photon illustration



Dr. Alok Dewangan , MD Radiation Oncology

•Photons are electromagnetic radiation with zero mass, zero charge, and a velocity that is always the speed of light.

•x-rays and γ-rays may be considered as bundles of energy called photons.

•Because they are electrically neutral, they do not steadily lose energy via coulombic interactions with atomic electrons, as do charged particles.

• Photons travel some considerable distance before undergoing a more “catastrophic” interaction leading to partial or total transfer of the photon energy to electron energy.

• These will ultimately deposit their energy in the medium.

• Photons are far more penetrating than charged particles of similar energy.

If an x-ray photon enters a thin layer of matter, it is possible that it will pass through without interaction, or it may interact (usually with the atomic electrons, but sometimes with the atomic nuclei) in one of five different ways  photoelectric effect, Compton scattering, pair production, and photodisintegration coherent scattering .


Electromagnetic radiations are characterized by oscillating electric and magnetic fields, always perpendicular to each other and to the direction of their energy propagation.

Electromagnetic radiation can be represented by a varying electric and magnetic field that is conveniently described using a sine-wave model.

The sine wave is characterized by two parameters: the frequency, represented by the Greek letter v, and the wavelength, represented by the Greek letter λ.

The wavelength is the distance from one crest of the sine wave to another; the frequency is the number of complete cycles or oscillations per second and is measured in hertz (Hz).

The product of the frequency and wavelength is the speed (c)with which the wave is propagated- c= v λ


Quantum physics allows electromagnetic radiation to be represented as waves and also as particles, called photons. This is referred to as the wave–particle duality of nature. The photon energy is directly proportional to the classic wave frequency and is related to it through a constant of proportionality known as Planck constant (h), which has a numerical value of 6.625 × 10−34 J-sec. The relationship between energy, E, and frequency, v, is given by the following equation:


The relationship between photon energy and photon wavelength is given by the following equation:

  E=hc/ λ


•The thickness of material that reduces the number of photons transmitted to one-half the incident number is termed the half-value layer (HVL). The HVL is related to the linear attenuation coefficient (μ) by the following equation- •

HVL=0.693/ μ

Energy Loss Mechanisms

•  Photoelectric effect

•  Compton scattering

•  Pair production


•Coherent scattering


•The photoelectric effect is a phenomenon in which a photon is absorbed by an atom, and as a result one of its orbital electrons is ejected.

•In this process, the entire energy (hv) of the photon is first absorbed by the atom and then essentially all of it is transferred to the atomic electron.

•The kinetic energy of the ejected electron (called the photoelectron) is equal to hn–EB, where EB is the binding energy of the electron.

In the photoelectric absorption process, a photon undergoes an interaction with an absorber atom in which the photon completely disappears.

In its place, an energetic photoelectron is ejected from one of the bound shells of the atom.

After ejection of the electron, the neutral atom becomes a positively charged ion with a vacancy in an inner shell that must be filled.

The atom returns to a stable condition by filling the vacancy with a nearby, less tightly bound electron farther out from the nucleus, and characteristic x-rays or an Auger electron is emitted.


In this type of photon interaction, the incident photon disappears, and an electron is ejected with kinetic energy equal to the incident photon’s energy minus the binding energy of the electron. Characteristic x-rays and Auger electrons are emitted as the atom’s electrons cascade to fill the vacancy created by the ejected electron.


•the incident photon interacts with a loosely bound orbital electron in which part of the photon’s energy is transferred to the electron as kinetic energy and the remaining energy is carried away by another photon.

•The binding energy of the electron is insignificant compared with the incident photon’s energy and thus can be ignored.

•The energy of the Compton scattered photon is equal to the difference between the energy of the incident photon and the energy transferred to the electron.

•If the incoming photon’s energy is low (e.g., 100 keV), very little energy is transferred to the electron.

•As the photon’s energy increases, a greater proportion of the energy is transferred to the electron, so the scattered photon necessarily retains a smaller proportion of the incident energy. •The photon may be scattered at any angle with respect to the direction of the incident photon, but the Compton electron is confined to angles between 0 and 180 degrees with respect to the direction of the incident photon.

The photon transfers a portion of its energy to the electron (assumed to be initially at rest), which is then known as a recoil electron, or a Compton electron.

