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1. An incoming photon collides with an electron and has sufficient energy to overcome its binding energy and eject the elctron. This interaction occurs closer to the nucleus.

2. The photon is completely absorbed. E pe = E o - E b (where E pe is the kinetic energy of the photoelectron, E o is the energy of the incident photon & E b is the binding energy).

3. The ejected electron is known as a photoelectron.

4. The “hole” in the electron shell is filled by an outer shell electron “dropping down” with the emission of another photon of energy.

5. Probability of photoelectric absorption: (where Z = atomic number, E = photon energy).

6. Atomic number ↑ => photoelectric effect↑. Photon energy ↑ => photoelectric effect↓ Photoelectric effect is important with high Z materials at low photon energies.


1. Bushberg, J. T. (2012). The essential physics of medical imaging. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. Find it at Amazon
2. Heggie, J. C., Liddell, N. A., & Maher, K. P. (1997). Applied imaging technology. Melbourne: St. Vincents Hospital.

Ⓒ A. Manickam 2018

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