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1. If E of incident electron/photon > K binding energy, then the K-shell electron will be ejected from the atom.
2. Most likely: L-shell electron will drop down to fill the hole, emitted photon is termed K alpha radiation (energy = E K - E L ).
3. Less likely: M-shell electron drops down, termed K beta radiation (energy = E K - E M ).
4. L-radiation (when an electron is knocked out of the L-shell) also occurs but mostly generates heat and plays no significant part in radiology.
5. The generated X-ray photons therefore have a few discrete energy levels and constitute a spectrum that is termed the characteristic radiation.
6. Characteristic radiation is determined by atomic number and unaffected by tube voltage.


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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