Photoelectric interactions

A 350-keV 7 ray interacts with a X-shell electron by the photoelectric interaction. If the binding energy of the X-shell electron is 25 keV, what is the kinetic energy of the photoelectron ... 

X-ray detectors are transducers that count individual photons. In a photoelectric interaction, the entire incident energy of the interacting photon is stored up in the detector (while in Compton scattering, only a portion of the incident energy is deposited). The detector works with greater accuracy, as the photon flux is weaker. The two most current types are ... 

Figure 4.22 shows the relative importance of the three interactions as E and Z change. Consider a photon with E = 0.1 MeV. If this particle travels in carbon (Z = 6), the Compton effect is the predominant mechanism by which this photon interacts. If the same photon travels in iodine (Z = 53), the photoelectric interaction prevails. For a y of 1 MeV, the Compton effect predominates regardless of Z. If a photon of 10 MeV travels in carbon, it will interact mostly through Compton scattering. The same photon moving in iodine will interact mainly through pair production. 

The photoelectric interactions are caused by photons that are produced in the counter as a result of the ionization and excitation of the atoms and molecules of the gas. If the chamber is filled with a monatomic gas, these photons produce photoelectrons only when they strike the cathode (wall of cylinder) because they do not have enough energy to ionize the atoms of the gas. If the counter is filled with a gas mixture, however, photons emitted by molecules of one gas may ionize molecules of another. 

Figure 12.1 As a result of a photoelectric interaction, the photon disappears, (a) All the energy of the electron is deposited in the detector, (h) Part of the energy is deposited in the wall.

Photoelectric interaction in materials surrounding the detector can result in characteristic X rays in the lower energy region of the gamma-ray spectrum. For example, the Ka (72 keV) and Kp (85 keV) X rays are almost always part of the background in a spectrum of a detector shielded with lead. Commercially available lead shields for gamma-ray-spectrometer detectors are lined with thin cadmium and copper layers to attenuate these lead X rays. 

The spectral response of a detector is more complex than described in Section 2.4.4 because of the bulk of the detector. The observed Compton continuum consists of single plus multiple successive scattering interactions. When such multiple Compton scattering interactions are terminated by a photoelectric interaction, the pulse is added to the full-energy peak. Most of the counts in a full-energy peak for gamma rays above 100 keV are due to such multiple scattering plus a final photoelectric interaction. 

Figure 2.9 The photoelectric interaction, (a) Before photoelectric interaction a photon of energy E encounters the atom, (b) In the photoelectric interaction the photon is absorbed by a K-shell electron, and the electron is ejected with an energy equal to the photon energy less the K-shell electron-binding energy, (c) the K-shell vacancy is filled by an L-shell electron, and the difference in binding energies is given off as either (c) a characteristic x-ray photon or (d) an Auger electron. (Reprinted by courtesy of EG G ORTEC.)...

In passing through the differential volume element the x>rays travel through a thickness dx esc Therefore the number of photoelectric interactions in this element per unit time is given by... 

In some situations the entire photon energy E is not absorbed within the active detector volume. Where the initial photoelectric interaction takes place near the detector entrance window, there is a high probability that the K x-ray emitted by the excited argon atom will escape the sensitive detector volume. Consequently, the energy deposited in the counter will be... 

It should be emphasized that the absorption coefficient is a much more restricted concept than the attenuation coefficient. Attenuation also includes the purely elastic process in which the photon is merely deflected and does not give up any of its initial energy to the absorber. In a photoelectric interaction, the entire energy of the incident photon is absorbed by an atom of the medium, while in the Compton effect, some energy is absorbed and appears in the medium as the kinetic energy of a Compton recoil electron the balance of the incident energy is not absorbed and is present as a Compton scattered photon. 

The photoelectric interaction is the most probable process up to a few hundred keV and that is the only interaction that results in a complete absorption of the photon. Compton scattering may occur at all energies, but is the most important between a few hundred keV and a few MeV, while at high energies pair production becomes the most important interaction (see e Fig. 31.4). 

The most troublesome photoelectric interactions will be those with the shielding, usually lead. As shown in... 

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