For the annihilation into two photons the cross-section formula of Heitler is used. The total cross-section is given by [,]
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For compounds, the cross-section is calculated using an effective atomic number as explained in [PHYS010].
Positrons can annihilate in a single photon if the electron with which it interacts is bound to a nucleus. The total cross-section for such a process is
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In the derivation of this formula, only the interactions with the K-shell electrons are taken into account. As the cross-section depends on , a special value of
is computed in GPROBI:
The notation of [PHYS010] is used.
The total cross-section for the positron annihilation is . The value of is at it largest for heavy materials, for example, it is 20 % of for a positron of 440 keV of kinetic energy in lead. For lower and higher energies the probability is lower.