bremsstrahlung
/ BREMS-shtrah-loong /
Fire a fast electron at a metal target and, as it is yanked sideways and slowed by the electric fields of the atomic nuclei, it sheds energy as a spray of X-rays. Bremsstrahlung — German for 'braking radiation' — is the electromagnetic radiation emitted when a charged particle is decelerated or deflected by another charge.
The mechanism is Larmor radiation applied to the acceleration a charge suffers in the Coulomb field of a nucleus: the closer the pass and the heavier the target nucleus (the deflecting force scales with the nuclear charge Z), the harder the swerve and the more radiation. Because each electron loses a different, random fraction of its energy in a single encounter, the emitted photons span a CONTINUOUS spectrum, unlike the sharp lines of atomic transitions. The spectrum has a sharp high-frequency cutoff: no photon can carry more than the electron's entire kinetic energy, giving the Duane-Hunt limit, a shortest wavelength lambda_min = hc / (eV) for electrons accelerated through voltage V.
Bremsstrahlung is the continuous background in every X-ray tube, on top of which the target's characteristic line spectrum sits. It dominates the energy loss of high-energy electrons in matter (defining the radiation length of a material) and produces the gamma-ray glow of astrophysical plasmas and even lightning. Because the radiated power for a given force scales as 1/m^2, bremsstrahlung is far stronger for light electrons than for heavy protons.
In an X-ray tube run at 50 kV, no bremsstrahlung photon can exceed 50 keV of energy, so the continuous spectrum stops abruptly at a shortest wavelength lambda_min = hc/(eV) = 1240 eV·nm / 50000 eV = 0.0248 nm. Raise the tube voltage and this cutoff slides to shorter wavelengths.
Braking in a nucleus's field gives a continuous X-ray spectrum with a sharp short-wavelength (Duane-Hunt) cutoff.
Bremsstrahlung is continuous, unlike the discrete characteristic lines of atomic transitions; the two appear together in an X-ray tube spectrum. Because radiated power ~ 1/m^2, it is enormous for electrons and negligible for protons under the same force.