bremsstrahlung
/ BREM-stra-loong /
Bremsstrahlung is German for 'braking radiation,' and that name says it all: when a fast charged particle is suddenly slowed or deflected, it gives off light. Slam your brakes in a car and the kinetic energy turns into heat in the brake pads; do the analogous thing to a speeding electron — bend its path sharply near an atomic nucleus — and a chunk of its energy is shed as a photon. The deflection is the braking, and the emitted photon is the radiation.
Physically, any accelerating electric charge radiates electromagnetic waves, and in the quantum picture that radiation is a stream of photons. When an electron whizzes past the strong electric field of a heavy nucleus, the nucleus's pull yanks it off course, accelerating it sideways, and the electron emits a bremsstrahlung photon to carry away the lost energy and momentum. Unlike Compton scattering or pair production, the emitted photon can come out with almost any energy up to the electron's entire kinetic energy, producing a smooth, continuous spread of photon energies rather than sharp lines.
Bremsstrahlung is everywhere in practice. It produces the broad continuous background in every medical and dental X-ray tube, where electrons are fired into a metal target and braked hard. It is the dominant way high-energy electrons lose energy as they plow through matter, which shapes how particle detectors and calorimeters work. And it limits electron accelerators and helps explain X-ray and gamma-ray emission from hot, energetic regions of the cosmos, from the Sun's corona to clusters of galaxies.
The X-ray machine at the dentist works by firing electrons into a tungsten target. As the electrons brake hard in the metal's electric fields, they emit bremsstrahlung — a broad spread of X-ray photons that pass through your jaw to make the image.
An X-ray tube makes its rays by braking fast electrons in a metal target.
Bremsstrahlung produces a continuous spectrum of photon energies, unlike the sharp lines from atomic transitions; an X-ray tube's spectrum is actually the smooth bremsstrahlung background with a few sharp characteristic lines on top.