Nuclear data

Nuclide identifiers, name dialects, and data sources in Nucleide.

Nucleide carries a small, self-contained set of nuclear reference data and a canonical nuclide identifier that can be translated into the naming dialects used by MCNP, Serpent, FLUKA, and other codes.

Canonical nuclide identifier

Every nuclide is stored as a single u32 called the nucid:

\text{nucid} = (Z \times 1000 + A) \times 10000 + S,

where:

$Z$ is the atomic number,
$A$ is the mass number,
$S$ is the metastable state index ( $0$ = ground state).

Examples of the nucid layout.
Nuclide	Z	A	S	nucid	zzaaam
H-1	1	1	0	10010000	10010
U-235	92	235	0	922350000	922350
Am-242m	95	242	1	952420001	952421
Ba-137m	56	137	1	561370001	561371

Where this lives in the code

The identifier and dialect converters are in crates/nuclei/src/lib.rs and crates/nuclei/src/dialects.rs. Data tables live in crates/nuclei/src/data/.

Name dialects

Nucleide converts between several legacy code conventions.

Examples of name dialect conversions for U-235 and Am-242m.
Dialect	U-235	Am-242m
GNDS / canonical	U235	Am242_m1
MCNP ZAID	92235	95242
zzllaaam	92-U-235	95-Am-242m
Serpent	U-235	Am-242m
FLUKA	235-U	— (no entry)
NIST	235U	242Am
Cinder	2350920	2420951
ALARA	u:235	am:242
SZA	92235	1095242

Metastable quirks

MCNP swaps the meanings of Am-242 and Am-242m: 95242 maps to the metastable state and 95642 maps to the ground state. FLUKA names are looked up in a vendored table and only a subset of isotopes has an explicit entry; Am-242m is not representable.

Serpent and zzllaaam emission use m for state 1 (PyNE-compatible) and extended letters (n, o, …) for higher states; parsing accepts both cases.

Data sources

Nucleide embeds three reference tables at compile time and parses them lazily on first access.

Atomic masses

Source: AME2020 atomic mass evaluation (Huang et al., Chinese Physics C 45, 030002/030003, 2021)^huang-2021^wang-2021.
Coverage: 3,557 ground-state nuclides with $Z \geq 1$ .
File: crates/nuclei/src/data/ame2020.tsv.
Units: unified atomic mass units (u).

Natural abundances

Source: standard isotopic compositions from the ENDF/B-VIII.0 evaluation^brown-2018, expressed as fractions.
Coverage: 289 naturally occurring nuclides, including the isomer Ta-180m.
File: crates/nuclei/src/data/natural_abundance.tsv.
Units: fraction in $[0, 1]$ .

Half-lives

Source: ENDF/B-VIII.0 decay evaluations (IAEA / BNL-NNDC).
Coverage: 3,561 radionuclides; stable nuclides are absent.
File: crates/nuclei/src/data/half_life.tsv.
Units: seconds.

Derived quantities

Decay constant

From the half-life $t_{1/2}$ :

\lambda = \frac{\ln 2}{t_{1/2}}.

Q-values from atomic masses

Nucleide computes Q-values using AME2020 atomic masses and the neutron mass constant:

Constants

$1\,\mathrm{u}\,c^2 = 931.49410242\,\mathrm{MeV}$ and $m_n = 1.00866491595\,\mathrm{u}$ .

Neutron radiative capture

Q_{n,\gamma} = \big[m(Z,A) + m_n - m(Z, A+1)\big] \, c^2.

Example: H-1 $(n,\gamma)$ gives $Q \approx 2.2246\,\mathrm{MeV}$ .

Alpha decay

Q_\alpha = \big[m(Z,A) - m(Z-2, A-4) - m(^4\mathrm{He})\big] \, c^2,

with $m(^4\mathrm{He}) = 4.00260325413\,\mathrm{u}$ .

Assumptions and limitations

Ground-state masses only: metastable-state ids have no separate mass entry; query the ground-state id.
Natural elements are unrepresentable: a bare symbol such as "U" cannot be stored as a NuclideId because $A \geq Z$ is required.
Electron binding neglected: capture Q-values use neutral atomic masses but do not correct for electron binding differences.
No ENSDF decay data: detailed decay schemes beyond half-life and branching ratios are out of scope.

Adjacent data capabilities found in PyNE and OpenMC that Nucleide does not embed:

Decay schemes: PyNE derives decay modes, branching ratios, state energies, and radiation spectra from ENSDF; OpenMC parses full decay modes from ENDF files. Nucleide carries only half-lives.
Fission product yields: PyNE exposes WIMSD and NDS yield tables; OpenMC reads yields into depletion chains.
Q-values and energy release: PyNE’s q_val and OpenMC’s FissionEnergyRelease go beyond the capture/alpha formulas here.
Reaction and scattering data: cross sections, thermal S(α,β), and scattering lengths are available in both codes but are outside Nucleide’s scope.
Natural elements: PyNE and OpenMC accept bare element symbols and compute atomic weights from abundances; Nucleide requires a mass number.
Living references: NuDat 3 and the IAEA LiveChart track structure and decay data between ENDF releases.