1. What are X-rays?
X-rays are high-energy electromagnetic waves with wavelengths shorter than ultraviolet rays but longer than gamma rays. Their high frequency gives them the ability to penetrate many materials, including human tissue, which makes them extremely useful in medical and security applications.
X-rays are invisible to the human eye and can pass through soft materials while being absorbed by denser ones like bones and metals.
1.1. Position in the electromagnetic spectrum
X-rays lie between ultraviolet rays and gamma rays. Their wavelengths range approximately from 10 nanometers down to 0.01 nanometers.
1.2. General characteristics
- Wavelength: ~10 nm to 0.01 nm
- Frequency: \(3 \times 10^{16}\) to \(3 \times 10^{19}\,\text{Hz}\)
- Very high energy and penetrating power
- Invisible to the eye
2. How X-rays are produced
X-rays are created when high-energy electrons strike a metal target or when certain atomic transitions occur inside atoms. These processes release photons with extremely high energy.
2.1. X-ray tubes (Bremsstrahlung radiation)
In most medical and industrial machines, X-rays are produced by rapidly decelerating high-speed electrons. When these electrons hit a metal target, their sudden slowdown produces a broad spectrum of X-ray energies. This is known as Bremsstrahlung radiation.
2.2. Characteristic X-ray emission
If the incoming electron knocks out an inner electron from the metal atom, an outer electron falls into its place. This transition releases X-rays with specific energies called characteristic X-rays.
2.3. Natural sources
- Radioactive materials
- Astronomical sources like neutron stars and black holes
- Cosmic rays interacting with the atmosphere
3. Interaction of X-rays with matter
X-rays interact with different materials differently depending on their density and atomic composition. This is why X-rays can create contrast images.
3.1. Penetration ability
X-rays penetrate soft tissues like skin and muscles easily, but are strongly absorbed by dense materials like bone and metal.
3.2. Absorption and scattering
As X-rays pass through matter, some are absorbed and some are scattered. The amount of absorption depends on thickness and density. Bones absorb more X-rays, producing a clear contrast on X-ray films or detectors.
3.3. Ionisation effects
Because of their high energy, X-rays can ionise atoms, knocking electrons out of them. This makes excessive exposure harmful to living tissue.
4. Uses of X-rays
X-rays have many practical uses because of their ability to penetrate material and reveal internal structures.
4.1. Medical imaging
- Detecting bone fractures
- Dental imaging
- Chest scans to identify infections or lung conditions
- Mammography
X-rays reveal internal structures by capturing differences in absorption between soft tissue and bone.
4.2. Security scanning
X-ray scanners are used in airports, railway stations, and other security checkpoints. They detect concealed metal objects, weapons, and items inside bags by showing different absorption patterns.
4.3. Industrial applications
- Checking cracks in metal structures
- Inspecting weld joints
- Non-destructive testing of manufactured parts
4.4. Scientific research
- X-ray diffraction for studying crystal structures
- X-ray spectroscopy for analysing chemical composition
- Astronomy uses X-ray telescopes to study high-energy cosmic phenomena
5. Safety and hazards of X-rays
X-rays can damage living tissues because they are ionising radiation. Long or repeated exposure can cause harmful biological effects. Controlled and minimal exposure is essential in medical use.
5.1. Harmful effects
- Cell damage and mutation
- Risk of cancer with repeated exposure
- Skin burns at very high doses
5.2. Safety measures
- Lead aprons and shields
- Minimal exposure time
- Standing behind protective barriers
- Regular calibration of X-ray machines
6. Example: X-ray imaging of a broken bone
When an X-ray beam passes through the body, bones absorb much more of the radiation than muscles or skin. The X-rays that pass through reach a detector or film, creating a contrasting image. Dense regions (like bone) appear white, while soft tissues appear darker. This makes it easy to locate fractures or foreign objects.