2. Isotopes & Decay Chains

Curie provides two classes for working with radioactive decay. The Isotope class looks up nuclear data for a single isotope: masses, abundances, half-lives, the energies and intensities of its emissions (gammas, betas, alphas, conversion electrons), and dose rates. The DecayChain class does the bookkeeping of decay itself: starting from a parent isotope it builds the full chain of decay products, computes every member’s activity as a function of time — during production as well as decay — and can fit production rates or initial activities to measured data.

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225Ra during and after production: the daughter 225Ac grows in as the parent decays.

Workflow. Isotope needs no workflow — construct it and read off the data (see Isotope & Decay Chain How-to Guide). Decay-chain problems come in two directions:

  • Forward — you know (or assume) how much of an isotope was made, and want activities at later times: build the chain with an initial activity A0 or a production-rate history R, then call dc.activity(), dc.decays() or dc.plot().

  • Inverse — you measured decays (typically peaks in gamma-ray spectra) and want the production rate or activity that explains them: load the measurements with dc.get_counts() and fit with dc.fit_R() (for production) or dc.fit_A0() (for decay-only).

Both directions share one time convention: t = 0 is the end of production (the end of bombardment in an activation experiment), and all times are in the chain’s units.

See the Isotope & Decay Chain How-to Guide for each task in detail, the Decay Chain Worked Examples for both directions worked on real examples, and Isotope & Decay Chain Troubleshooting for the common pitfalls.

Uses and limitations. Isotope serves quick lookups (a half-life in sensible units, a table of gamma lines above some intensity) and provides the decay data that Spectrum, Calibration and DecayChain use internally. The decay data are compiled from ENSDF (via the IAEA LiveChart interface), with NUBASE2020/AME2020 closures and ENDF/B-VIII.1 fission yields.

DecayChain solves the Bateman equations exactly (see Radioactive Decay Chains), including chains with branching and same-half-life members, for one parent isotope at a time with a piecewise-constant production history. Production physics is not computed — the production rate is an input, which you can fit from measured counts, or estimate yourself from cross sections and particle fluxes (the Reactions pages cover the cross-section side).