What are the photon interactions with matter?
Description: The various ways in which high-energy photons interact with matter are introduced—photoelectric effect, Compton scattering, pair production, and nuclear reactions. Energetics of each are discussed so one can read a gamma spectrum like we saw in the NRL lab last week. We also explain how the gamma spectrometer detector works. The ashes of 1000 bananas are counted and interpreted. Prof. Short irradiates his cell phone to demonstrate “digital snow” noise in the camera’s detector. Show
Instructor: Michael Short Note: To report potential content errors, please use this form. Photons are quanta of electromagnetic energy that exhibit both wave-like and particle-like properties. A photon can be considered to have a wavelength and frequency (like a wave), as well as momentum and energy (like a particle). Despite carrying electromagnetic energy, a photon has no 'charge' and has a much lower chance of interacting with matter than charged particles such as electrons and protons.
These may cause the photon to attenuate (lose some of its energy and/or disappear). Photon interactions are very important when considering how a photon beam interacts with a patient. Coherent Scattering - σcohCoherent (or Rayleigh) scattering occurs at low photon energies. A photon may interact with an orbital electron and is then deflected (or scattered) at a small angle. There is no change in energy of the photon and no other effects occur. Photoelectric Effect - τThe photoelectric effect occurs when a photon interacts with an orbital electron whose binding energy is close to that of the photon energy. In this scenario, the photon disappears and all of its energy is given to the orbital electron, which is then ejected from the atom with kinetic energy equivalent to the photon energy minus the binding
energy.
Therefore, as photon energy increases, the likelihood of the photoelectric effect decreases rapidly. Absorption EdgesAn important concept is that when the photon energy is very closely aligned with the binding energy of a particular electron shell, the likelihood of photoelectric interactions jumps. This is an absorption edge, and reflects the ability of photons above that energy to interact more frequently. Incoherent Scattering (Compton Effect) - σincIncoherent scattering is the most
important interaction in radiotherapy. It occurs when a photon has a much greater amount of energy than the binding energy of the electron, effectively considering the electron as 'free'. In this interaction, the photon interacts with the 'free' electron, giving up some of its energy and undergoing scattering. The electron receives the energy and is set in motion in a different direction.
Importantly, incoherent scattering is not directly related to atomic number but rather the concentration of electrons in tissue. As atomic number rises, the density of electrons falls slowly, so incoherent scattering becomes slightly less likely in high Z materials. Incoherent scattering also decreases with energy, but nowhere near as rapidly as the photoelectric effect. This makes it more relevant as photon energies rise above the K-shell binding energies of orbital electrons. Pair Production - κPair production occurs when a photon passes very close to the nucleus of an atom. If the energy of the photon is high enough, the photon may disappear and 'create' an electron and a positron. The new particles move away with the remaining energy of the photon converted to kinetic energy. Situations in which pair production is possiblePair production only occurs when photon energy is high enough and when there is an object of sufficient mass to take on the momentum gained by the new particles.
Relation of pair production with atomic number and photon energyPair production does not occur with photon energy under
1.022 MeV. Once this threshold is reached, it becomes more likely as photon energy increases. Triplet Production - κtrTriplet production is a special case of pair production which occurs in the vicinity of an orbital electron. The photon disappears and the energy is used to create an electron and positron. The orbital electron also receives energy and is freed from the atom. The threshold for this to occur is a photon of 2.044 MeV. Photodisintegration - πPhotodisintegration is an uncommon event that occurs when a photon is absorbed by the nucleus of an atom. The photon is destroyed and a nucleon (either a proton or a neutron) is released. The threshold for this effect is over 10 MeV for most nuclei (with the exception of beryllium and deuterium, where it is 2 MeV). Even at high energies, photodisintegration is an uncommon event and does not attenuate a substantial portion of a photon beam. It is more important for radiation protection concerns - if neutrons are released they are highly penetrating and can convert atoms into unstable isotopes. The release of a nucleon from the atom in question also usually results in a radioactive daughter product. The production of neutrons in high energy linear accelerators means that bunkers must be regularly ventilated to prevent buildup of radioactive gasses. LinksWhat are the four types of photon interactions with matter?The main processes of interaction of photons with matter are the following: Photoelectric absorption, • Compton scattering, • Pair production, • Rayleigh scattering.
What particles do photons interact with?Photons passing through matter transfer energy to charged particles, which in turn affects the material. These photons are indirectly ionizing. Photons and charged particles interact primarily with the electrons in atoms.
How do protons interact with matter?A proton has three main interactions with matter; Inelastic Coulomb interaction with atomic electrons, Elastic Coulomb scattering with atomic nuclei, and non-elastic nuclear interaction. The dominating interaction is Inelastic coulomb interaction with atomic electrons.
What are the 5 interaction of matter?Five main interactions can cause attenuation of photons: (1) coherent scattering, (2) photoelectric effect, (3) Compton scattering, (4) pair producion, and (5) photodisintegration. How much of the beam gets attenuated depends on which interaction process dominates.
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