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Pair production in shielding
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Pair production in the surrounding material of the detector gives rise to the 'annihilation peak' at 511 keV in the energy spectrum. This is due to the escape of one of the 511 keV gamma-rays to the detector. The mechanism is similar to the double and single escape peaks in the detector but only one of the 511 keV photons can ever reach the detector because they are emitted in opposite directions. The annihilation peak in a spectrum only arises if the source emits at an energy greater than 1022 keV.

[Would be nice to have a 'real' spectrum on this page showing the annihilation peak.]


The annihilation peak can also be seen whenever a radioisotope emits positrons as part of its decay process.

 There are three main sources of 511 keV gammas:

  • positron decay of a radioactive isotope;
  • pair production in the shielding by high energy gamma-rays from the source; and also
  • pair production in the shielding by high energy cosmic rays.

It is thus important to think about the source of 511 keV peaks in a spectrum, before dismissing them.

The annihilation peak is always broader than expected at that energy due to Doppler broadening.


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1999, 2000 The University of Liverpool, Department of Physics

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