Return discrete lines exactly in lin-lin tabular energy sampling - #4069
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Return discrete lines exactly in lin-lin tabular energy sampling#4069dallonby wants to merge 1 commit into
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ContinuousTabular, KalbachMann, and CorrelatedAngleEnergy all guard the histogram interpolation branch with k >= n_discrete, returning a sampled discrete line's energy exactly — but none of them guard the lin-lin branch. A discrete hit leaves the walk with r1 < c_k, so the lin-lin inversion runs with a negative (r1 - c_k) and shifts the sampled energy below the line (bounded by the max(0, ...) clamp at E_l_k - p_l_k/frac). In ENDF/B-VIII.0 the discrete-lines-with-lin-lin combination appears in 19,913 incident-energy tables — all secondary-photon distributions (discrete gammas from (n,2n), (n,3n), (n,n') continuum and similar), so coupled neutron-photon transport samples smeared gamma lines. No neutron-product table in the library uses the combination, so neutron-only results are unaffected. The fix mirrors the existing histogram guard in all three samplers; for Kalbach-Mann the r/a parameters use the line's tabulated values, which is what the interpolation reduces to at E_out = E_l_k. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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CHI_DELAYED -> PR openmc-dev#4067; discrete-line lin-lin sampling (with the ENDF/B-VIII.0 photon-table census) -> issue openmc-dev#4068 + PR openmc-dev#4069; MG density_mult tally asymmetry -> issue openmc-dev#4070. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Description
Fixes #4068.
ContinuousTabular,KalbachMann, andCorrelatedAngleEnergyguard their histogram interpolation branch withk >= n_discrete(a sampled discrete line is returned at exactly its tabulated energy), but not the lin-lin branch — a discrete hit leaves the CDF walk withr1 < c_k, so the lin-lin inversion runs with a negative(r1 - c_k)and shifts the sampled energy below the line.This PR mirrors the histogram guard in all three samplers. For Kalbach-Mann, the discrete branch takes
r[k]/a[k]directly — identical to what the parameter interpolation reduces to atE_out = E_l_k, so no behavior change beyond the energy fix.Impact (measured against the official ENDF/B-VIII.0 HDF5 library; details in #4068): the discrete-lines-with-lin-lin combination appears in 19,913 secondary-photon tables (discrete gammas of (n,2n)/(n,3n)/(n,n′)-continuum reactions across ~half the nuclides) and in zero neutron-product tables — so coupled neutron–photon runs currently sample smeared gamma lines, while neutron-only results are unaffected by this change.
Found while porting the transport engine to Apple Silicon GPUs (dallonby/openmc-metal).
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