5.2.9. Adjoint (backward-in-time) mode

Table 5.2.85 iadjoint

Value

Explanation
(D=0)
Option for adjoint mode.
=0
Turn off adjoint mode (conventional forward transport).
=1
Use adjoint mode for photon transport (see “/phits/utility/adjoint/adjoint”).
=2
Use adjoint mode for charged-particle transport (see “/phits/utility/adjoint/charged”)

Adjoint mode is a calculation mode that performs particle transport in backward time. Adjoint mode calculations can be more efficient than conventional forward mode calculations for transport problems where the source region is much larger than the tally region. The source and tally regions are switched in an adjoint mode calculation compared to the corresponding forward mode calculation. Adjoint photons start within the tally region of the forward calculation and are tallied in its source region. The PHITS adjoint mode support the transport of only photons below 3 MeV (iadjoint=1) and charged-particles excluding electrons and positrons (iadjoint=2).

The adjoint mode for photon (iadjoint=1) should be used in tandem with [t-adjoint]. Adjoint photons undergo reserve interactions with matter. The interactions are modeled for continuous energies up to 3.0 MeV. See reference [1] for more details. When using the adjoint mode, you must also set these parameters as follows:

dmax(14) = 3.0
negs = -1
file(7) = file(1)/data/xsdir.adj

The geometry of the [source] in an adjoint mode calculation should match the geometry of the tally region in the corresponding forward mode calculation. Within the adjoint mode [source] section, set proj=photon and define a uniform source energy distribution with minimum and maximum energies covering the range of energies you want to tally. In adjoint mode, the minimum and maximum energies of the e-type mesh in [t-adjoint] should match the minimum and maximum energies set in the adjoint [source] section. Use e-smin and e-smax in [t-adjoint] to set the minimum and maximum energies, respectively, of the real photon source.

The adjoint mode for charged particles (iadjoint=2) should be used in tandem with conventional tallies such as [t-track] and [t-cross]. In this mode, charged particles gain the energy equal to what they should lose in the forward mode, as if they had negative stopping power. There is no energy restriction in this mode, but nuclear reaction cannot be considered. The maximum flight mesh size should be small in this mode, and thus, deltm should be set to a small number (less than 0.01) while confirming that changing this value will not significantly change the results.