X-rays are high-energy electromagnetic waves produced when fast electrons suddenly lose energy inside a metal target. In an X-ray tube, a heated cathode releases electrons, a high voltage accelerates them, and a tungsten anode stops them. The lost electron energy becomes mostly heat, but a small fraction becomes X-ray photons.
This process matters because it makes medical imaging, security scanning, and materials testing possible.
Two main kinds of X-rays are produced in the target: bremsstrahlung and characteristic X-rays. Bremsstrahlung radiation is made when electrons slow down or change direction near atomic nuclei, producing a continuous range of photon energies. Characteristic X-rays are made when an incoming electron knocks out an inner-shell electron and another electron falls into the empty lower-energy state.
The maximum X-ray photon energy is controlled by the tube voltage, while the intensity depends strongly on the tube current and exposure time.
Understanding Physics: X-Ray Production
Inside the tube, the cathode has a small filament that works much like the wire in an old light bulb. Heating it gives some electrons enough energy to escape the metal surface. This is called thermionic emission.
A focusing cup helps direct the electrons into a narrow beam, so they strike a small region of the target. Tungsten is useful because it has a very high melting point and its atoms have many protons. More protons create a stronger electric field near the nucleus, which makes the slowing process more effective.
Even so, the target becomes extremely hot. Many medical tubes use a rotating anode. Spreading impacts around a circular track prevents one spot from overheating too quickly.
The X-ray output is not a single, perfectly controlled beam. It contains photons with a range of energies. Low-energy photons are usually not useful for making an image because they are absorbed near the skin and add dose without reaching the detector.
For this reason, the beam passes through filters, often made from aluminium. Filtration removes many of these weaker photons. Changing the tube voltage changes the beam quality.
A higher voltage produces photons that can pass through thicker or denser material more easily. Changing the tube current mainly changes how many electrons travel across the tube each second. A longer exposure similarly increases the total number of photons produced.
An image forms because different materials weaken X-rays by different amounts. Dense materials, such as bone, contain many atoms in a small volume and absorb or scatter more photons than soft tissue. Air absorbs very little.
The detector records the number of photons that arrive at each position. Areas receiving fewer photons are displayed differently from areas receiving more. The result is a shadow image, not a direct picture of the body.
Overlapping structures can hide each other, which is one reason doctors may take images from more than one angle. In computed tomography, many views around the body are processed to build cross-sectional images.
Students should separate photon number from photon energy. These are related to different tube settings and have different effects on an image. Higher-energy photons penetrate better, but can reduce contrast between tissues.
More photons can make an image less grainy, but increase radiation exposure. Scatter is another important idea. A photon can change direction after interacting with matter and still reach the detector, where it can blur the image.
Collimators restrict the beam to the needed area, while grids can reduce scattered photons before they reach the detector. Radiation workers use shielding, distance, and short exposure times because X-rays can ionise atoms in living cells. The aim is always to obtain enough information while keeping dose as low as reasonably achievable.
Key Facts
- Electron kinetic energy from the tube voltage is K = eV.
- Maximum X-ray photon energy is Emax = eV.
- Minimum X-ray wavelength is λmin = hc / eV.
- Photon energy and frequency are related by E = hf.
- Most electron energy in the anode becomes heat, and only about 1 percent becomes X-rays in typical medical tubes.
- Bremsstrahlung gives a continuous spectrum, while characteristic X-rays give sharp energy peaks set by the target element.
Vocabulary
- Cathode
- The negative electrode in an X-ray tube that heats a filament to release electrons by thermionic emission.
- Anode
- The positive electrode and metal target that attracts electrons and produces X-rays when they strike it.
- Bremsstrahlung
- X-ray radiation produced when fast electrons are decelerated or deflected by the electric fields of atomic nuclei.
- Characteristic X-ray
- An X-ray photon with a specific energy produced when an electron falls into an inner-shell vacancy in an atom.
- Tube voltage
- The potential difference across the X-ray tube that determines the maximum kinetic energy of the accelerated electrons.
Common Mistakes to Avoid
- Thinking all electron energy becomes X-rays is wrong because most of the energy is converted into heat in the metal target.
- Confusing tube voltage with tube current is wrong because voltage controls maximum photon energy, while current mainly controls the number of photons produced.
- Assuming bremsstrahlung photons all have the same energy is wrong because electron slowing can produce a continuous range of photon energies up to a maximum.
- Forgetting that characteristic X-rays depend on the target material is wrong because their energies are set by the electron energy levels of atoms such as tungsten.
Practice Questions
- 1 An X-ray tube operates at 80 kV. What is the maximum energy of an X-ray photon in keV?
- 2 Find the minimum wavelength of X-rays from a 100 kV tube using hc = 1240 eV·nm.
- 3 A radiographer increases the tube current but keeps the tube voltage the same. Explain how the X-ray beam changes and why the maximum photon energy does not change.