  The XIFU X-ray spectrometer instrument on the future Athena mission needs
X-ray calibration sources to calibrate the gains of the individual detector
pixels. For this purpose, electronically controlled Modulated X-ray Sources
(MXS) are proposed, similar to the calibrations sources used on the Hitomi
spacecraft and which will also fly on its successor, XARM. Here we present a
simulation package based on the particle transport GEANT4 toolkit. Using this
package, we compute the results for different targets and window configurations
for the MXS's. The simulations expose the trade-offs to be made to select the
optimum source configuration for the Athena/XIFU and XARM/Resolve spectrometer
instruments.
