Physics of X-rays By Dr. Amr A. Abd-Elghany.

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Physics of X-rays By Dr. Amr A. Abd-Elghany

Has no mass, and moves through space at the speed of light (3x108 m/sec). EM radiation can be described by two models: Wave model Photon model. EM Radiation: Wave model EM radiation is a pair of perpendicular, time varying electric and magnetic fields traveling through space with velocity of light. ν = c / λ

EM radiation can also be described as discrete packets of energy called photons. The energy (E) is related to the wavelength (λ) in the model through Plank constant and the speed of light (c). E = h c / λ E = h ν

X-ray Energy X-rays are electromagnetic waves have the ability to penetrate an object. EM radiation with wavelength shorter than 100 nm can remove electrons from the outer shell. The higher the energy, the more easily the wave will pass through the object. The shorter the wavelength, the greater the energy will be and the higher the frequency, the greater the energy will be.

Which of the above x-rays has the highest energy?
A: It has the shortest wavelength, highest frequency

X-ray Characteristics
X-rays are high energy waves, with very short wavelengths, and travel at the speed of light. X-rays have no mass (weight) and no charge (neutral). You cannot see x-rays; they are invisible. X-rays travel in straight lines; they can not curve around a corner. An x-ray beam cannot be focused to a point; the x-ray beam diverges (spreads out) as it travels toward and through the patient. This is similar to a flashlight beam.

X-ray Characteristics (continued)
X-rays are differentially absorbed by the materials they pass through. More dense materials (like a bone) will absorb more x-rays than less dense material (like skin tissue). This characteristic allows us to see images on an x-ray film. X-rays will cause certain materials to fluoresce (give off light). We use this property with intensifying screens used in extra oral radiography. X-rays can be harmful to living tissue. Because of this, you must keep the number of films taken to the minimum number needed to make a proper diagnosis.

X-rays are produced in the x-ray tube, which is located in the x-ray tube head.
X-rays are generated when electrons from the filament cross the tube and interact with the target. The two main components of the x-ray tube are the cathode and the anode.

The cathode is composed of :
Focusing cup Filament (tungsten) side view (cross-section) front view (facing target) The cathode is composed of : (1) a tungsten filament (high melt (2) a circuit to provide filament current (3) negative charged focusing cup. The focusing cup has a negative charge, like the electrons, and this helps direct the electrons to the target (“focuses” them; electrons can be focused, x-rays cannot).

Thermionic Emission When you depress the exposure button, electricity flows through the filament in the cathode, causing it to get hot. The hot filament then releases electrons which surround the filament (thermionic emission). The hotter the filament gets, the greater the number of electrons that are released. (Click to depress exposure button and heat filament). hot filament electrons

Anode Anode material should have:
side view front view Anode Target Copper stem Target Anode material should have: High conversion efficiency for electrons into x-rays (x-ray intensity is proportional to the atomic number Z). At 100KeV lead (Z=82) converts 1% of the energy into x-rays but aluminum (Z= 13) converts only 0.1%. High melting point to tolerate the heat produced. High conductivity to remove heat rapidly. Suitable mechanical properties.

side view front view Anode Target Copper stem Target The anode in the x-ray tube is composed of a tungsten (Z=74, melting point 3370 oC) target embedded in a copper stem (due to its high conductivity). When electrons from the filament enter the target and generate x-rays, a lot of heat is produced. The copper helps to take some of the heat away from the target so that it doesn’t get too hot.

1. focusing cup 6. copper stem 2. filament 7. leaded glass
4 3 6 2 5 1 7 9 8 1. focusing cup copper stem 2. filament leaded glass 3. electron stream x-rays 4. vacuum beryllium window 5. target (for description, see next slide)

X-ray Tube Components (continued)
1. Focusing cup: focuses electrons on target. 2. Filament: releases electrons when heated. 3. Electron stream: electrons cross from filament to target during length of exposure. 4. Vacuum: no air or gases inside x-ray tube that might interact with electrons crossing tube. 5. Target: x-rays produced when electrons strike target. 6. Copper stem: helps remove heat from target. 7. Leaded glass: Keeps x-rays from exiting tube in wrong direction. 8. X-rays produced in target are emitted in all directions. 9. Beryllium window: this non-leaded glass allows x-rays to pass through. The PID would be located directly in line with this window.

Photo of an X-ray Tube Leaded glass Beryllium Window Target
Photo of an X-ray Tube Leaded glass Beryllium Window Target Focusing cup (filament located inside)

X-ray Exposure 1. Depress exposure button
X-ray Exposure 1. Depress exposure button 5. X-ray production stops when exposure time ends. Release exposure button 3. Activate high-voltage circuit to pull electrons across tube 2. Activate low-voltage circuit to heat filament 4. Electrons cross tube, strike target and produce x-rays

X-ray Production There are two types of x-rays produced in the target of the x-ray tube. 1.The majority are called Bremstrahlung radiation And 2.The others are called Characteristic radiation.

