|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
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
[Would be nice to have a 'real' spectrum on this page showing the
The annihilation peak can also be seen whenever a
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
- 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.