9.1 Production of X-rays

X-rays are electromagnetic waves of very short wavelength, about 10⁻¹³ m to 10⁻⁸ m, carrying high energy photons that can pass through materials opaque to light. They were discovered by Wilhelm Röntgen in 1895. X-rays are produced when fast-moving electrons are suddenly stopped by striking a metal target: most of the electrons' kinetic energy becomes heat, but a small fraction is given out as X-ray photons. The device that does this is the X-ray tube, found in hospital radiography units, airport baggage scanners and analytical instruments such as X-ray diffractometers and X-ray fluorescence spectrometers. Example: the radiology department of a sub-county hospital in Kiirua takes chest X-rays with a tube run at about 100 kV, set by the radiographer for each patient.

Parts of an X-ray tube

How X-rays are produced

1. Thermionic emission: Current through the filament heats it to a high temperature, and electrons are emitted from its surface.
2. Acceleration: The high voltage between cathode and anode accelerates the electrons across the vacuum to very high speeds.
3. Focusing: The focusing cup directs the electrons onto a small area of the target, the focal spot.
4. Impact: The electrons strike the target and are rapidly slowed and stopped by the target atoms.
5. Energy conversion: About 99 % of their kinetic energy becomes heat in the target; only about 1 % is converted into X-ray photons.
6. Emission: The X-rays leave the target in all directions; the shielding lets only the useful beam out through the window.

Energy of the electrons and X-rays

An electron accelerated through a potential difference V gains kinetic energy:

KE = eV

If all of this is converted into a single photon, the photon has the maximum energy and therefore the minimum wavelength:

eV = hc ÷ λmin, so λmin = hc ÷ (eV)

Worked example: an X-ray tube operates at 30 kV. Find the kinetic energy of the electrons arriving at the target and the shortest wavelength of the X-rays produced. (e = 1.6 × 10⁻¹⁹ C, h = 6.626 × 10⁻³⁴ J s, c = 3.0 × 10⁸ m/s)

KE = eV = 1.6 × 10⁻¹⁹ × 30,000 = 4.8 × 10⁻¹⁵ J

λmin = hc ÷ (eV)

λmin = (6.626 × 10⁻³⁴ × 3.0 × 10⁸) ÷ (4.8 × 10⁻¹⁵)

λmin = 4.14 × 10⁻¹¹ m (0.0414 nm)

Controlling the X-ray beam

• Intensity (quantity): Controlled by the filament current (the tube current, in mA). A hotter filament emits more electrons, so more X-ray photons are produced, without changing their energy.
• Penetrating power (quality): Controlled by the accelerating voltage (kV). A higher voltage gives faster electrons, higher energy photons and shorter wavelengths, which penetrate further.
• Exposure: Set by the tube current multiplied by the exposure time (mA s).
• Filtration: Thin aluminium filters absorb the low energy X-rays that would only be absorbed by the skin, reducing the patient's dose.

Heat in the X-ray tube

Because about 99 % of the electron energy becomes heat, the target gets extremely hot. A tube at 100 kV and 20 mA receives 100,000 × 0.020 = 2,000 W, of which about 1,980 W is heat. The target is therefore made of tungsten (melting point 3,422 °C), set in copper to conduct the heat away, and in powerful tubes the anode is a rotating disc so that the heat is spread over a ring rather than a single spot.

Properties of X-rays

• Travel in straight lines at the speed of light: 3.0 × 10⁸ m/s in a vacuum.
• Not deflected by electric or magnetic fields: They carry no charge.
• Penetrate matter: Pass through soft tissue, paper and wood, but are absorbed by dense, high atomic number materials such as bone and lead.
• Ionise gases: They knock electrons out of atoms, which makes them harmful to living cells.
• Affect photographic film and cause fluorescence: The basis of X-ray images.
• Are diffracted by crystals: Their wavelength is similar to the spacing of atoms, the basis of X-ray crystallography.