G4GeneralParticleSource Class:

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• 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.

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G4GeneralParticleSource Features
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• G4 Radioactive Decay Model
• Long-term (gt1?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.

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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.