7 Fixed External Sources

External sources injected into a Radiant calculation are represented by a Fixed_Sources object, which is a collection of Surface_Source and Volume_Source objects.

7.1 The Fixed_Sources Object

A Fixed_Sources object is instantiated by passing it the cross-sections, geometry, and solvers it must be consistent with. Sources are then added one at a time with add_source:

s = Fixed_Sources(cs,geo,solvers)
s.add_source(surface_source)         # surface_source :: Surface_Source
s.add_source(volume_source)          # volume_source  :: Volume_Source

When a Computation_Unit is run, the Fixed_Sources object is built automatically: the contributions of every source are mapped onto the multigroup angular-flux representation expected by the solver, and added to the appropriate spatial moments of the source term.

After a calculation, a normalization factor representing the sum of all source intensities can be retrieved with s.get_normalization_factor().

7.2 Surface Sources

A Surface_Source represents a directional source applied to a face of the geometry. In Cartesian geometries, the available boundaries are "x-", "x+", "y-", "y+", "z-" and "z+".

7.2.1 1D Cartesian Geometry

ss = Surface_Source()
ss.set_particle(photon)              # Particle to be emitted
ss.set_intensity(1.0)                # Intensity [#/cm^(N-1)], N is the geometry dimension
ss.set_energy_group(1)               # Energy group index in which particles are emitted
ss.set_direction([1.0,0.0,0.0])      # Direction cosines (μ,η,ξ); must be a unit vector
ss.set_location("x-")                # Face on which the source is located

In 1D, the source covers the whole face and only the previous calls are required.

7.2.2 2D and 3D Cartesian Geometries

In multidimensional geometries, the extent of the source on the chosen face must be set with set_boundaries(axis,boundaries). The two boundary values are given in ascending order, in centimetres:

ss.set_boundaries("y",[2.0,3.0])     # 2D: extent along y
ss.set_boundaries("z",[0.5,1.5])     # 3D: extent along z (in addition to the y extent)

7.2.3 Optional Legendre Truncation

By default the boundary angular flux is expanded to a high Legendre order (64). The truncation can be reduced if a coarser representation of the directional source is acceptable:

ss.set_legendre_order(7)

7.2.4 Direction Cosines

set_direction(v) expects a three-component unit vector [μ,η,ξ]. The intensity is interpreted as the integral of the angular flux over the source area, multiplied by the direction cosine of the source with respect to the inward normal.

7.3 Volume Sources

A Volume_Source represents an isotropic source distributed within an axis-aligned box inside the geometry.

7.3.1 Setting up a Volume Source

vs = Volume_Source()
vs.set_particle(photon)
vs.set_intensity(1.0)                # Intensity [#/cm^N]
vs.set_energy_group(1)
vs.set_boundaries("x",[0.0,2.0])     # Always required (1D, 2D, 3D)
vs.set_boundaries("y",[0.0,2.0])     # Required in 2D and 3D
vs.set_boundaries("z",[0.0,2.0])     # Required in 3D

The boundaries for each axis are an ascending two-element vector giving the lower and upper bound of the source box along that axis, in centimetres.

7.4 Mixing Multiple Sources

Several surface and volume sources can be added to the same Fixed_Sources object. For example:

s = Fixed_Sources(cs,geo,solvers)

# Photon pencil beam entering from x-
ss = Surface_Source()
ss.set_particle(photon)
ss.set_energy_group(1)
ss.set_direction([1.0,0.0,0.0])
ss.set_location("x-")
s.add_source(ss)

# Distributed electron source in a region
vs = Volume_Source()
vs.set_particle(electron)
vs.set_energy_group(5)
vs.set_boundaries("x",[2.0,3.0])
s.add_source(vs)

Different particles, different energy groups and different intensities can be combined freely in a single calculation.

7.5 Summary of the Source API

Fixed_Sources

Surface_Source

Volume_Source