Methods and data
Sources
Nuclear decay data — half-lives, decay modes, photon energies and emission probabilities, beta maximum and mean energies with branch intensities, and alpha energies and intensities — are retrieved from the IAEA Nuclear Data Section's Livechart interface, which serves the evaluated nuclear structure data file (ENSDF). Photon mass attenuation coefficients and mass energy-absorption coefficients are taken from the NIST X-Ray Mass Attenuation Coefficients tables, covering 1 keV to 20 MeV, and are interpolated logarithmically in both energy and coefficient.
Both datasets are harvested once and committed to the source repository rather than fetched at runtime. Results therefore do not change between visits, the site does not depend on an external service being up, and the data version is fixed and auditable.
Gamma constants
The air kerma rate constant is computed as Γ = Σ Ei·yi·(µen/ρ)air(Ei) / 4π, summed over photon lines above the low-energy cutoff δ. The default δ is 20 keV, the conventional value used by published compilations. Because the sum is performed at calculation time rather than baked into the data, δ is adjustable in the shielding calculator.
Validation
The engine is covered by 48 automated tests that run on every change. Computed gamma constants are checked against published values for Cs-137, I-131, Cs-134, Co-60 and Ir-192 and agree within 2%. Specific activities for Co-60, Cs-137, Pu-239, U-238, Am-241, Sr-90 and H-3 agree within 1%. Beta maximum energies for fifteen nuclides and alpha principal energies for eight agree within 1% and 0.01% respectively. Attenuation coefficients are checked at five points against published values within 0.5%. Field rules of thumb are also tested: one curie of Co-60 at one metre computes to 1.29 R/h against the remembered 1.32, and Cs-137 to 0.326 against 0.33, the small deficit being the absence of buildup.
Analytic identities are tested rather than asserted — one half-life leaves exactly half, one half-value layer transmits exactly 0.5, doubling the distance quarters the dose rate, and quadrupling the count time improves the detection limit by exactly two.
Known limitations
- Point-source geometry in air. No room scatter, skyshine, source self-absorption or capsule.
- Attenuation without buildup is a narrow-beam result and underestimates behind thick shields.
- Buildup coefficients are not supplied; the Berger form accepts values you provide.
- Skin dose from surface contamination is not calculated.
- Molar mass is approximated by mass number — within 0.03% for every nuclide included.
- Decay chains assume a simple unbranched series.
- Beta range uses the Katz–Penfold empirical fit, accurate to roughly 10%.
- Bremsstrahlung yield uses a thick-target approximation and is an order-of-magnitude guide.
Corrections
If a value here disagrees with a source you trust, the disagreement is worth reporting — several figures in the test suite exist because a check found an error during development. Contact details are on the contact page.