The mass attenuation coefficient (m m ) of iron is shown on the right as a funct
ID: 1582217 • Letter: T
Question
The mass attenuation coefficient (mm) of iron is shown on the right as a function of photon energy. All but two of the curves are labeled. The two curves that are not labeled correspond to the two largest components of attenuation over the range of 1 keV to 1 MeV.
What is the curve labeled A? Why is it shaped the way it is?
What is the curve labeled B? Why is it shaped the way it is?
In what sense does it make sense to say that one effect dominates in terms of finding the mass attenuation coefficient?
Photon Mass Attenuation Coefficiants for Iron 10 _Coherent Scattering 10 Pair Production In Nuclear Field Pair Production In Electron Field 10 Total Attenuation E 10 10 10 10 10 10 10 10 10 10 10 Energy in MeVExplanation / Answer
Curve A represents the phenomenon of Compton scatterscattering for any material.
Curve B represents the phenomenon of Photoelectric effect produced for any material.
As photons pass through matter, they can engage in either photoelectric or Compton interactions with the material electrons. The photoelectric interaction captures all photon energy and deposits it within the material, whereas the Compton interaction removes only a portion of the energy, and the remainder continues as scattered radiation. The combination of the two types of interactions produces the overall attenuation of the x-ray beam. We now consider the factors that determine which of the two interactions is most likely to occur in a given situation.
The energy at which interactions change from predominantly photoelectric to Compton is a function of the atomic number of the material. The figure shows this crossover energy for several different materials. At the lower photon energies, photoelectric interactions are much more predominant than Compton. Over most of the energy range, the probability of both decreases with increased energy. However, the decrease in photoelectric interactions is much greater. This is because the photoelectric rate changes in proportion to 1/E3, whereas Compton interactions are much less energy dependent. In soft tissue, the two lines cross at an energy of about 30 keV. At this energy, both photoelectric and Compton interactions occur in equal numbers. Below this energy, photoelectric interactions predominate. Above 30 keV, Compton interactions become the significant process of x-ray attenuation. As photon energy increases, two changes occur: The probability of both types of interactions decreases, but the decrease for Compton is less, and it becomes the predominant type of interaction.
In higher-atomic-number materials, photoelectric interactions are more probable, in general, and they predominate up to higher photon energy levels. The conditions that cause photoelectric interactions to predominate over Compton are the same conditions that enhance photoelectric interactions, that is, low photon energies and materials with high atomic numbers.
The total attenuation coefficient value for materials involved in x-ray and gamma interactions can vary tremendously if photoelectric interactions are involved. A minimum value of approximately 0.15 cm2/g is established by Compton interactions. Photoelectric interactions can cause the total attenuation to increase to very high values. For example, at 30 keV, lead (Z = 82) has a mass attenuation coefficient of 30 cm2/g.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.