1. G4GeneralParticleSource Class:
Developed by ESA as the space radiation environment is often quite complex in energy and angular distribution, and requires more sophisticated sampling algorithms than for typical high-energy physics studies.
It is used as a substitute to G4ParticleGun, with all original features retained.
It allows the user to define a source particle distribution in terms of:
spectrum (defined in terms of energy or momentum)
angular distribution with respect to a user-defined axis or surface normal
spatial distribution of particles from 2D or 3D planar surfaces or beam line in Gaussian profile or generated homogeneously within a volume.
It also provides the option of biasing the sampling distribution. This is advantageous, for example, for sampling the area of a spacecraft where greater sensitivity to radiation effects is expected (e.g. where radiation detectors are located) or increasing the number of high-energy particles simulated, since these may produce greater numbers of secondaries.

2. G4GeneralParticleSource Features:

3. RDM-specific user inputs
G4 Radioactive Decay Model
Long-term (>1s) radioactive decay induced by spallation interactions can represent an important contributor to background levels in space-borne -ray and X-ray instruments, as the ionisation events that result often occur outside the time-scales of any veto pulse. The Radioactive Decay Model (RDM) treats the nuclear de-excitation following prompt photo-evaporation by simulating the production of , -, +,  and anti-, as well as the de-excitation -rays. The model can follow all the descendants of the decay chain, applying, if required, variance reduction schemes to bias the decays to occur at user-specified times of observation.
ENSDF2
Photon-
evaporation
Radioactive
decay
Ion from G4 tracking
defined by A, Z, Q, nuclear
and atomic excitation state
Sample decay profile to
determine time of decay. If
stable do not process
Sample branching ratios
Sample secondaries ( a , b n
) and commit to stack.
Determine nuclear recoil
Apply photonuclear de-
excitation process
User input defines
times of observation
for biasing decay
curve, splitting
of nuclei, and nuclide
decays to ignore
Biasing of branching ratios
Geant4 simulation
Geant4 Tracking
& process control
Geant4 data sources
RDM-specific user inputs

4. Variance Reduction in RDM
The branching ratio and decay scheme data are based on the Evaluated Nuclear Structure Data File (ENSDF), and the existing Geant4 photo-evaporation model is used to treat prompt nuclear de-excitation following decay to an excited level in the daughter nucleus. (Atomic de-excitation following nuclear decay is treated by the Geant4 EM physics processes.)
The RDM has applications in the study of induced radioactive background in space-borne detectors and the determination of solar system body composition from radioactive - ray emission. On ground it is used in the Dark Matter Experiments
Variance Reduction in RDM
Times sampled in Monte Carlo simulation of radioactive decay often may not correspond to the times of observation leading to inefficient simulation. Biasing the sampling process and modifying the weights of the decay products significantly improves efficiency.