Pair production

So far, the Monte Carlo code mcapm does not track electrons. What it does is to say that the positron from pair production will collide with an electron to produce two 0.511 Mev photons. The data for pair production is therefore handled as follows.

Cross section:

The cross sections for pair and triplet production are separated in the `epdl.asc' file. They are combined by mcfgen and log-log interpolated onto a fixed grid of energy points.

Available energy:

The available energy is the energy of the incident photon, so long as this exceeds the threshold.

Energy distribution of secondary photon:

The energy distribution is an approximate $\delta$-function at 0.511 Mev. The code mcfgen prints corresponding equiprobable energy bins.

Energy of secondary photons:

The energy of secondary photons is twice 0.511 Mev for all incident energies above the threshold.