What is a Photon and How They Interact with Matter

Everything you need to know about photons and their interaction process: Photoelectric effect, Compton and Rayleigh scattering & Pair production

Photons are the quanta or discrete packets of electromagnetic radiation across the entire electromagnetic spectrum, including radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.

As the force carriers of the electromagnetic force, photons mediate the interactions between electrically charged particles. The nature and behavior of photons has been studied for centuries, with key developments in understanding light as both a wave and particle occurring in the 17th through early 20th centuries.

In 1690, Christiaan Huygens proposed a wave theory of light propagation through an omnipresent ether. However, in 1704 Isaac Newton favored a corpuscular theory, describing light particles or “corpuscules” with different sizes corresponding to different colors.

Thomas Young’s 1801 double slit experiment demonstrated interference patterns consistent with a wave view. Foucault showed in 1850 that light travels slower in water than air, as expected for a wave. Maxwell’s prediction and subsequent detection of electromagnetic waves in the 1860s was further evidence for a wave theory.

In quantum field theory, particles can be described as waves in a field (Image: Piotr Traczyk/CERN)

In 1900 Planck described blackbody radiation through quantized energy packets, and Einstein’s 1905 explanation of the photoelectric effect established the dual wave-particle duality of photons. The term “photon” was later coined by Gilbert Lewis in 1926.

Photons are now considered bosonic excitations in a quantum electromagnetic field that permeates space, carrying the electromagnetic force between charged particles.

When two electrons interact, they exchange a photon, the particle of light. (Image: Ana Tovar/CERN)

Across the electromagnetic spectrum, photons serve as an omnipresent probe and tool with broad research and practical applications. Low energy radio photons transmit information through fiber optic cables and free space. Higher energy ultraviolet, X-ray and gamma ray emissions from celestial sources provide information on stellar lifecycles and galactic evolution. The cosmic microwave background gives insight into early universe conditions. In particle physics, the 2012 discovery of the Higgs boson involved reconstruction of its photon decay signature.

Interaction of Photons with Matter

The behavior of photons in matter is completely different from that of charged particles. In particular, the photon’s lack of an electric charge makes impossible the many inelastic collision with atomic electrons so characteristic of charged particles. For this kind of radiation the most important mechanism of interaction are:

a) Photoelectric effect
b) Compton and Rayleigh scattering
c) Pair production

As consequence of such kind of interactions a photon that interacts with the target is completely removed from the incident beam, in other words a beam of photons that cross a medium is not degraded in energy but only attenuated in intensity.

Moreover, due to the smallest cross section of all this kind of reactions,x‑ray or γ-ray are many times more penetrating than charged particles. The attenuation of the incident beam is exponential with the thickness of the absorbing medium and can be expressed by the following relation:

I(x)= I0 exp (-xμl)

where μl is the linear attenuation coefficient, I0 is the incident beam intensity and x the thickness. The linear attenuation coefficient is related to the cumulative cross section by the relation:

μl =ησtot

where ηA is the number of atoms per unit of mass and σtot is the total cross section. The total or cumulative cross section σtot is the sum of all the cross sections of the interactions mentioned above. A plot of this quantity is shown in Figure 1 where the different components have been highlighted.

Interaction of Particles with Matter: From Theory to Measurements

Interaction of Particles with Matter: From Theory to Measurements

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Photoelectric Absorption

In photoelectric absorption, a photon disappears being absorbed by an atomic electron. The process results in ionization by subsequent ejection of the electron from the atom. The energy of the liberated electron is the difference between the photon energy and the energy needed to extract the electron from the atom i.e. the binding energy of the electron.

The recoil momentum is absorbed by the nucleus to which the ejected electron was bound. If the resulting photoelectron has sufficiently enough of kinetic energy, it may be a source of a secondary ionization occurring along its trajectory, and in the case of the semiconductor material, it may create further e-h pairs.

If the electron does not leave the detector the deposited energy corresponds to the energy possessed by the incident photon.

