  In high-intensity (> 10^21W/cm^2) laser-matter interactions gamma-ray photon
emission by the electrons can strongly affect the electron's dynamics and
copious numbers of electron-positron pairs can be produced by the emitted
photons. We show how these processes can be included in simulations by coupling
a Monte-Carlo algorithm describing the emission to a particle-in-cell code. The
Monte-Carlo algorithm includes quantum corrections to the photon emission,
which we show must be included if the pair production rate is to be correctly
determined. The accuracy, convergence and energy conservation properties of the
Monte-Carlo algorithm are analysed in simple test problems.
