Cross Section Data in SCONE

You do not type cross sections by hand — point aceLibrary to a processed library and list nuclides in Materials. This page explains what that library contains and how ZAID suffixes select tables. Blocks built from tutorial_nuclear_data_basic_testlib match that verify file line-for-line (testLib .03 and $SCONE_ACE). The ACE overview paragraph is commentary only. For a full machine-checked deck see Nuclear Data or Basics.

Nuclear cross sections form the foundation of Monte Carlo neutron transport simulations. These fundamental quantities describe how neutrons interact with different atomic nuclei, determining their behavior as they travel through materials. SCONE uses this data to calculate mean free paths, determine interaction types, and generate secondary particles with appropriate properties.

When a neutron travels through matter, cross section data guides every aspect of its journey. The data determines how far the neutron will travel before interacting, what type of interaction occurs (such as scattering or absorption), and the resulting changes in neutron energy and direction.

SCONE reads cross-section data from the nuclearData block. The handles section declares database connections. For continuous-energy neutron transport, use an ACE-format library via aceNeutronDatabase. Point aceLibrary to your ACE cross-section file (e.g., from JEF, ENDF/B, or JENDL processed with NJOY).

nuclearData {
  handles { ce { type aceNeutronDatabase; aceLibrary $SCONE_ACE; } }
}

SCONE primarily works with ACE (A Compact ENDF) format data, optimized for Monte Carlo calculations. This format efficiently organizes nuclear data into a structured form, including energy grids, reaction cross sections, and distribution data for secondary particles.

// ACE format description (conceptual)
// Header: ZAID.NN  nuclide name  library  temperature (as processed — must match your aceXS rows)
// e.g. 92235.03c ... (suffix + temperature come from the evaluation/processing route, not from this comment)
//
// ACE files contain: energy grids, cross sections (total, elastic, fission, etc.),
// angular distributions, energy distributions, and probability tables for URR.
// Process ENDF data to ACE using NJOY or similar codes.

Processing tools play a crucial role in preparing nuclear data for SCONE. The industry-standard NJOY code converts ENDF data to ACE format, handling Doppler broadening and thermal scattering law processing. SCONE expects ACE libraries in the format produced by such processing.

Nuclides are identified by ZAID (Z-A ID) plus a suffix (often two digits and sometimes a letter such as c in raw ACE listings). The format is ZZZAAA.NN: ZZZ is the atomic number (padded to 3 digits), AAA is the mass number (padded to 3 digits), and the suffix selects which processed table to use from your aceLibrary. The mapping from suffix to Kelvin is defined by how that library was built—check its index file or processing notes, not this guide.

// ZAID.NN: ZZZAAA.NN — ZZZ=atomic no., AAA=mass no., NN=table index in YOUR aceLibrary (meaning is library-specific)
nuclearData {
  handles { ce { type aceNeutronDatabase; aceLibrary $SCONE_ACE; } }
  materials {
    Water {
      temp 600;
      composition { 1001.03 6.67e-02; 8016.03 3.33e-02; }
    }
  }
}

Material compositions are defined in the nuclearData.materials block. Each material specifies a temperature and a composition of ZAID.NN pairs with number densities. The ZAID values must match nuclides available in the ACE library referenced in handles.

nuclearData {
  handles { ce { type aceNeutronDatabase; aceLibrary $SCONE_ACE; } }
  materials {
    UO2-31 {
      temp 600;
      composition { 92235.03 4.50e-02; }
    }
  }
}

Temperature significantly influences neutron cross sections, particularly in the resonance region. SCONE uses pre-processed ACE data at discrete tables. The ZAID suffix selects one of those tables from your library file. Ensure your library actually contains tables appropriate for your material temp values.

Maintain consistency across data libraries. Use ZAID indices that match your material temperatures. Verify that all nuclides in your material compositions exist in the ACE library. Document the library path and version for reproducibility.