This feature of the photoelectric effect allows calibrating the gain of the detector chained with its readout system if the energy required to create a single e-h pair is known. The range R of the electron having the kinetic energy E is of the order of some micrometers, as given by the follow equation:

R[um] = 40.8 10^(-3) x ( E[keV] )^1.5

Thus the cloud of generated charge is confined close to the photon absorption point. The clear image may be smeared by escape photons, which can leave the detector volume leading to less amount of energy deposited. These photons are actually the fluorescence photons emitted by de-exciting atoms. Photons of fluorescence radiation are emitted by atoms after the ejection of a deep shell (K, L) electron.

The incident photon creates a vacancy in the shell, thus leaving an atom in an excited state. Then, the vacancy can be filled by an outer orbital electron, giving rise to the emission of the characteristic X-rays photons of the fluorescence radiation.

The missing energy, which is conveyed by the escape photons leads to, so called escape peaks in the measured energy spectrum. Photon interaction coefficient for photoelectric absorption depends strongly on the atomic number of the absorbing material. The relevant cross section increases roughly as Z^3. For silicon, the photoelectric effect is a dominant process for photon energies below 100 keV.

Figure 1 Cross sections of photons in Carbon (a) and Lead (b) in barns/atom; 1barn=10-24 cm2.
Figure 1 Cross sections of photons in Carbon (a) and Lead (b) in barns/atom; 1barn=10-24 cm2.

Compton and Rayleigh scattering

The Compton scattering, instead of photoelectric effect, involves the free electrons. In matter of course, the electrons are bound to an atom; however, if the photon energy is high with respect to the binding energy, this latter energy can be ignored and the electrons can be treated as essentially free.

When Compton scattering occurs, the electron is scattered away in conjunction with a new photon that have a lower energy than the incoming one.

In Rayleigh scattering the photon interact with the whole atom and the only effect of this interaction is a deflection of the incoming photon; it does not participate to the absorption and for most purposes can be neglected.

Pair Production

At very high energy another effect starts to be relevant: the pair production. In this process the photon interacts with an electron or a nucleus producing a positron-electron pair. In order to produce the pair the photon must have at least an energy of 1.022 MeV.

Figure 1, with knuc and ke, shows the two components of the pair production cross section, respectively for the interaction with nuclei or electrons.

Another interaction, but usually negligible compared to the previous ones is the Photonuclear reaction, in this case the photon interact directly with the nucleus.

Interaction of Particles with Matter: From Theory to Measurements

Interaction of Particles with Matter: From Theory to Measurements

Understand the Mysteries of Radiation: Master the Interaction with Matter in a Few Simple Steps

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Figure 1 shows the related cross section in dotted line (σg.d.r.). The above cross section in barns/atom (1barn = 10-24 cm2, approximately the section of an uranium nucleus) expresses the probability of an interaction. A more suitable quantity, often used to characterize the absorption of a photon shower, is the mass attenuation coefficient. The mass attenuation coefficient is defined as:

μmσtot/ρ

where ρ is the density of the material. Figure 2 shows the mass attenuation coefficient of the silicon with the indication of its different components.

Figure 2 Mass attenuation coefficient of the silicon and its components.
Figure 2 Mass attenuation coefficient of the silicon and its components.

References

  1. Yung-Su Tsai, Pair production and bremsstrahlung of charged leptons, Reviews of Modern Physics, vol. 46, no. 815, 1974
  2. M.Bronshtein, B.S. Fraiman, “Determination of the Path Lengths of Slow Secondary Electrons”, Sov. Phys. Solid State, Vol.3, (1961), pp.1188-1197.
  3. R. Wunstorf, Systematische Untersuchungen zur Strahlenresistenz von Silizium-Detektoren fur die Verwendung in Hochenergiephysik-Experimenten, PhD Thesis, Universitat Hamburg, Germany (1992)
  4. S. Meroli “Interaction of radiation with matter: from the theory to the measurements”
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Stefano Meroli
Stefano Meroli

CERN scientist. PhD in Nuclear Physics.

Articles: 7

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