Electromagnetic Spectrum. X-rays and Gamma rays are part of the Electromagnetic Spectrum with higher energies (shorter wavelength) and are capable of penetrating material that visible light can not penetrate.
The Electromagnetic Spectrum spans a very wide range of wavelengths. Our human visual system is limited to seeing a very narrow band within the Electromagnetic Spectrum ranging from Violet (~380 nm) to Red (~700 nm). Frequencies slightly below 380 nm are called Infrared, and those slightly higher than 700 are called Ultraviolet. There are multiple bands within Infrared and Ultraviolet, which are outside the scope of this discussion.
Like visible light, x-rays are part of the Electromagnetic Spectrum but have significantly higher frequencies—hence higher energies. Unlike visible light, x-rays can penetrate through material due to their high energies. As one would expect, the higher the energy of the x-rays the thicker the material they can penetrate. It is this ability to penetrate through material which makes x-rays attractive and very common in medical imaging and Non-Intrusive Inspection (NII) in general.
The high-energy of x-rays makes them ionizing. That is, x-rays can interact with atoms and molecules causing electrons to be detached hence causing that atom or molecule to be ionized. Exposure to ionizing radiation can impact human health as it can cause cell mutations which in turn can cause cancer. Hence, it is generally advisable to avoid exposure to elevated levels of ionizing radiation.
Gamma Rays are generated as part of the decay of radioactive nuclei.
Gamma rays and x-rays are both parts of the Electromagnetic Spectrum and overlap in energy. That is, Gamma and x-rays may have the same energy. The differentiation between the two is often made based on the origin of the radiation: Generally speaking, Gamma rays can occur naturally while x-rays are man-made.
A Gamma ray is emitted by a radioactive nucleus following radioactive decay. The daughter nucleus that results after the decay is usually left in an excited state, which can then decay to a lower energy state by emitting a Gamma ray in a process called Gamma decay.
X-Rays are generated when accelerated electrons hit a target inside an x-ray generator.
X-rays are generated by an x-ray generator, such a tube or an accelerator. An x-ray generator is typically composed of the following elements:
Cathode with a heated filament which emits electrons
An electrical field which focuses and accelerate the electrons to the Anode
An anode with a target made of material with high melting temperature, such as Tungsten, which electrons hit
When electrons hit the target on the anode, most of their energy (~99%) is converted into heat in addition to generating x-rays. There are multiple types of x-ray generators, but they all share the same main components described above. Some x-ray generators produce a constant beam of x-rays, just like a light bulb and hence are commonly referred to as Continuous generators. Others generate pulses of x-rays and are commonly referred to as Pulsed generators.
The energy of an X-ray generator depends on the voltage difference between the cathode and anode. Generated x-rays do not have a monochromatic energy but span a range of energies with a peak equal to the voltage difference between the cathode and anode. These x-rays are referred to as poly-chromatic and their energy spectrum is known as the Bremsstrahlung spectrum. In contrast, Gamma-rays are monochromatic.
X-ray energy is generally measured in Kilo Electron Volt (KeV) or Mega (Million) Electron Volt (MeV). X-ray generators in the MeV range are generally pulsed, not continuous.
X-ray Interactions with Matter
As presented above, x-rays are part of the Electromagnetic spectrum. Due to their high-energy, x-rays can penetrate through material other parts of the Electromagnetic Spectrum, such as visible light, can not. This makes x-rays suitable for non-intrusive imaging through closed objects without physically opening them, i.e., Non-Intrusive Inspection (NII).
X-rays used in most x-ray imaging applications interact with matter mainly in two ways: Photoelectric Absorption and Compton Scatter. Understanding these two different interactions is key to understanding how different x-ray imaging modalities work.
Photoelectric absorption is one of two primary processes in x-ray imaging:
When an x-ray or Gamma ray photon is traveling through material, it interacts with an inner shell electron of an atom and the photon is fully absorbed in the process.
The electron which the x-ray photon interacted with is called a Photoelectron and is ejected from the atom.
The shell from which the photoelectron was ejected is left in an excited state and an outer-shell electron moves to fill in the vacant inner shell electron. Given the outer shell now has less photons, the atom is Ionized, hence x-rays and Gamma rays are iononizing.
Finally, the quantum energy difference between the binding energies of outer and inner shell electrons is emitted as a Gamma-ray photon with a distinct energy specific that difference.
Photoelectric Absorption. An x-ray photon interacts with an inner shell electron which is then fully absorbed. This process is key for x-ray Transmission imaging.
The photoelectric absorption process is key for x-ray Transmission Imaging, where a beam of x-ray photons are attenuated while traveling through the matter. As one would expect, the photoelectric absorption depends on the density and thickness of material the x-ray beam is going through. That is, denser, and/or thicker material causes higher attenuation of x-rays than lower density or thinner material. More discussion on x-ray Transmission imaging will follow.
The Compton Scatter process is another primary process governing x-ray imaging:
When an x-ray or Gamma ray photon is traveling through material, it interacts with an outer shell electron.
The incident photon is scattered (deflected). Given the energy conservation principle, the incident photon loses some of its energy hence the scattered photon has lower energy. The energy of the scattered photon is inversely correlated with the scatter angle. That is, x-ray photons scattered in the backward direction (Backscatter) have lower energies than those scattered in the forward direction (Forward-scatter).
The outer shell electron might get ejected out of the atom, leaving it in an ionized state, hence Ionizing Radiation.
Compton Scatter. An x-ray photon interacts with an outer shell electron and is deflected. This process is key for x-ray Scatter imaging.
Compton scattering is generally correlated with the presence of Hydrogen atoms in the material. Hence, Compton scatter effect occurs more in Organic materials where Hydrogen is more abundant. Compton scattering principle is the enabler of Scatter (forward and back-scatter) imaging as will be discussed herein.
Finally, note that Photoelectric Absorption and Compton Scattering may occur independent of each other. Hence, the same x-ray beam can cause both phenomena, and an imaging system can place detectors in a way such that both Transmission and Scatter images are collected simultaneously.