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Sampling an interaction

After the free flight the particle reaches its next interaction. Depending on the particle energy the interaction types have different probabilities the data thereof can be found here(external link).

For photons (gamma or x-ray) the most important interactions are the absorption, the Compton scattering and the pair production. Using a new random number we can select the interaction, with probability, for example of the Compton scattering:
$  \frac{\mu_{Compton} }{\mu_{Compton}+\mu_{Abs}+\mu_{Pair}}

If this 'roulette' results in an absorption the particle life terminates and a new particle is started. If pair production is sampled the particle looses its energy but 1.022 MeV of it, then from the interaction point two photons of 511keV energy are born, one started according to a uniform distribution the other in the opposite direction.During the simulation both particles are followed.

When Compton scattering happens the differential Klein-Nishina cross section defined here we chose new energy and angle for the particle.

During the particle history we count the detector contributions.

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