Q.The energy of a photon of characteristic X-ray from a Coolidge tube comes from :

X-rays arise from the conversion of the kinetic energy of fast electrons into electromagnetic radiation when they are suddenly decelerated at a target.

In an X-ray tube, electrons are accelerated through a high potential difference and made to strike a heavy metal target (e.g., tungsten, molybdenum). When these fast electrons are suddenly decelerated (or stopped) inside the target, most of their kinetic energy converts to heat, and a fraction converts into electromagnetic radiation in the X-ray region — this is the continuous (Bremsstrahlung) X-ray spectrum; sharp characteristic lines are produced separately from inner-shell atomic transitions. The fundamental mechan …

Characteristic X-ray photons get their sharply-defined energy from an electron falling between two specific inner-shell energy levels of a target atom — an atomic transition — not from the bombarding electron's own kinetic energy.

In a Coolidge tube, electrons are accelerated through a high potential difference and made to strike a metal target. Two distinct kinds of X-ray spectra result: a continuous spectrum (bremsstrahlung), produced when the striking electrons are decelerated by the target's nuclear/atomic electric fields and radiate away part or all of their kinetic energy as a photon of any energy up to their full kinetic energy; and a superimposed line/characteristic spectrum, consisting of sharp peaks at specific wavelengths that depend only on the target element, not on the accelerating voltage (above a threshold).

The characteristic lines arise through a two-step process: first, an incident high-energy electron collides with and ejects a tightly bound inner-shell electron (say, from the K-shell) of a target atom, creating a vacancy there; second, an electron from a higher, less tightly bound shell (L, M, ...) 'falls' down to fill this vacancy. Because the electron drops between two well-defined, quantized atomic energy levels $E_i$ and $E_f$ specific to that element, the photon emitted in this transition has a precisely fixed energy $h\nu = E_i - E_f$ — exactly analogous to how optical spectral lines arise from outer-shell electron transitions in atoms, except here the transitions involve the much more tightly bound inner shells, which is why the emitted photons are in the X-ray (rather than visible) part of the spectrum.

…