X-rays are a form of electromagnetic radiation that are produced in X-ray tubes. These tubes typically use a tungsten target because tungsten has a high atomic number (Z=74) and a high melting point 3,422°C (or 6,192°F), which makes it efficient at producing X-rays when bombarded by high-energy electrons. Below, we will explain the process of X-ray creation in a typical X-ray tube, along with the relevant equations and reactions that occur.
The basic components of an X-ray tube include:
In an X-ray tube, electrons are accelerated from the cathode and strike the tungsten anode target. The impact of high-energy electrons with tungsten atoms results in the emission of X-rays.
Characteristic X-ray radiation occurs when a high-energy electron from the cathode collides with an inner electron of a tungsten atom, knocking the inner electron out of its orbit. This creates a vacancy in the electron shell. Electrons from higher energy levels then drop down to fill this vacancy, and in the process, they release energy in the form of X-rays.
K- + W → K+ + W+ + X-ray
Where: - K- represents an incoming electron. - W is the tungsten atom. - K+ represents the outgoing electron after being knocked out. - The X-ray produced corresponds to the energy difference between the shells.
When an electron from the cathode collides with a tungsten atom and ejects an electron from the K-shell (the innermost electron shell), the atom becomes unstable. The vacancy is filled by an electron from the L-shell, and the energy difference between the K and L shells is emitted as an X-ray photon. The energy of the photon corresponds to the difference in binding energies between these two shells.
Ephoton = Ebinding(K) - Ebinding(L)
The energy of the X-ray is characteristic of the element (tungsten in this case), and it depends on the difference in binding energy between the two electron shells involved.
Bremsstrahlung (meaning "braking radiation") occurs when the high-energy electron from the cathode is slowed down or deflected by the positively charged nucleus of the tungsten atom. As the electron decelerates, it loses energy, which is emitted as X-ray radiation.
e- + W → e- + W + X-ray
Where: - The electron (e-) is deflected by the nucleus of the tungsten atom (W). - The energy lost by the electron is emitted as a Bremsstrahlung X-ray.
When a high-energy electron passes close to the tungsten nucleus, it is deflected by the Coulomb force. This deflection results in a loss of kinetic energy, which is emitted in the form of X-rays. The amount of energy emitted depends on the degree of deflection and can result in a spectrum of X-ray energies.
The X-rays produced in a tungsten X-ray tube form a spectrum that includes both characteristic and Bremsstrahlung radiation. The characteristic radiation produces discrete X-ray lines at specific energies corresponding to electron transitions between the tungsten atom's electron shells. The Bremsstrahlung radiation produces a continuous spectrum of X-ray energies.
The energy of the characteristic X-rays is specific to the material (tungsten in this case), while the energy of the Bremsstrahlung X-rays depends on the energy of the incident electrons and the degree of deflection near the nucleus.
The X-ray spectrum from a tungsten target typically includes two main peaks: the Kα line and the Kβ line, which are characteristic of tungsten. These peaks represent the energy differences between the K and L shells, and the K and M shells, respectively.
The total energy (E) of the X-ray produced can be expressed as the sum of the energy lost during electron deceleration (Bremsstrahlung) and the energy released during electron transitions (Characteristic radiation):
EX-ray = EBrem + Echar
Where: - EBrem is the energy of the Bremsstrahlung radiation. - Echar is the energy of the characteristic X-ray radiation.
X-rays are generated in X-ray tubes when high-energy electrons collide with a tungsten target. These collisions produce two main types of X-ray radiation: characteristic and Bremsstrahlung. Characteristic radiation occurs when electrons from the cathode knock inner electrons from the tungsten atom's electron shells, leading to the emission of X-ray photons as other electrons fill the vacancies. Bremsstrahlung radiation, on the other hand, occurs when electrons are deflected by the nucleus of tungsten atoms, emitting X-rays as a result of their deceleration. These processes are responsible for the X-ray spectrum observed in X-ray tubes, which is used for a wide range of applications, from medical imaging to material analysis.