A Geant4/Garfield++ and Geant4/Degrad Interface for the Simulation of Gaseous Detectors

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. The present paper illustrates different possibilities to interface Geant4 with two such packages, Garfield++ and Degrad. The basic idea is to use the Geant4 physics parameterization feature and to implement a Garfield++ or Degrad based detector simulation as an external model. With the Geant4/Degrad interface, detailed simulations of the X-ray interaction in gaseous detectors, including shell absorption by photoelectric effect, subsequent Auger shake-off, and fluorescence emission, become possible. The Geant4/Garfield++ interface can be used for photons and charged particles of all kinetic energies. Depending on the particular physics case, either the Geant4 PAI model, the Heed PAI model or both Geant4 and Heed are responsible for primary ionization and the production of the conduction electrons. For the case in which the Geant4 PAI model is used in conjunction with the Heed PAI model, a more detailed analysis is performed. Parameters, such as the lower production cut of the PAI model and the lowest electron energy limit of the physics list have to be set correctly. The paper demonstrates how to determine these parameters with the help of the W value 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 standalone software packages.