THE BASES X-RAY RELATED PHYSICS

 

Recommended Book:

Walter Huda, REVIEW OF RADIOLOGIC PHYSICS

By: Maisa Alhassoun

maisa@inaya.edu.sa

II. Matter

A. Atoms

-Matter is made up of atoms, which are composed of protons, neutrons, and electrons.

-Protons have a positive charge and are found in the nucleus of atoms.

-Neutrons are electrically neutral and are also found in the nucleus.

-The number of neutrons in an atom affects the stability of the nucleus.

-Electrons have a negative charge and are found outside the nucleus.

-Electrons are much lighter than protons and neutrons.

-The atomic number (Z) is the number of protons in the nucleus of an atom and is unique for each element.

-The mass number (A) is the total number of protons and neutrons in the nucleus.

XChemical symbol for the elementA

Z

Mass number=A = Z + N

Atomic number =Number of

protons

N Number of neutrons

-In the notation AZX or AX, X is the unique letter or letters

designating the element, A is the mass number, and Z is the atomic number.

-Electrically neutral atoms have Z electrons and Z protons.

B. Electronic structure-The nucleus of an atom is made up of tightly bound

protons and neutrons, which are called nucleons.

-The nucleus contains most of the atomic mass.

-In the Bohr model of an

atom, electrons surround

the nucleus in shells (e.g.,

K-shell and L-shell) as

shown for tungsten.

-Each shell is assigned a principal quantum number (n), beginning with one for the

K-shell, two for the L-shell, and so on.

-The number of electrons each shell can contain is 2n2.

-The K-shell in tungsten (n = 1) has 2 electrons, the L-shell (n = 2) has 8 electrons, the M-shell (n = 3) has 18 electrons, and so on.

-The number of electrons in the outer shell (valence electrons) determines the chemical properties of the atom.

C. Electron binding energy

-The work that is required to remove an electron from an atom is called the electron binding energy.

 

-The binding energy of outer-shell electrons is small and equal to approximately several electron volts.

-The binding energy of inner-shell electrons is large, that is, thousands of electron volts (keV).

-K-shell binding energies increase with atomic number (Z), as listed in Table

-Energetic particles can knock out inner-shell electrons only if their energy is greater than the electron binding energy.

-A 100 keV electron can eject a K-shell electron from a tungsten atom.

-A 50 keV electron cannot, as it does not have sufficient energy to overcome the 69.5 keV binding energy.

-A vacancy in the K-shell will be filled by an electron from a higher shell.

-Electrons moving from an outer shell to an inner shell may emit excess energy as electromagnetic radiation.

D. Nuclear binding energy

-Nucleons are held together by strong forces.

-The total binding energy of the entire nucleus is the energy required to separate all of the nucleons.

-The binding energy of a single nucleon (i.e., neutron or proton) is the energy required to remove it from the nucleus.

-The average binding energy per nucleon is the total binding energy divided by the number of nucleons.

III. Radiation

A. Electromagnetic radiation

-Radiation is the transport of energy through space.

-Wavelength (λ) is the distance between successive crests of waves.

-Amplitude is the intensity defined by the height of the wave.

-Frequency (f) is the number of wave oscillations per unit of time expressed in cycles per second, or in hertz (Hz).

-The period is the time required for one wavelength to pass (1/f).

-For any type of wave motion, velocity (v) = f x λ m/second, where f is measured in hertz and λ in meters.

-Electromagnetic radiation travels in a straight line at the speed of light, c (3 x 108 m/second in a vacuum).

-X-rays are an example of electromagnetic radiation.

-The product of the wavelength (λ) and frequency (f) of electromagnetic radiation is equal to the speed of light

(c = f x λ).

-Electromagnetic radiation represents a transverse wave, in which the electric and magnetic fields oscillate perpendicular to the direction of the wave motion.

-Fig. 1.3 shows the electromagnetic spectrum from radio waves (long wavelength) to x-rays and gamma rays (short wavelength).

B. Photons -Electromagnetic radiation is quantized, meaning that it

exists in discrete quantities of energy called photons.

-Photons may behave as waves or particles but have no mass.

-Photon energy (E) is directly proportional to frequency and inversely proportional to wavelength.

-The wavelength of an x-ray may be measured in angstroms (Å), where 1 Å is 10−8 cm, or 10−10 m.

-Photon energy is E = h x f = h x (c/λ) = 12.4/λ, where E is in keV, h is Planck's constant, and λ is the wavelength in angstroms.

-A 10 keV photon has a wavelength of 1.24 Å, which is equal to the diameter of a typical atom.

-A 100 keV photon has a wavelength of 0.124 Å.

-By convention, photons are called x-rays if produced by electron interactions, and gamma rays if produced in nuclear processes.

C. Inverse square law -X-ray beam intensity decreases with distance from the tube

because of the divergence of the x-ray beam.

-The decrease in intensity is proportional to the square of the distance from the source and is an expression of energy conservation.

-This nonlinear fall-off in intensity with distance is called the inverse square law.

-For example, doubling the distance from the x-ray source decreases the x-ray beam intensity by a factor of 4; increasing the distance by a factor of 10 decreases the beam intensity by a factor of 100.

-In general, if the distance from the x-ray source is changed from x1 to x2, then the x-ray beam intensity changes by (x1/x2)2.

I1/I2 = (D2/D1)2