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1. Only happens at very high energies (at least 1.02 MeV, which is 2 * 511 keV).
2. Incoming photon passes close to the nucleus.
3. A positron and an electron are formed from the photon’s energy (E = mc 2 ).
3. If the incoming photon had > 1.02 MeV of energy then what’s left over is given to the electron and positron as kinetic energy.
4. The electron causes excitations and ionisations and gradually loses energy.
5. The positron combines with a “free” electron and is annihilated & produces two 511 keV photons that are emitted at 180° to each other.
6. Net effect of pair production is that all of the initial incoming photon’s energy is transferred to the material.


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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