The K-series refers to a specific set of X-ray emissions that are produced when an electron transitions to the K shell (the innermost energy level) of an atom. This process occurs when inner-shell electrons are ejected, usually by high-energy photons or particles, causing electrons from higher energy levels to fall into the K shell, resulting in the emission of X-rays characteristic of that element. These X-rays are crucial for various applications, including medical imaging and material analysis.