Particle Sources

A mcdc.Source describes the position, direction, energy, time, particle type, and relative probability of particles introduced into a simulation. Position and spatial bounds use cm, physical energy uses eV, time uses seconds, and direction vectors are dimensionless. Only sources passed to mcdc.Simulation.set_sources() participate in transport.

Energy Distributions

A scalar energy defines a mono-energetic source:

physical_source = mcdc.Source(energy=1.0e6)

An energy array with shape (2, N) defines a continuous probability density with energy values in the first row in eV and density in the second row in eV^-1. Use discrete_energy for a probability mass function over physical emission lines:

emission_lines = mcdc.Source(
    particle_type="electron",
    discrete_energy=(
        [1.0e5, 2.0e5],
        [0.8, 0.2],
    ),
)

The values in the second row are dimensionless relative probabilities. The energy and discrete_energy inputs are alternative source-energy specifications and cannot be combined.

Standard Neutron Multigroup Sources

Standard neutron multigroup transport uses a dimensionless group coordinate in place of physical source energy. A scalar integer selects one group:

group_zero_source = mcdc.Source(energy=0)

Use discrete_energy to sample among groups:

group_mixture = mcdc.Source(
    discrete_energy=(
        [0, 1],
        [0.25, 0.75],
    ),
)

Group coordinates must be finite, integer-valued, and satisfy 0 <= energy < G. Continuous energy distributions are not valid in standard neutron multigroup transport. Native and hybrid transport use physical source energy in eV, as described in Materials and Transport Data.