Radiology Physics CH 1 : Discovery Of X-Ray
X-rays were discovered by WC Roentgen, the German physicist in 1895 when he was investigating the conduction of electricity through gases at low pressure in glass tubes. He noticed that the positive electrodes in the tubes gave off invisible rays which made fluorescent screens (Barium platinocyanide screen kept near the tube) to glow and fogged photographic plates. The rays were highly penetrating, they passed through black paper and even thicker objects. They were not deflected in magnetic field. Therefore, Roentgen concluded that they were not charged particles. As their nature was not known he called them X-rays; later, they were shown to be electromagnetic radiation of very short wavelength. Roentgen received the first Nobel prize in physics in 1901 for his discovery.
PROPERTIES OF X-RAYS
1. X-rays are electromagnetic radiation of shorter wavelength (few nm).
2. They travel in straight line with a velocity equal to light.
3. X-rays are not influenced by electric and magnetic fields.
4. X-rays penetrate through substances that are opaque to visible
light.
5. X-rays produce fluorescence in materials like calcium tungstate,
and cesium iodide, etc.
6. X-rays affect the photographic film and form latent image.
7. X-rays produce ionization and excitation in the substances through
which they pass.
8. X-rays produce chemical changes in substances through which
they pass.
9. X-rays produce biological effects in living organisms. The cells can
be either damaged or killed due to X-ray exposure.
PRODUCTION OF X-RAYS
X-rays are produced when fast moving electrons are stopped by means of a target material. The moving electrons possess kinetic energy. When the electron is suddenly stopped, its kinetic energy is converted into heat and X-rays. This conversion is taking place in the target material. Therefore, the interaction of electron with the target is the basis for X-ray production.
ELECTRON INTERACTION WITH THE TARGET
When the electron arrives at the target, it interacts in four ways as follows (Fig). When the electron arrives at the target, it interacts in four
The electron interaction involves ionization collisions (i) and radiative collisions (ii), (iii) and (iv).
i. Ionization of target atoms: The fast moving electron enters the surface layer of the target and undergoes collisions. In this process, the incident electron transfers sufficient energy and removes an electron from the atom. This involves small energy transfer, resulting in ionization of target atoms. The incident electron may undergo number of such collisions and each time its direction gets altered. A 100 keV electron may encounter 1000 of such interactions, before coming to rest and most of its energy appears as heat in the target. The displaced electron, known as a secondary electron, may have sufficient energy and produce further ionization of target atoms. They are few in number and produce their own track, known as delta rays.
ii. Characteristic X-rays: This is an interaction between the incident electron and the electron in the K shell. In this process, the incident electron directly hit the K shell, transfers sufficient energy and removes the K shell electron. The vacancy in the K shell is filled by an electron moving inwards from the outer shell. During this transition, the difference in binding energies of the two shells is given out as X-ray photon. This photon is known as the characteristic X-ray. The ejected electron may produce further interaction in other target atoms.
iii. Interaction with nuclear field: The incident electron occasionally reaches nearer to nucleus of an atom in the target. Since the electron is a negative particle, it is attracted by the positive nucleus. It is made to orbit partially around the nucleus, decelerates and goes out with reduced energy. The loss of energy appears in the form of X-ray photons, known as Bremsstrahlung. The energy of the X-ray photon depends on the degree to which the electron is decelerated by the nuclear attraction. The photon energy can take any value from zero to a maximum. This process is unlikely at low energies, but dominant at high energies.
iv. The electron may hit the nucleus directly and is stopped completely in a single collision. The entire electron energy appears as bremsstrahlung radiation. This type of interaction is very rare, but capable of giving high energy X-rays.
In general, the interaction of B, C and D are very rare in the diagnostic range of energies, leading to lesser amount of X-ray production. The international collision dominate (> 99%) the interaction process and produce heat. Thus, a X-ray tube is inefficient in the conversion of electron energy into X-rays.
BREMSSTRAHLUNG
The bremsstrahlung is a German word meaning braking radiation. It is a process of radiative collision between the electron and a nucleus in the target (Fig. 3.3). The electron while passing near the nucleus may suffer a sudden deflection and acceleration by the action of coulomb forces of attraction. As a result, the electrons may lose their kinetic
energy, in the form of bremsstrahlung X-rays. The electron may have one or more such interactions and this may result in partial or complete loss of energy.
The amount of bremsstrahlung production is determined by the distance between the bombarding electron and the nucleus. At very large distance, the columbic force is weak, only low energy X-rays are created, but this process has higher probability to occur. When the electron is very close to the nucleus, columbic force is strong, electron lose more kinetic energy, resulting production of high energy X-rays. But this process has lower probability to occur. When the electron is in the middle, the electron interaction is moderate and the X-ray energy is also moderate. If the electron hit the nucleus directly, it lose all its kinetic energy, but the probability of this type of interaction is very low (5%). To conclude low energy X-rays are produced in greater abundance compared to high energy X-rays.
Thus, the bremsstrahlung radiation will have all possible energy from zero to maximum. The maximum energy is determined by the maximum kinetic energy of the incident electron. Also the direction of emission of bremsstrahlung photons depends on the energy of the incident electron. At electron energies below 100 keV, X-rays are emitted equally in all directions. As the kinetic energy of the electron increases, the direction of X-ray emission becomes increasingly forward.
CHARACTERISTIC X-RAYS
Electron incident on a target may produce characteristic X-rays. An electron with kinetic energy E0 may interact with the atoms of the target, by ejecting an orbital electron from the K shell. Now, there is a vacancy in the K shell and the atom is said to be ionized. The original electron will have energy E0 -E, where, E is the energy given to the orbital electron. The outer orbital electrons (from M or L) will fell down to fill the vacancy in the K shell (Fig. 3.4). In doing so, the difference in binding energy of the two shells is radiated as X-ray photons, which is called the characteristic radiation.
0 Comments