(Also known as braking radiation or general radiation) Bremstrahlung x-rays are produced when high-speed electrons from the filament are slowed down as they pass close to, or strike, the nuclei of the target atoms. The closer the electrons are to the nucleus, the more they will be slowed down. The higher the speed of the electrons crossing the target, the higher the average energy of the x-rays produced. The electrons may interact with several target atoms before losing all of their energy.

+ Bremsstrahlung X-ray Production
Electron slowed down by positive charge of nucelus; energy released in form of x-ray + High-speed electron from filament enters tungsten atom Electron continues on in different direction to interact with other atoms until all of its energy is lost

+ Bremsstrahlung X-ray Production Maximum energy
+ High-speed electron from filament enters tungsten atom and strikes target, losing all its energy and disappearing The x-ray produced has energy equal to the energy of the high-speed electron; this is the maximum energy possible

The amount of bremsstrahlung produced for a given number of electrons striking the anode depends upon two factors: (1) the Z no. of the target (i.e. the more protons in the nucleus, the greater the acceleration of the electrons), and (2) the kilovolt peak (i.e. the faster the electrons, the more likely they will penetrate into the region of the nucleus).

Characteristic x-rays are produced when a high-speed electron from the filament collides with an electron in one of the orbits of a target atom; the electron is knocked out of its orbit, creating a void (open space). This space is immediately filled by an electron from an outer orbit. When the electron drops into the open space, energy is released in the form of a characteristic x-ray. The energy of the high-speed electron must be higher than the binding energy of the target electron with which it interacts in order to eject the target electron. Both electrons leave the atom.

Characteristic x-rays have energies “characteristic” of the target material. The energy will equal the difference between the binding energies of the target electrons involved. For example, if a K-shell electron is ejected and an L-shell electron drops into the space, the energy of the x-ray will be equal to the difference in binding energies between the K- and L-shells. The binding energies are different for each type of material; it is dependent on the number of protons in the nucleus (the atomic number). K-shell L-shell M-shell

Characteristic X-ray Production
Characteristic X-ray Production Electron in L-shell drops down to fill vacancy in K-shell X-ray with 59 keV of energy produced. 70 (binding energy of K-shell electron) minus 11 (binding energy of L-shell electron) = 59. Ejected electron leaves atom vacancy M High-speed electron with at least 70 keV of energy (must be more than the binding energy of k-shell Tungsten atom) strikes electron in the K shell, knocking it out of its orbit L K Recoil electron (with very little energy) exits atom

X-ray Spectrum An x-ray beam will have a wide range of x-ray energies; this is called an x-ray spectrum. The average energy of the beam will be approximately 1/3 of the maximum energy. The maximum energy is determined by the kVp setting. If the kVp is 90, the maximum energy is 90 keV (90,000 electron volts).

X-ray Interaction with Matter
The higher the energy of the x-ray, the shorter the wavelength. Low energy x-rays interact with whole atoms. Moderate energy x-rays interact with electrons. High energy x-rays interact with the nuclei.

Five forms of x-ray Interactions
Classical or Coherent Scattering Compton Effect Photoelectric Effect Pair production Photodisintegration

Two Forms of X-ray Interactions Important to Diagnostic X-ray
Compton Effect Photoelectric Effect

Compton Effect Moderate energy x-ray photon through out the diagnostic x-ray range can interact with outer shell electron. X-ray photon transfer some of its energy E=mc2 to the electron and the rest of the energy is given to the Compton scattered photon. The electron uses this amount of photon energy to leave the atom.

Scattered X-ray photon ƛ
Incident X-ray photon λ

Photoelectric Effect X-ray photon interacts with the most inner electron (i.e. k-electron). The photon transfer all of its energy to the electron and disappears. The electron uses some of this energy to overcome the binding energy of the nucleus and the rest of energy as kinetic energy to leave the atom.

Photoelectric Effect

Electron-positron pair production
In nuclear physics, this occurs when a high-energy photon interacts in the vicinity of a nucleus. The energy this (mass-less) photon has can be converted into mass through Einstein's equation E=mc² where E is energy, m is mass and c is the speed of light. Thus if the energy of the photon is high enough so that it can make the mass of an electron plus the mass of a positron (basically twice the mass of an electron which is 9.11 × 10−31 kg) then an electron-positron pair may be created. If there is more energy in the photon than just enough to create the mass of the electron-positron pair then the electron and positron will have some kinetic energy - meaning they will be moving.

The electron and positron can move in opposite directions (at an angle of 180 degrees) meaning they have a total momentum of zero or they can move at an angle of less than 180 degrees resulting in a combined momentum which is very small (since momentum is a vector quantity). However, if the photon only just had enough energy to create the mass of the electron-positron pair then the electron and positron will be at rest. This could violate the conservation of momentum since the photon has momentum and the two resulting electrons have none if they are stationary (since momentum = mass x velocity). This means that the pair production must take place near another photon or the nucleus of at atom since they can absorb the momentum of the original photon i.e since the momentum of the initial photon must be absorbed by something, pair production cannot occur in empty space out of a single photon; the nucleus (or another photon) is needed to conserve both momentum and energy (consider the time reversal ofElectron-positron annihilation).[1]

Mobile x-ray