X-rays are high-energy electromagnetic waves produced when fast electrons suddenly lose energy inside a metal target. In an X-ray tube, a heated cathode releases electrons, a high voltage accelerates them, and a tungsten anode stops them. The lost electron energy becomes mostly heat, but a small fraction becomes X-ray photons.
This process matters because it makes medical imaging, security scanning, and materials testing possible.
Two main kinds of X-rays are produced in the target: bremsstrahlung and characteristic X-rays. Bremsstrahlung radiation is made when electrons slow down or change direction near atomic nuclei, producing a continuous range of photon energies. Characteristic X-rays are made when an incoming electron knocks out an inner-shell electron and another electron falls into the empty lower-energy state.
The maximum X-ray photon energy is controlled by the tube voltage, while the intensity depends strongly on the tube current and exposure time.
Key Facts
- Electron kinetic energy from the tube voltage is K = eV.
- Maximum X-ray photon energy is Emax = eV.
- Minimum X-ray wavelength is λmin = hc / eV.
- Photon energy and frequency are related by E = hf.
- Most electron energy in the anode becomes heat, and only about 1 percent becomes X-rays in typical medical tubes.
- Bremsstrahlung gives a continuous spectrum, while characteristic X-rays give sharp energy peaks set by the target element.
Vocabulary
- Cathode
- The negative electrode in an X-ray tube that heats a filament to release electrons by thermionic emission.
- Anode
- The positive electrode and metal target that attracts electrons and produces X-rays when they strike it.
- Bremsstrahlung
- X-ray radiation produced when fast electrons are decelerated or deflected by the electric fields of atomic nuclei.
- Characteristic X-ray
- An X-ray photon with a specific energy produced when an electron falls into an inner-shell vacancy in an atom.
- Tube voltage
- The potential difference across the X-ray tube that determines the maximum kinetic energy of the accelerated electrons.
Common Mistakes to Avoid
- Thinking all electron energy becomes X-rays is wrong because most of the energy is converted into heat in the metal target.
- Confusing tube voltage with tube current is wrong because voltage controls maximum photon energy, while current mainly controls the number of photons produced.
- Assuming bremsstrahlung photons all have the same energy is wrong because electron slowing can produce a continuous range of photon energies up to a maximum.
- Forgetting that characteristic X-rays depend on the target material is wrong because their energies are set by the electron energy levels of atoms such as tungsten.
Practice Questions
- 1 An X-ray tube operates at 80 kV. What is the maximum energy of an X-ray photon in keV?
- 2 Find the minimum wavelength of X-rays from a 100 kV tube using hc = 1240 eV·nm.
- 3 A radiographer increases the tube current but keeps the tube voltage the same. Explain how the X-ray beam changes and why the maximum photon energy does not change.