X-rays are high-energy electromagnetic waves with wavelengths typically in the range of 0.01 nm to 10 nm (frequencies ~1017 to 1019 Hz). They were discovered by Wilhelm Röntgen in 1895 and are therefore also called Röntgen rays.
X-rays are produced in a Coolidge tube (hot-cathode X-ray tube):
- A tungsten filament (cathode) is heated by a low-voltage current, releasing electrons by thermionic emission.
- A high potential difference V (typically 10 kV – 150 kV) accelerates the electrons toward a heavy metal anode (target), usually tungsten or molybdenum.
- When the high-speed electrons strike the target, most of their kinetic energy is converted to heat, but a small fraction (~1%) is emitted as X-ray photons.
| Parameter | Controlled by | Effect |
|---|
| Intensity (number of X-rays) | Filament current | More current → more electrons → more X-rays |
| Energy / Hardness (penetrating power) | Accelerating voltage V | Higher V → higher energy → shorter wavelength |
- Produced when electrons are decelerated by the electric field of the target nucleus.
- The electron loses kinetic energy, which is emitted as a photon: Ephoton=ΔKE.
- Because electrons lose varying amounts of energy, a continuous spectrum of wavelengths is produced.
- There is a minimum wavelength (cutoff) corresponding to an electron losing all its kinetic energy in a single collision.
- Produced when an incoming electron knocks out an inner-shell electron (e.g., from the K-shell) of a target atom.
- An outer-shell electron drops down to fill the vacancy, emitting a photon of a specific energy (characteristic of the target element).
- Transitions are labelled:
- Kα: L-shell → K-shell transition
- Kβ: M-shell → K-shell transition
- Lα: M-shell → L-shell transition
When an electron accelerated through voltage V gives up all its kinetic energy as a single photon:
eV=hfmax=λminhc
λmin=eVhc
where:
- e=1.6×10−19 C (electron charge)
- h=6.63×10−34 J s (Planck's constant)
- c=3×108 m s−1 (speed of light)
- V = accelerating voltage (V)
Key relationship: λmin∝V1 — doubling the voltage halves the minimum wavelength.
An X-ray tube operates at 50 kV. Calculate the minimum wavelength of X-rays produced.
λmin=eVhc=(1.6×10−19)(50×103)(6.63×10−34)(3×108)
λmin=8×10−151.989×10−25=2.49×10−11 m≈0.025 nm
Different tissues absorb X-rays to different degrees depending on their density and atomic number:
| Tissue | Absorption | Appearance on film |
|---|
| Bone (calcium-rich) | High | White (radiopaque) |
| Muscle / soft tissue | Moderate | Grey |
| Air / lungs | Very low | Black (radiolucent) |
This differential absorption creates contrast on the X-ray image (radiograph).
- X-rays pass through the patient and strike a photographic film or digital detector.
- Dense structures (bones) block more X-rays → less exposure → appear white.
- Used to detect: fractures, lung conditions (pneumonia, tuberculosis), dental problems.
- For soft tissues with similar densities (e.g., digestive tract), a contrast agent (e.g., barium sulfate for GI tract, iodine compounds for blood vessels) is introduced.
- These agents absorb X-rays strongly, making the target organ visible.
- X-rays are ionising radiation — excessive exposure can damage DNA.
- Poor contrast between soft tissues of similar density (CT scanning addresses this).
- Produces a 2D projection image (overlapping structures).
| Feature | Detail |
|---|
| Nature of X-rays | Electromagnetic radiation, λ≈0.01–10 nm |
| Production | High-speed electrons striking heavy metal target |
| Minimum wavelength | λmin=hc/eV |
| Continuous spectrum | Bremsstrahlung (electron deceleration) |
| Characteristic spectrum | Inner-shell electron transitions |
| Medical use | Differential absorption creates contrast images |