X-ray production and spectrum

Medical Physics | AQA A-Level Physics

X-rays are produced in a rotating anode X-ray tubeA vacuum tube in which electrons are accelerated from a heated cathode to a metal target (anode) at high voltage (up to 200 kV). When the electrons decelerate in the target, they produce X-rays. The anode rotates at about 3000 rpm to spread the heat.. Electrons are released from a heated cathode by thermionic emission and accelerated across a high potential difference (up to 200 kV) towards a tungsten target.
When the high-speed electrons decelerate in the target, they produce X-rays by two mechanisms:
- BremsstrahlungBraking radiation. X-rays produced when electrons are decelerated by the electric field of target nuclei. Produces a continuous spectrum of energies up to a maximum determined by the accelerating voltage. (braking radiation): electrons are decelerated by the electric field of target nuclei. This produces a continuous spectrum of X-ray energies.
- Characteristic radiationX-rays produced when an incoming electron knocks out an inner-shell electron from a target atom. When a higher-shell electron drops down to fill the vacancy, a photon of specific energy is emitted. This produces discrete spikes on the X-ray spectrum.: an incoming electron ejects an inner-shell electron from a target atom. When a higher-energy electron fills the vacancy, an X-ray photon of a specific energy is emitted. This gives discrete spikes superimposed on the continuous spectrum.
The maximum X-ray energy equals the kinetic energy of the electrons: $E_{\max} = eV$, and the minimum wavelength is $\lambda_{\min} = \frac{hc}{eV}$.
Only about 1% of the electron kinetic energy is converted to X-rays. The rest becomes heat, which is why the anode is rotated at about 3000 rpm to spread the thermal load.
The tube is enclosed in a vacuum (so electrons are not stopped by air) and shielded with lead (to prevent stray radiation).