X-ray Production and Tube Components

X-rays are produced when high-speed electrons interact with a target material, usually tungsten. Electrons are released from the cathode, accelerated by a high potential difference and strike the anode. Their kinetic energy is converted mainly into heat, with a small proportion becoming X-ray photons.

An X-ray tube contains a cathode, an anode and an evacuated enclosure. The cathode supplies electrons, the anode acts as the target, and the vacuum allows electrons to travel without frequent collisions with gas molecules. A high voltage accelerates electrons between the electrodes.

The cathode is the negative electrode of the X-ray tube. It contains a filament that is heated to release electrons through thermionic emission. In many conventional tubes, a focusing cup helps direct electrons towards a small region of the anode.

The anode is the positive electrode and contains the target struck by accelerated electrons. When electrons interact with target atoms, they produce Bremsstrahlung and characteristic X-rays. Most of the incoming electron energy becomes heat, so the anode must tolerate high temperatures.

Tungsten is commonly used as an X-ray target because it has a high atomic number (Z = 74), which favours X-ray production, and a high melting point, which helps it withstand intense heating. Its electron-shell binding energies also allow characteristic X-ray emission.

The X-ray tube is evacuated so electrons can travel from the cathode to the anode with minimal collisions with gas molecules. This helps electrons retain their kinetic energy before reaching the target and allows the accelerating voltage to operate effectively.

Thermionic emission is the release of electrons from a heated metal filament. Heating supplies enough energy for some electrons to overcome the attraction holding them in the metal. These released electrons form a cloud near the cathode and can be accelerated towards the anode.

A high potential difference between the cathode and anode accelerates negatively charged electrons towards the positive anode. The electrical energy gained becomes electron kinetic energy. Increasing the accelerating voltage increases the maximum energy available for X-ray photon production.

Kilovoltage describes the potential difference used to accelerate electrons in a conventional X-ray tube. An electron accelerated through 100 kV can gain up to 100 keV of kinetic energy. Therefore, the maximum possible Bremsstrahlung photon energy is 100 keV, assuming negligible initial electron energy.

Tube current measures the rate of electron charge flowing from the cathode to the anode. Increasing mA increases the number of electrons striking the target each second and therefore increases X-ray photon output. At a fixed kV, it does not change the maximum photon energy.

Milliampere-seconds (mAs) equals tube current in mA multiplied by exposure time in seconds. It describes the total electron charge delivered during an exposure. At constant kV, increasing mAs increases the total number of X-ray photons produced.

When electrons strike the target, their energy is transferred through interactions with target atoms. Most energy becomes heat through collisions and excitation. A much smaller fraction becomes X-ray radiation through Bremsstrahlung and characteristic processes.

Bremsstrahlung means braking radiation. A fast electron passing near a positively charged target nucleus is deflected and decelerated by the electric field. The electron loses kinetic energy, which is emitted as an X-ray photon. Different amounts of energy loss produce different photon energies.

Bremsstrahlung produces a continuous range of photon energies because electrons can lose different amounts of kinetic energy during interactions near nuclei. A photon may receive a small fraction or almost all of the electron's energy. The maximum photon energy is limited by the incident electron energy.

Bremsstrahlung production depends on incident electron energy and target atomic number. Higher electron energies increase the maximum possible photon energy and generally increase output. Higher-Z targets increase the probability of Bremsstrahlung interactions.

Characteristic X-rays are produced when an incoming electron ejects a tightly bound inner-shell electron from a target atom. An electron from a higher shell fills the vacancy, releasing the difference in binding energies as an X-ray photon. The photon energy is characteristic of the target element.

Inner-shell ionisation occurs when an incoming electron transfers enough energy to eject an electron from an inner atomic shell, such as the K shell. The atom is left with a vacancy and becomes unstable. An outer-shell electron may then move down to fill the vacancy.

An electron moving from a higher energy shell to a lower shell releases energy equal to the difference between the two binding energies. This energy may be emitted as a characteristic X-ray photon. For example, an L-to-K transition can produce K-series characteristic radiation.

The cascade effect occurs when an electron fills an inner-shell vacancy and leaves a vacancy in its original shell. Another electron may then move down to fill that vacancy, causing successive transitions. The released energy may appear as characteristic photons or be transferred to Auger electrons.

Characteristic X-ray energies are discrete because electrons occupy specific atomic energy levels. Photon energy equals the difference between the binding energies of the two shells involved. The pattern of characteristic energies depends on the target material.

Bremsstrahlung results from the deceleration or deflection of electrons near target nuclei and produces a continuous spectrum. Characteristic radiation results from electron-shell transitions following inner-shell ionisation and produces photons at specific energies. Both mechanisms contribute to X-ray output.

An X-ray emission spectrum plots photon output against photon energy. The broad continuous distribution represents Bremsstrahlung, while narrow peaks represent characteristic X-rays. The high-energy endpoint is determined by the maximum kinetic energy of the electrons striking the target.

Increasing kV accelerates electrons to higher energies. This raises the maximum Bremsstrahlung photon energy and generally increases total X-ray output. It also changes the energy distribution, making the beam more penetrating. Characteristic peaks appear only when electrons have enough energy to create the required shell vacancies.

Increasing mA increases the number of electrons striking the anode per second. More electron interactions produce more X-ray photons, increasing beam quantity. If kV and other conditions remain unchanged, the energy distribution and maximum photon energy remain approximately the same.

Radiotherapy equipment uses accelerated electrons to produce therapeutic X-ray beams. In a medical linear accelerator, electrons are accelerated to high energies and directed onto a high-Z target to produce megavoltage Bremsstrahlung photons. This differs from a conventional heated-filament diagnostic X-ray tube, although the basic Bremsstrahlung mechanism is shared.