Since photon production from neutron reactions is usually stronger than photo-neutron production, the same downscatter structure can be obtained by defining a composite group structure with N neutron groups arranged as groups 1 through N, and G photon groups attached as groups N+1 through N+G. The arrangement is shown schematically here. Photoneutron production could be included with fission.
A coupled set provides the convenience of solving both the
neutron and photon problems simultaneously with a single
transport code run. Because more than one photon is normally
produced by a neutron reaction (i.e., cascades), the
particle-balance absorption edit $\sigma_{ag}$ will usually be
large and negative for a coupled set. Special features are
available in some transport codes to allow the photon part of a
response to be computed separately from the neutron part.
The GOXS interface format includes a
parameter LNG to define the point between the
neutron and photon parts of the group structure.
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