  For several years, attempts have been made to interface Geant4 and other
software packages with the aim of simulating the complete response of a gaseous
particle detector. In such a simulation, Geant4 is always responsible for the
primary particle generation and the interactions that occur in the non-gaseous
detector material. Garfield++ on the other hand always deals with the drift of
ions and electrons, amplification via electron avalanches and finally signal
generation. For the ionizing interaction of particles with the gas, different
options and physics models exist. The present paper focuses on how to use
Geant4, Garfield++ (including its Heed and SRIM interfaces) and Degrad to
create the ionization electron-ion pairs in the gas. Software-wise, the
proposed idea is to use the Geant4 physics parameterization feature, and to
implement a Garfield++ or Degrad based detector simulation as an external
model. With a Degrad model, detailed simulations of the X-ray interaction in
gaseous detectors, including shell absorption by photoelectric effect,
subsequent Auger cascade, shake-off and fluorescence emission, become possible.
A simple Garfield++ model can be used for photons (Heed), heavy ions (SRIM) and
relativistic charged particles or MIPs (Heed). For non-relativistic charged
particles, more effort is required, and a combined Geant4/Garfield++ model must
be used. This model, the Geant4/Heed PAI model interface, uses the Geant4 PAI
model in conjunction with the Heed PAI model. Parameters, such as the lower
production cut of the Geant4 PAI model and the lowest electron energy limit
have to be set correctly. The paper demonstrates how to determine these
parameters for certain values of the W parameter and Fano factor of the gas
mixture. The simulation results of this Geant4/Heed PAI model interface are
then verified against the results obtained with the stand-alone software
packages.
