Lec.1 د. غسان علي
Radiation physics
Introduction:-
X-rays and their ability to penetrate human tissues were discovered by Roentgen in 1895. He called them X-rays because their nature was then unknown. They are a form of high – energy electromagnetic spectrum.
A basic knowledge of atomic physics is essential to understand the production and interactions of X-rays.
Atomic particles:
Nuclear particles (nucleons).
Protons:
• Mass = 1.66 × 10 -27 Kg.
• Charge = positive.
Neutrons:
• Mass = 1.70 × 10-27 Kg.
• Charge = nil.
• Neutrons act as binding agents within the nucleus
Electrons:
• Mass = 1/ 1840 of the mass of a proton.
• Charge = negative.
• Electrons move in elliptical shells or orbits around the nucleus.
• The shells represent different energy levels and are labeled K, L, M, N, O, P, Q outwards from the nucleus.
• Electrons can move from shell to shell but can t exist between shells.
• To remove an electron from the atom, additional energy is required to overcome the binding energy of attraction which keeps the electrons in their shells.
Electron binding energy :-
The amount of electrostatic attraction between appositively charged nucleus and its negatively charged electrons balances the centric fugal force of the rapidly revolving electron and maintains them in their orbit.
1 And it is specific for each shell of each element.
2 For an electron to move from specific orbit to another orbit farther from the nucleus, energy most be supplied in an amount equal to the difference in binding energies between the tow orbits and vice Versa.
Ionization:-
Is the production of ions, or the process of converting an atom into ions, ionization deal with electrons only requires sufficient energy to over come the electron binding energy.
When electron is removed, an ion pair results. The atom become the positive ion and the ejected electron become the negative ion. This ion pair reacts with other ions until electrically stable, neutral atoms are formed.
Electrons can be removed by heating or interaction (collisions) with high energy X-ray or particles such as protons.
Gamma ray or high – energy particles or X-ray can remove the electrons from inner shell (K, L and M). In contrast, electrons in outer shell easily displaced by photons of lower energy (e.g. ultra violet or visible light).
Radiation:- Is the emission and propagation of energy through space or a substance in the form of waves particle.
Radioactivity:- Process by which certain unstable atoms or elements undergo spontaneous disintegration in an effort to attain amore balanced nuclear state.
Ionizing radiation:- Radiation that is capable of producing ion by removing or adding an electron to an atom.
Electromagnetic radiation:
Is a propagation of wave – like energy (without mass) through space or matter, this propagation is accompanied by oscillating electrical and magnetic fields positioned at right angles to one another.
Electromagnetic radiation are believed to move through space as both a particle and a wave,
Electromagnetic radiation can be classified as ionizing "e.g. cosmic rays, gamma rays, X-rays" or non – ionizing radiations.
X – Rays properties:
1 Electromagnetic radiation originate at atomic level.
2 Invisible and can t be detected by any of the human senses.
3 Have no mass or weight, charge.
4 Electromagnetic rays with short wave lengths and high frequency (shorter wave length greater energy great penetration.
5 Travel with speed of light and straight line and can be deflected or scattered.
6 Can t be focused to a point and always diverge from a point.
7 Can be penetrate liquid, solids and gases, and cause.
8 Can be absorbed by matter (absorption depend on atomic structure of matter and the wave length of the X – rays.
9 Interact with the materials and cause ionization.
10 No medium is required for propagation.
11 Can affect film emulsion to produce a visual image (radiograph) and can cause certain salts fluoresce and to emit light.
Heat – producing collisions.
* The incoming electron is deflected by the cloud of outer – sell tungsten electrons, with a small loss of energy, in the form of heat.
* The incoming electron colloids with an outer shell tungsten electron displacing in to an even more peripheral shell (excitation) it from the atom (ionization), again with a small loss of energy in the form of heat.
* 99% of electrical energy converted to heat. because there are millions of incoming electrons and many outer – shell tungsten electrons with which to interact.
X – ray – producing collisions.
The kinetic energy of the electrons is converted to X – ray photons via one of two mechanism and production 2 types of
X – radiation.
General radiation:-
Speeding electrons slow down because of their (many) interactions with the tungsten target in the anode, so radiation produced by this manner called general (Bremsstrahlung), or braking radiation.
G.R. represent approximately 70% of the X – ray energy produced at the anode.
This radiation produced when an electron hits the nucleus (rarely occur; when it does, all kinetic energy is connected to a high – energy
X – ray photon) or when electron pass very close to the nucleus (Mostly occur and lower energy X – ray photon results the electron that misses the nucleus continues to penetrate many atoms, producing lower energy
X – rays before it imparts all of it s kinetic energy. So the general radiation consists of X – rays of many different energies and wavelengths.
Characteristic radiation:-
Is produced when a high – speed electron dislodges an inner shell electron and cause ionization of the atom, the remaining orbiting electrons are rearranged to fill the vacancy. This rearrangement produces a loss of energy that result in the production of an X – ray photon.
This type of X – radiation occur only at 70 Kvp and above because the binding energy of K–shell electron is nearly 70 Kev.
Intactions of X – radiation with matter:-
There are four main interactions with matter at atomic level:-
? No interaction.
? Absorption or photoelectric effect.
? Compton scatter.
? Coherent scatter.
Absorption of energy and photoelectric effect:-
Absorption refers to the total transfer of energy from x-ray photon to the atoms of matter through which the x-ray beam passes.
It is occurs when x-ray photon collides with a tightly bound, inner shell electron and give up all of its energy to eject the electron from its orbit. This electron termed a photoelectron which has a negative charge; it is absorbed by other atom because it has very little penetration power.
Photoelectric effect accounts for 30 % of interaction of matter with the dental x-ray beam.
Compton scatter:
Scatter: refer to a type of radiation when the photon be deflected from its path during its passage through a matter.
Compton effect accounts for most of the scatter radiation (Compton scatter accounts for 62 % of scatter that occurs in diagnostic radiology).
In Compton scatter, ionization takes place, an x-ray photon collide with a loosely bound, outer shell electron and give up part of its energy to eject electron from its orbit and this photon continues in a different direction (scatter) at a lower energy level and it will interacted with other atoms until all of its energy is gone.
The ejected electron is termed a Compton electron or recoil electron.
Coherent scatter (unmodified scatter):
It is occur when a low-energy x-ray photon interacts with an outer shell electron. No change in atom occurs, and an x-ray photon of scattered radiation is produced and in a direction differ from the incident photon, no loss of energy, no ionization occurs.
Coherent scatter account for 8 % of interaction of matter with the dental x-ray beam.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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