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.

2D Surfacesources

3D Surfacesources

Volume sources Angulardistribution

Energy spectrum

circle ellipse square rectangle Gaussian

beam profile

sphere ellipsoid cylinder paralellapiped

(incl. cube &cuboid)

sphere ellipsoid cylinder paralellapiped

(incl. cube &cuboid)

isotropic cosine-law user-defined

(throughhistograms)

mono-energetic Gaussian Linear Exponential power-law bremsstrahlung black-body CR diffuse user-defined

(throughhistograms orpoint-wisedata)

G4GeneralParticleSource Features:

G4 Radioactive Decay ModelLong-term (>1s) 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.

ENSDF2http://ie.lbl.gov/ensdf

Photon-evaporation

Radioactivedecay

Ion from G4 trackingdefined by A, Z, Q, nuclearand atomic excitation state

Sample decay profile todetermine time of decay. Ifstable do not process

Sample branching ratios

Sample secondaries ( , ,) and commit to stack.Determine nuclear recoil

Apply photonuclear de-excitation process

User input definestimes of observationfor biasing decaycurve, splittingof nuclei, and nuclidedecays to ignore

User input defines Biasing of branching ratios

Geant4 simulation

Geant4 Tracking& process control

Geant4 data sources

RDM-specific user inputs

Unbiassed and Biassed Probability of Decay

0

0.2

0.4

0.6

0 5 10 15 20 25 30Time [hr]

Pro

babi

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of d

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uni

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Unbiassed probability

Biased probability

Biassed probability corresponds to times

Variance Reduction in RDMTimes 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.

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