macs

Command-line tool to compute Maxwellian-Averaged Cross Sections (MACS) for astrophysical nucleosynthesis studies. Cross-section data is fetched live from the IAEA EXFOR nuclear database.

Physics background

The MACS at thermal energy $kT$ is defined as:

$$\langle\sigma v\rangle / v_T = \frac{2}{\sqrt{\pi}} \frac{\mu^2}{(kT)^2} \int_0^\infty \sigma(E), E, e^{-\mu E / kT}, dE$$

where $\mu = A/(1+A)$ is the reduced mass in atomic mass units and $E$ is the centre-of-mass energy. The integral is evaluated numerically with the trapezoidal rule over the energy grid returned by EXFOR.

Requirements

• C++23 compiler (GCC 14+ or Clang 18+)
• CMake 3.20+
• vcpkg (dependencies are managed via vcpkg.json)

Dependencies installed automatically by vcpkg:

Package	Purpose
cpr	HTTP client
nlohmann-json	JSON parsing
spdlog	Logging
bfgroup-lyra	CLI argument parsing
doctest	Unit testing

Build

export VCPKG_ROOT=/path/to/vcpkg

cmake -S . -B build -G "Ninja Multi-Config" \
  -DCMAKE_TOOLCHAIN_FILE="$VCPKG_ROOT/scripts/buildsystems/vcpkg.cmake"

cmake --build build --config Release

Install

cmake --install build --config Release
# or to a custom prefix
cmake --install build --config Release --prefix ~/.local

The binary is installed as macs-run.

Usage

macs-run [options]

  -e, --element <nuclide>      Nuclide to query, e.g. Er-166 (repeatable)
  -l, --library <lib>          Nuclear data library, e.g. ENDF/B-VIII.1 (repeatable)
  -t, --temperature <keV>      Temperature in keV for MACS computation (repeatable)
  -r, --reaction <reaction>    Reaction type [default: N,G]
      --xs                     Print cross-section table (energy vs sigma) instead of MACS
  -v, --verbose                Enable debug logging to stderr
  -h, --help                   Show this help

Nuclides must be in Symbol-A format (e.g. Er-166, Fe-56, Au-197). Logs and errors go to stderr; data goes to stdout — safe to redirect or pipe.

Examples

MACS for Er-166 at stellar temperatures:

macs-run -e Er-166 -l ENDF/B-VIII.1 -t 8 -t 25 -t 30 -t 90

# Er-166 ENDF/B-VIII.1 N,G
# temperature[keV]	MACS[mb]
8.0	1199.056606
25.0	665.818215
30.0	606.106442
90.0	324.175037

Compare multiple libraries:

macs-run -e Er-166 -l ENDF/B-VIII.1 -l JEFF-4.0 -l JENDL-5 -t 30

# Er-166 ENDF/B-VIII.1 N,G
# temperature[keV]	MACS[mb]
30.0	606.106442
# Er-166 JEFF-4.0 N,G
# temperature[keV]	MACS[mb]
30.0	659.520829
# Er-166 JENDL-5 N,G
# temperature[keV]	MACS[mb]
30.0	681.549681

Compare multiple nuclides:

macs-run -e Er-164 -e Er-166 -e Er-168 -l ENDF/B-VIII.1 -t 30

Dump raw cross-section data (no temperature needed):

macs-run -e Er-166 -l ENDF/B-VIII.1 --xs

# Er-166 ENDF/B-VIII.1 (N,G)
# energy[eV]	cross_section[barn]	d_cross_section[barn]
1.000000e-05	1.234567e+03	0.000000e+00
...

Pipe to a file or plotting tool:

macs-run -e Er-166 -l ENDF/B-VIII.1 --xs > er166_xs.tsv
macs-run -e Er-166 -l ENDF/B-VIII.1 -t 30 2>/dev/null | gnuplot ...

Tests

cmake --build build --config Debug
ctest --test-dir build --build-config Debug --output-on-failure

Integration tests (require network access) are skipped by default:

./build/tests/Debug/macs-tests --test-suite="*integration*"

CI

Automated builds and tests run on GitHub Actions on every push and pull request to main. See .github/workflows/ci.yml.