v0.7.0 — real Born excitation, parameter-free, sub-keV CSDA closed
This release came out of a Geant4-DNA expert review of the project. It addresses
the three top concerns at once, and the headline is a physics win where it matters
most: the low-energy end of the track.
The audit (reviewer #1: physics-list consistency)
A source audit (E29) settled whether the cascade and chemistry were validated
against the same Geant4 physics list. They are: both dnaphysics (cascade
oracle) and chem6 (chemistry oracle) register G4EmDNAPhysics_option2 —
no seam. And for option2, G4EmDNABuilder never enables the Emfietzoglou
models (the energy limit is 0; only option4 enables them), so the reference
excitation is Born, across the whole electron range.
The fix (reviewer #2: remove the σ_exc fudge)
The codebase had been approximating Born excitation by flat-scaling an
Emfietzoglou table (SIGMA_EXC_SCALE = 0.39). But the Emfietzoglou/Born ratio is
energy-dependent (~2.5× at keV, ~10× at ~10 eV), so one scalar left
low-energy secondaries over-excited ~4×. v0.7.0 loads the real
sigma_excitation_e_born cross section and removes the scalar — the
track-structure physics is now parameter-free (with RECOMB_BOOST already
gone since v0.5.0).
The result (reviewer #3: the sub-keV CSDA)
| 100 eV | 300 eV | 500 eV | 1 keV | 10 keV | |
|---|---|---|---|---|---|
| before (scaled-Emf) | 0.782× | 0.852× | 0.894× | 0.933× | 0.997× |
| v0.7.0 (Born) | 0.956× | 0.986× | 0.994× | 0.987× | 0.997× |
The chronic sub-keV deficit — the project's weakest spot for its whole history,
and the energy range where track-structure codes earn their keep (the track ends,
the dense terminal clusters break DNA) — is closed. All 8 energies are now
0.956–1.005×. Cascade ions 0.937→0.942×; chemistry RMS flat at 7.0% (the G(H)
overshoot is gone); energy conservation 99.89%; primary bit-exact; 46/46 tests.
Honest notes
- The SSB indirect/direct ratio drifted 2.72→3.26 with the new physics.
P_indirect
is a calibrated fit; we report the drift honestly rather than re-tune it to
the band — it demonstrates exactly why we treat DNA damage as methodology, not
validated absolute physics. - Scope: electrons, 100 eV–20 keV, low LET. Protons/heavier ions and realistic
chromatin geometry are future work.