Before Reading Photoelectric effect we should revise Interaction of ionising radiation with matter again in short.
Interaction of ionising radiation with matter –
When ionising radiation interacts with matter, it can cause changes in the physical and chemical properties of the material. Depending on the type of radiation and the material, interactions can lead to changes in the structure of atoms and molecules, the formation of new compounds, or the release of energy. In biological materials, ionising radiation can cause changes in DNA, leading to mutations and other cellular damage.
When ionising radiation interacts with matter, it can cause changes in the structure of atoms. This is known as ionisation. When an atom is ionised, it can become unstable and cause a chain reaction, resulting in the production of secondary particles such as electrons and protons. The interaction of ionising radiation with matter can also cause changes in the molecular structure of molecules, leading to the production of new molecules. This process is known as radiolysis. Finally, ionising radiation can also cause changes in the physical properties of materials such as those of metals, plastics, and even living cells, resulting in changes in their electrical, optical and thermal properties.
The Processes of Interaction
There are following type of Radiation Interaction With Matter process
Photoelectric effect –
The photoelectric effect is a phenomenon in which electrons are ejected from the surface of a material when it is illuminated by light or other electromagnetic radiation.
When light of a particular frequency is shone on a metal, the photons transfer their energy to electrons in the metal, causing them to be ejected from the surface. An electron is ejected from its orbit around a nucleus as a result of this event, in which the photon is totally absorbed. The electron’s binding energy is partially overcome using photon energy; the remaining portion is transferred to the electron as kinetic energy and is locally wasted. Despite the fact that electrons in any shell can interact with light in a photoelectric way, the interaction is most likely to happen with the electron that is most firmly bound. The energy changes are described by the following equation.
The energy of the ejected electrons is related to the frequency of the light. This effect is used in many modern applications such as solar cells, photo detectors, and photocopiers.
hf = W + 1/2meu²
where
hf = photon energy )(E = hf = hc/λ)
W = Binding Energy Of Electron
meu² = Kinetic Energy of Ejected Electron
The likelihood of this effect is greatest when:
- The incident photon’s energy is equal to or slightly higher than the electron’s binding energy at the k edge (absorption), indicating that the electron is strongly bound (as in K shell)
The ejected electron is subsequently referred to as a photoelectron, and the incoming photon is entirely absorbed throughout the process. Therefore, as the x-ray beam travels through materials, the photoelectric effect helps to attenuate it.
An outer shell electron fills the hole in the inner shell to stabilise the atom. This electron loses energy as it descends to the inner shell, which is then released as either characteristic radiation (an x-ray photon) or an Auger electron.4
An ion pair is a term that is occasionally used to describe the freed electron and this positively charged ion. The fluorescence yield is the ratio of the number of primary vacancies produced in the atomic electron shells to the number of X-rays that are actually released. The fluorescence yield is considerable in high atomic number materials, and fairly significant re-radiation of characteristic radiation in a manner comparable to X-ray generation may occur. The selection of appropriate materials for X-ray beam filtering should take this into consideration.
- The likelihood of photoelectric absorption is inversely related to the cube of incident photon energy (E),
- proportionate to the physical density of the attenuating medium (p), and
- inversely proportional to the attenuating medium’s atomic number (Z)
Photoelectric absorption ~ p·(Z³/E³)
Any X-rays generated in materials with a lower atomic number have a low energy (equivalent to a low K shell energy) and are locally absorbed. Low fluorescence yield is another issue.
It is now more likely that Auger electrons will be produced after being expelled from the atom’s outer shell.
Since the ranges of these electrons in tissue are limited by their energies, which range from a few to several hundred electron volts, the photoelectric interaction process now completely absorbs the energy of the original photon.
It is important to note that the energy from the intense Auger electron shower that is released is deposited just next to the decay site.
A particle’s track can match or even exceed the high local energy density that results, causing radiobiological harm.
The photoelectric cross section, however, also depends on the binding of the electrons and grows quickly as the atomic number rises.
As a result of the Z³ factor, lead (Z = 82) has a 300 times higher photoelectric coefficient than bone (Z = 12.3) at the same photon energy. This explains why these two materials have such a large disparity in their values of at low photon energy.
With a little amount of phosphorus, soft tissue (such as muscle and skin) is mostly composed of hydrogen, oxygen, carbon, and nitrogen. These elements all have very low atomic numbers, making it simple to calculate photon attenuation. Photons impacting on muscle with energy less around 10 keV are practically completely absorbed in 1 cm. About 30% of the incoming photons penetrate 1 cm at 20 keV. The percentage that penetrates 1 cm increases to 80% at 50 keV. As a result, the pictures from backscatter represent the top 1 cm or so of the material. Although it frequently lies under more than 1 cm of soft tissue and includes calcium and has a substantially greater rate of photon attenuation, bone is rarely given additional consideration.
With rising photon energy, the cross-section for a photoelectric interaction decreases sharply, however the drop is not completely regular due to absorption edges.
As a result, at the low end of the diagnostic X-ray energy spectrum, the photoelectric effect is the main interaction activity.
