Understanding Photoelectric Interaction for Ionization Chamber: Science Detectors

Consider the diagram below of an ionization chamber with a cathode, an anode, and a neutral gas contained within a sealed radiolucent compartment. Bombarding the chamber with ionizing radiation will yield a source of electrons through which interaction?Image credit to Jeffrey Tillotson. Permission to reproduce given by Jeffrey Tillotson. ACoherent scattering BPhotoelectric interaction CBremsstrahlung DPair production

The answer is B.The five interactions of x-ray photons are with matter, not just the atoms and molecules inside the patient. A novice student may not realize that those reactions apply to AEC ionization chambers and image receptors as well. The key to understanding is recognizing which of the four choices meets these criteria:-The phenomenon described is initiated by an x-ray photon.-A free electron is available after the phenomenon has occurred.Coherent scattering begins with an x-ray photon but does not produce a negative ion through ionization (A). Bremsstrahlung doesn’t fit either criterion, as it is a means of x-ray production, not an interaction (C). Both photoelectric interaction and pair production fit the criteria (D). The photoelectric interaction is the correct choice because pair production occurs at higher energy levels than energies found within the diagnostic range (B). When a photon enters the cell, it ionizes an air molecule, perhaps nitrogen, and creates both a positive and a negative ion. The positive ions will feel a coulomb force attraction to the cathode, and the negative ions (photoelectrons) will be pulled toward the anode. Notice that ions don’t begin forming in meaningful numbers until x-rays enter the chamber.

The interaction that will yield a source of electrons in the ionization chamber shown in the diagram is photoelectric interaction. Photoelectric effect is a type of interaction between ionizing radiation (e.g. x-rays or gamma rays) and matter, in which a photon is absorbed by an atom and an electron is ejected from its shell (usually the K-shell) as a result of the interaction. The ejected electron is called a photoelectron and it can produce ionization of other atoms or molecules it encounters in the gas-filled chamber, resulting in the production of an electrical current that can be measured. This is the basis for the operation of an ionization chamber, which is a type of radiation detector that measures the amount of ionization produced by the interaction of ionizing radiation with a gas.

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