All angles of scattering are possible.

The energy transferred to the electron can vary from zero to a large fraction energy.


In this type of photon interaction, the incident photon

interacts with one of the atom’s outer electrons, and the energy is shared between the ejected electron and a scattered photon




•Pair production is possible only with photons having energies >1.02 MeV. When such an energetic photon approaches closely enough to the nucleus of the target atom, the incident photon energy may be converted directly into an electron–positron pair.

•Energy possessed by the photon in excess of 1.02 MeV appears as kinetic energy, which may be distributed in any proportion between the electron and the positron.


In this type of photon interaction, the incident photon interacts with the electromagnetic field of the nucleus. The incident photon disappears, and two energetic electrons (a positron and a negatron) are produced. Two annihilation photons of energy 0.511 MeV then are produced when the positron interacts with its antiparticle, another electron

When the positron comes to rest, it combines with an electron, and both particles then undergo mutual annihilation, with the appearance of two photons with energy of 0.511 MeV traveling in opposite directions.

The photon, passing near the nucleus of an atom, is subjected to strong field effects from the nucleus and may disappear as a photon and reappear as a positive and negative electron pair.

The two electrons produced, e- and e+, are not scattered orbital electrons, but are created, de novo, in the energy/mass conversion of the disappearing photon.


•A high-energy photon interacts with the nucleus of an atom, totally disrupting the nucleus, with the emission of one or more nucleons.

•It typically occurs at photon energies much higher than those encountered in radiation therapy. However, it is important to account for this in designing shielding around high-energy accelerators, as this interaction is a source of low energy neutrons.


•The coherent scattering, also known as classical scattering or Rayleigh scattering. •The process can be visualized by considering the wave nature of electromagnetic radiation. This interaction consists of an electromagnetic wave passing near the electron and setting it into oscillation.

•The oscillating electron reradiates the energy at the same frequency as the incident electromagnetic wave.

These scattered x-rays have the same wavelength as the incident beam.

Thus, no energy is changed into electronic motion and no energy is absorbed in the medium.

The only effect is the scattering of the photon at small angles.

The coherent scattering is probable in high-atomic-number materials and with photons of low energy.

illustrating the process of coherent scattering.

The scattered photon has the same wavelength as the incident photon. No energy is transferred.






•Cellular damage may occur directly when the interacts with the atom directly, or indirectly when interaction occurs by secondary electrons.

•The biologic effects of radiation result principally from damage to deoxyribonucleic acid (DNA).

If any form of radiation—x- or -rays, charged or uncharged particles—is absorbed in biologic material, there is a possibility that it will interact directly with the critical targets in the cells. The atoms of the target itself may be ionized or excited, thus initiating the chain of events that leads to a biologic change. This is called direct action of radiation.

The radiation may interact with other atoms or molecules in the cell (particularly water) to produce free radicals that are able to diffuse far enough to reach and damage the critical targets. This is called indirect action of radiation.

Incident x-ray photon

Fast electron (e)

Ion radical

Free radical

Chemical changes from the breakage of bonds

Biologic effects

Direct and indirect actions of radiation





•The process by which a neutral atom acquires a positive or negative charge is known as ionization •Removal of an orbital electron leaves the atom positively charged, resulting an ion pair- 

  1. Molecule with a net positive charge

   2. Free electron with net negative charge


•Energy that has to enough to move atom to vibrate , but not enough energy to remove electrons.


•Photoelectric effect produces a positive ion, a photoelectron, and a photon of characteristic radiation. •Photoelectric effect used as diagnostic x-ray and responsible for contrast effect.

Photoelectric effect: produces a scattered photon and an electron, varies as Z4

Compton effect: produces an electron, varies as Z

Pair production: produces an electron and a positron, varies asZ2

Relative importance of the three principal modes of interaction as a function of photon energy and atomic number of absorber.


•Absorbed Dose Definition: the amount of energy absorbed per unit weight of the matter.

•Unit of absorbed dose is Gray (Gy) 1Gy =1 Joule radiation energy absorbed per kilogram .

•Old unit is rad. 1Gy=100 rads COMPTON EFFECT used in radiation therapy as therapeutic significance.

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