X-Ray Production in Radiotherapy: Bremsstrahlung and Characteristic Radiation Explained

X-rays are a form of electromagnetic radiation produced when high-energy electrons interact with matter. In diagnostic and therapeutic settings, X-rays are generated within an X-ray tube when electrons accelerated by a high voltage strike a target material (usually tungsten). Understanding this process is essential for FRCR Part 1 Physics.

Two distinct mechanisms produce X-rays: bremsstrahlung radiation and characteristic radiation. Together, they create the X-ray spectrum used in medical imaging and radiotherapy.

The X-Ray Tube: Basic Components

An X-ray tube consists of:

• Cathode: A heated tungsten filament that releases electrons through thermionic emission
• Anode: A tungsten target that electrons strike to produce X-rays (rotating in diagnostic tubes to dissipate heat)
• Vacuum: The tube is evacuated so electrons don't collide with air molecules
• High voltage supply: Creates the electric field that accelerates electrons from cathode to anode (measured in kVp)

Why tungsten? Tungsten is used because of its high atomic number (Z=74), which increases X-ray production efficiency, and its high melting point (3,422°C), allowing it to withstand the heat generated. Only about 1% of electron energy is converted to X-rays—the remaining 99% becomes heat.

Bremsstrahlung Radiation

Bremsstrahlung (German for "braking radiation") accounts for approximately 80% of X-rays produced in a diagnostic tube. It occurs through the following mechanism:

1. A high-energy electron from the cathode passes close to a tungsten nucleus in the anode
2. The electron is deflected and decelerated by the attractive force of the positively charged nucleus
3. The kinetic energy lost during this deceleration is emitted as an X-ray photon
4. The electron continues on a deflected path with reduced energy

Key Properties of Bremsstrahlung

• Continuous spectrum: X-ray photons are produced with a range of energies from zero up to the maximum energy of the incident electrons
• Maximum photon energy: Equals the peak tube voltage (kVp). For example, a 100 kVp tube produces X-rays with maximum energy of 100 keV
• Energy distribution: Low-energy photons are more numerous; the number of photons decreases with increasing energy
• Dependence on Z: The probability of bremsstrahlung production varies with Z² of the target material

Clinical note: At low electron energies (diagnostic radiology), bremsstrahlung X-rays are emitted in all directions. At high electron energies (linear accelerators in radiotherapy), X-rays are emitted preferentially in the forward direction.

Characteristic Radiation

Characteristic radiation accounts for approximately 20% of useful X-rays in a diagnostic tube (at typical kVp settings). The mechanism is:

1. An incident electron knocks an inner-shell electron (usually K-shell or L-shell) out of a tungsten atom
2. This creates a vacancy in the inner shell
3. An electron from a higher energy shell drops down to fill the vacancy
4. The energy difference between shells is released as an X-ray photon

Key Properties of Characteristic Radiation

• Discrete energies: X-ray photons are produced at specific energies corresponding to the binding energy differences between electron shells
• Material dependent: The characteristic energies depend on the atomic structure of the target material, not the tube voltage (hence "characteristic")
• Threshold energy: Incident electrons must have at least the binding energy of the inner shell to eject an electron

Tungsten Characteristic X-Ray Energies

For tungsten (K-shell binding energy ≈ 69.5 keV):

• Kα radiation: ~59.3 keV (L→K transition)
• Kβ radiation: ~67.6 keV (M→K transition)

Characteristic X-rays from tungsten only appear in the spectrum when the tube voltage exceeds ~70 kVp (above the K-shell binding energy).

The X-Ray Spectrum

The complete X-ray spectrum is a combination of:

• The continuous bremsstrahlung spectrum (shaped like a ramp, decreasing from low to high energies)
• Characteristic peaks superimposed at specific energies

Factors Affecting the X-Ray Spectrum

Tube Voltage (kVp)

• Increasing kVp increases the maximum photon energy
• Increases the quantity of X-rays produced
• Increases the average energy (beam quality)
• X-ray quantity is proportional to kVp²

Tube Current (mA)

• Increasing mA increases the number of electrons hitting the target
• Increases X-ray quantity proportionally
• Does not change the spectrum shape or maximum energy

Target Material (Atomic Number)

• Higher Z materials produce more X-rays (efficiency ∝ Z)
• Changes the characteristic peak positions

Filtration

• Filters (usually aluminium in diagnostic imaging, higher Z materials in therapy) preferentially absorb low-energy X-rays
• This "beam hardening" increases the average beam energy
• Reduces patient skin dose by removing photons that wouldn't penetrate to the detector
• Minimum total filtration of 2.5 mm aluminium equivalent is required for diagnostic tubes >110 kVp

Comparison: Diagnostic vs. Therapeutic X-Ray Production

Feature	Diagnostic	Therapeutic (Linac)
Electron energy	25-150 keV	4-25 MeV
Target type	Thick target at 90°	Thin transmission target
X-ray direction	All directions	Forward directed
Characteristic radiation	Significant contribution	Negligible (high energies)

Key Points for FRCR Part 1

1. Bremsstrahlung produces a continuous spectrum; characteristic radiation produces discrete peaks
2. Only ~1% of electron energy is converted to X-rays; 99% becomes heat
3. Maximum photon energy equals the tube voltage (kVp)
4. X-ray quantity ∝ kVp² × mA × Z
5. Filtration removes low-energy photons, increasing average beam energy
6. Characteristic radiation from tungsten only appears above ~70 kVp

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