Which two of the following photon interactions result in the ionization of an atom? (select two)

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 effectb) 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 =ηA σtot

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 1.6 where the different components have been highlighted.

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.

Which two of the following photon interactions result in the ionization of an atom? (select two)

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

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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. In Figure 1, with knuc and ke, are shown the two components of the pair production cross section, respectively for the interaction with nuclei or electrons. Another possible interaction, but usually negligible compared to the previous ones is the Photonuclear reaction, in this case the photon interact directly with the nucleus.

The related cross section is shown in Figure 1 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:

μm=ηA σtot/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.

Which two of the following photon interactions result in the ionization of an atom? (select two)

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)

The attenuation that results due to the interaction between penetrating radiation and matter is not a simple process.  A single interaction event between a primary x-ray photon and a particle of matter does not usually result in the photon changing to some other form of energy and effectively disappearing.  Several interaction events are usually involved and the total attenuation is the sum of the attenuation due to different types of interactions. These interactions include the photoelectric effect, scattering, and pair production. The figure below shows an approximation of the total absorption coefficient, (µ), in red, for iron plotted as a function of radiation energy. The four radiation-matter interactions that contribute to the total absorption are shown in black. The four types of interactions are: photoelectric (PE), Compton scattering (C), pair production (PP), and Thomson or Rayleigh scattering (R). Since most industrial radiography is done in the 0.1 to 1.5 MeV range, it can be seen from the plot that photoelectric and Compton scattering account for the majority of attenuation encountered.

Which two of the following photon interactions result in the ionization of an atom? (select two)

Summary of different mechanisms that cause attenuation of an incident x-ray beam

Which two of the following photon interactions result in the ionization of an atom? (select two)
Photoelectric (PE) absorption of x-rays occurs when the x-ray photon is absorbed, resulting in the ejection of electrons from the outer shell of the atom, and hence the ionization of the atom. Subsequently, the ionized atom returns to the neutral state with the emission of an x-ray characteristic of the atom. This subsequent emission of lower energy photons is generally absorbed and does not contribute to (or hinder) the image making process. Photoelectron absorption is the dominant process for x-ray absorption up to energies of about 500 KeV. Photoelectron absorption is also dominant for atoms of high atomic numbers.

Which two of the following photon interactions result in the ionization of an atom? (select two)
Compton scattering (C) occurs when the incident x-ray photon is deflected from its original path by an interaction with an electron.  The electron gains energy and is ejected from its orbital position.  The x-ray photon loses energy due to the interaction but continues to travel through the material along an altered path.  Since the scattered x-ray photon has less energy, it, therefore, has a longer wavelength than the incident photon. The event is also known as incoherent scattering because the photon energy change resulting from an interaction is not always orderly and consistent.  The energy shift depends on the angle of scattering and not on the nature of the scattering medium.

Which two of the following photon interactions result in the ionization of an atom? (select two)
Pair production (PP) can occur when the x-ray photon energy is greater than 1.02 MeV, but really only becomes significant at energies around 10 MeV. Pair production occurs when an electron and positron are created with the annihilation of the x-ray photon. Positrons are very short lived and disappear (positron annihilation) with the formation of two photons of 0.51 MeV energy.  Pair production is of particular importance when high-energy photons pass through materials of a high atomic number.

Below are other interaction phenomenon that can occur. Under special circumstances these may need to be considered, but are generally negligible.

Which two of the following photon interactions result in the ionization of an atom? (select two)
Thomson scattering (R), also known as Rayleigh, coherent, or classical scattering, occurs when the x-ray photon interacts with the whole atom so that the photon is scattered with no change in internal energy to the scattering atom, nor to the x-ray photon. Thomson scattering is never more than a minor contributor to the absorption coefficient. The scattering occurs without the loss of energy. Scattering is mainly in the forward direction.

Which two of the following photon interactions result in the ionization of an atom? (select two)
Photodisintegration (PD) is the process by which the x-ray photon is captured by the nucleus of the atom with the ejection of a particle from the nucleus when all the energy of the x-ray is given to the nucleus. Because of the enormously high energies involved, this process may be neglected for the energies of x-rays used in radiography.

Effect of Photon Energy on Attenuation
Absorption characteristics will increase or decrease as the energy of the x-ray is increased or decreased. Since attenuation characteristics of materials are important in the development of contrast in a radiograph, an understanding of the relationship between material thickness, absorption properties, and photon energy is fundamental to producing a quality radiograph. A radiograph with higher contrast will provide greater probability of detection of a given discontinuity. An understanding of absorption is also necessary when designing x-ray and gamma ray shielding, cabinets, or exposure vaults.

The applet below can be used to investigate the effect that photon energy has on the type of interaction that the photon is likely to have with a particle of the material (shown in gray). Various materials and material thicknesses may be selected and the x-ray energy can be set to produce a range from 1 to 199 KeV. Notice as various experiments are run with the applets that low energy radiation produces predominately photoelectric events and higher energy x-rays produce predominately Compton scattering events. Also notice that if the energy is too low, none of the radiation penetrates the material.

This second applet is similar to the one above except that the voltage (KVp) for a typical generic x-ray tube source can be selected. The applet displays the spectrum of photon energies (without any filtering) that the x-ray source produces at the selected voltage. Pressing the "Emit X-ray" button will show the interaction that will occur from one photon with an energy within the spectrum.  Pressing the "Auto" button will show the interactions from a large number of photos with energies within the spectrum.