(b) production of X-ray photons from an X-ray tube

The idea

In an X-ray tube, electrons are thermionically emitted from a heated cathode and accelerated across a high potential difference towards a rotating tungsten anode in an evacuated envelope. Two distinct processes generate X-ray photons. When an accelerated electron interacts with a target nucleus, it decelerates and emits bremsstrahlung (braking) radiation; the photon energy depends on how much kinetic energy is lost, producing a continuous spectrum with a sharp minimum wavelength set by the tube voltage. When an electron ejects an inner-shell electron from a tungsten atom, an outer-shell electron drops down and emits a characteristic photon whose energy equals the shell energy difference. Increasing tube current raises the number of photons; increasing tube voltage raises photon energy and shifts the minimum wavelength.

What earns the marks

• Electrons are emitted from a heated cathode by thermionic emission and accelerated by a high potential difference towards the anode.
• Bremsstrahlung (braking) radiation arises when an accelerated electron is decelerated by interaction with the target nucleus, producing a continuous range of photon energies.
• The minimum wavelength of the continuous spectrum corresponds to an electron converting all its kinetic energy into one photon, so λ_min depends on the tube voltage.
• Characteristic X-ray photons arise from electron transitions between inner atomic shells, giving discrete photon energies equal to shell energy differences.
• Increasing tube current increases the number of electrons and hence the intensity of X-rays; increasing tube voltage increases photon energy and reduces λ_min.
• The tube is evacuated so electrons travel to the anode without colliding with gas molecules, and the anode is often rotated to dissipate heat.

Common mistakes and how to put them right

• Thinking X-rays are produced by the nucleus decaying.

  Put it right: They arise from electron deceleration (bremsstrahlung) or electron shell transitions (characteristic) at the anode.

• Believing the continuous spectrum has a maximum wavelength limit.

  Put it right: It has a minimum wavelength λ_min set by the tube voltage, with no upper limit.

• Confusing the effect of tube current and tube voltage.

  Put it right: Current changes the number of photons (intensity), voltage changes photon energy and λ_min.

• Assuming characteristic X-rays form a continuous spectrum.

  Put it right: They appear as discrete peaks at energies fixed by the target element's shell structure.

Exam tips

• Sketch both the continuous bremsstrahlung curve and the characteristic peaks on the same axes, labelling λ_min and the discrete lines.
• State clearly that λ_min occurs when one electron converts all its kinetic energy into a single photon, and link it to the accelerating voltage.
• Use the correct terms: thermionic emission, bremsstrahlung (braking radiation), characteristic radiation, evacuated tube, rotating anode.
• When comparing spectra, refer to intensity (number of photons) versus photon energy, not just 'more X-rays'.