Physics Reference
NEREIDS implements exact SAMMY physics for neutron resonance imaging. This chapter is a navigation guide to the rustdoc API documentation, not a standalone physics textbook.
All implementations reference specific sections of the SAMMY manual and SAMMY Fortran source files. See the rustdoc for each module for detailed equations and citations.
Cross-Section Formalisms
| Formalism | ENDF LRF | Module | SAMMY Reference |
|---|---|---|---|
| Reich-Moore | LRF=3 | reich_moore | Manual Sec 2, rml/ |
| Single-Level Breit-Wigner | LRF=1,2 | slbw | Manual Sec 2, mlb/ |
| R-Matrix Limited | LRF=7 | rmatrix_limited | Manual Sec 2 |
| Unresolved Resonance Region | LRU=2 | urr | Hauser-Feshbach |
The penetrability and
channel modules provide the underlying
nuclear physics: hard-sphere phase shifts, penetrability factors, wave numbers,
and statistical spin weights.
Broadening Models
Doppler Broadening
Free Gas Model (FGM) convolution accounting for thermal motion of target nuclei.
- Module:
doppler - SAMMY reference:
dop/module, manual Sec 3.1 - Key function:
doppler_broaden()using psi/chi auxiliary functions on an adaptive grid
Resolution Broadening
Instrument resolution broadening from flight-path uncertainty, timing jitter, and moderator pulse width.
- Module:
resolution - SAMMY reference:
convolution/module, manual Sec 3.2 - Supports: Gaussian convolution, Gaussian + exponential tail, tabulated resolution functions
Transmission Model
Beer-Lambert transmission: T(E) = exp(-sum_i n_i sigma_i(E))
Where n_i is the areal density (atoms/barn) and sigma_i(E) is the broadened total cross-section for isotope i.
- Module:
transmission - SAMMY reference:
cro/,xxx/modules, manual Sec 2, Sec 5 - Handles multi-isotope samples with shared Doppler temperature (one global temperature parameter, optionally fitted jointly with densities)
Fitting Engines
Levenberg-Marquardt
Standard nonlinear least-squares minimization for Gaussian-distributed data.
- Module:
lm - SAMMY reference:
fit/module, manual Sec 4 - Parameters: areal densities with optional bounds, optional temperature fitting
Poisson KL Divergence
Maximum-likelihood fitting for low-count data where Gaussian statistics break down.
- Module:
poisson - Reference: TRINIDI approach (
trinidi/reconstruct.py) - Uses bounds-based preconditioning for joint density + temperature fits
ENDF Nuclear Data
Resonance parameters are sourced from the ENDF/B library via the IAEA API:
- Module:
retrieval-- download and cache - Module:
parser-- parse ENDF-6 File 2 - Module:
resonance-- data structures
Supported libraries: ENDF/B-VIII.0, ENDF/B-VIII.1, JEFF-3.3, JENDL-5.
Further Reading
- SAMMY User's Guide (ORNL/TM-9179/R8)
- ENDF-6 Formats Manual (BNL-203218-2018-INRE)
- ENDF/B-VIII.0 (Nuclear Data Sheets, 2018)