogen (N) found in soft tissue. These higher atomic number elements in bone have more tightly bound electrons that can absorb X-ray photons effectively, leading to the contrast seen in X-ray images.
X-rays are absorbed when their energy is sufficient to eject electrons from atoms via the photoelectric effect. Heavier atoms, like calcium (Ca) in bone, have more electrons compared to the lighter atoms (e.g., carbon, hydrogen, oxygen) found in soft tissue. This makes calcium much better at absorbing X-rays.
The Photoelectric Effect:
The photoelectric effect is a process where a photon (a particle of light, including X-rays) interacts with an atom and transfers its energy to an electron, causing that electron to be ejected from the atom. This occurs when the photon’s energy is greater than or equal to the binding energy of the electron in the atom.
X-rays are high-energy photons, and when they encounter an atom, they may interact with one of the atom's tightly bound electrons (usually from the inner electron shells, such as the K or L shells). If the X-ray photon's energy exceeds the binding energy of the electron, the photon is absorbed, and the electron is ejected from the atom.
The energy of the X-ray photon is used to:
Overcome the binding energy of the electron.
Give the ejected electron kinetic energy (the leftover energy after removing the electron).
The binding energy of electrons depends on the atomic number of the atom. Higher atomic number elements (like calcium or lead) have electrons that are more tightly bound, making them more effective at absorbing X-rays.
In a typical X-ray tube, electrons are emitted from a hot cathode (a negatively charged electrode) through a process called thermionic emission. These electrons are then accelerated toward the anode (a positively charged electrode) due to the high voltage applied between the cathode and anode.