Describe a problem

An actinv-spec-1 JSON problem contains data references, a material, a spectrum, and a schedule. The CLI, desktop, and Python interface use the same contract. Start with actinv new problem.json and edit that complete example.

Unknown fields are errors. Numbers must be finite; misspelt options are rejected instead of being ignored. See Problem specification for the full field reference.

Material

This fragment describes one gram of an iron/chromium mixture:

"material": {
  "mass_g": 1.0,
  "basis": "wt_percent",
  "composition": {"Fe": 90.0, "Cr": 10.0}
}

Composition keys may be natural elements such as Fe or explicit nuclides such as Fe56 and Ba137m1. A natural element and one of its explicit isotopes cannot appear in the same composition. Use explicit nuclides for elements without tabulated natural abundances.

Inventory, activity, and heat are reported per gram. mass_g scales total photon source quantities; it does not turn heat_W_per_g into total watts.

Spectrum

flux_per_group holds group-integrated flux in particles cm⁻² s⁻¹. It is not a spectral density per eV. The ordinary neutron library uses fispact-709 with 709 values. Standard proton, deuteron, and alpha libraries use fispact-162 with 162 values and temperature_K: 0.

descending: true means the values are supplied from highest energy to lowest. When total is present, ACTINV scales the vector to that total while preserving its shape. A positive total with an all-zero vector is rejected.

Custom spectra require increasing energy boundaries, one more boundary than group values, and a compatible activation library. The projectile, temperature, and group structure must match the library. Use transport import for supported transport tallies and review the source-rate normalization.

Irradiation and cooling

This fragment irradiates for five minutes and then cools for one hour:

"schedule": [
  {"dt": "5 min", "flux": 1.0},
  {"dt": "1 h", "flux": 0.0}
]

Each duration is incremental. The final time above is 3,900 seconds from the start, including the irradiation. flux multiplies the chosen spectrum; zero means cooling. Add separate cooling steps for the times you want reported.

Single-problem schedules can supply a per-step spectrum with the same group structure and count. Optional feed gives atoms s⁻¹ g⁻¹ and removal gives first-order rates in s⁻¹; these are described in the specification reference.

Data paths

Catalog references such as catalog:tendl-2025-neutron-709g resolve against ACTINV_DATA_DIR, or ./actinv-data by default. actinv new uses these references unless you supply --data-dir.

Literal relative paths in CLI problems use the current working directory. Desktop problems use Input base. Python Problem.from_file uses the problem’s directory; plain Python mappings use the current directory. JSON does not expand ~ to your home directory.

Use actinv doctor problem.json to inspect setup, then validate and run as shown in Your first calculation.