Common parameters#

Sources are the objects that create particles ex nihilo. The particles created from sources are called the Event in the Geant4 terminology, they got a EventID which is unique in a given Run.

Several sources can be defined and are managed at the same time. To add a source description to the simulation, you do:

source1 = sim.add_source('GenericSource', 'source1')
source1.n = 100

source2 = sim.add_source('VoxelSource', 'source2')
source2.activity = 10 * g4_units.Bq

There are several source types, each one with different parameter. In this example, source1.n indicates that this source will generate 100 Events. The second source manages the time and will generate 10 Events per second, so according to the simulation run timing, a different number of Events will be generated.

Information about the sources may be displayed with:

# Print all types of source
print(sim.dump_source_types())

# Print information about all sources
print(sim.dump_sources())

Some of the parameters are common to all types of sources, while others are specific to a certain type of source. Given a source source, use print(source) to display the source’s parameters and their default values.

Common parameters are:

  • attached_to: the name of the volume to which the source is attached (world by default) in the hierarchy of volumes.
    See Details: Volumes for more details.
  • position.translation: list of 2 numerical values, e.g. [0, 2 * cm, 3 * mm].
    It defines the translation of the source with respect to the reference frame of the attached volume.
    Note: the origin of the reference frame is always at the center of the shape in Geant4.
  • rotation: a 3×3 rotation matrix.
    Rotation of the volume with respect to the attached volume.
    We advocate the use of scipy.spatial.transform.Rotation to manage the rotation matrix.
  • n: the number (integer or a list) of particles to emit (the number of Geant4 Events).
  • activity: the number (real, in Bq) of particle to emit per second.
    The number of Geant4 Events will depend on the simulation time.

If you want to start a multi-run simulation using the parameter n, you can provide a list of the number of particles to simulate for each run (see test096)

A minimal working example would be:

source = sim.add_source("GenericSource", "mySource")
source.n = [10000, 150000, 36]

sim.run_timing_intervals = [[0, 1 * sec], [2 * sec, 2.5 * sec], [4 * sec, 5 * sec]]

Please note that sim.run_timing_intervals must have the same length as source.n. By default, all particles of a given run will be emitted at a timestamp equal to the minimum time of the corresponding timing interval.

Coordinate system#

The attached_to option indicates the coordinate system of the source. By default, it is the world, but it is possible to attach a source to any volume. In that case, the coordinate system of all emitted particles will follow the given volume. Using source.direction_relative_to_attached_volume = True will make your source direction change following the rotation of that volume.

Reference#

property SourceBase.attached_to[source]#

attached_to:

  • Default value: world

  • Description: Name of the volume to which the source is attached.