3.3. Reactions Troubleshooting

Most reaction-data problems are one of three things: the reaction was written in a form the library doesn’t use, the automatic library choice wasn’t what you assumed, or the target was natural where the library is isotopic (or vice versa).

“Reaction … not found or not unique”

Symptom: ValueError: Reaction 48TI(p,x) not found or not unique. for a target the library certainly evaluates.

Cause: the error covers two opposite situations — nothing matched (not found), or several things matched (not unique) — and each library organizes its reactions in one particular form. The common cases:

  • No product given to a residual-product library (“not unique”): '48TI(p,x)' alone matches every one of the ~200 products TENDL evaluates for that target. Specify the product.

  • A channel form the library doesn’t use (“not found”): 'natTI(p,n)48V' fails in the IAEA library — its entries are indexed by product, as (p,x) — while '90ZR(n,x)89ZR' fails in ENDF, which evaluates exclusive channels: '90ZR(n,2n)'.

  • The reaction genuinely isn’t in that library — the target isn’t evaluated, or the product is out of reach.

(Related, but not an error: a TENDL residual-product search without an isomer suffix quietly assumes the ground state — '86SR(p,x)86Y' means 86Yg, with a warning that prints once per library instance.)

Fix: ask the library what it has, with the loosest search that brackets your case:

>>> lb = ci.Library('tendl_p')
>>> print(lb.search(target='48TI'))
['48TI(p,x)44SCg', '48TI(p,x)44SCm1', ..., '48TI(p,x)48Vg', ...]

and pass one of the returned names verbatim to Reaction.

Which library am I actually using?

Symptom: results change when a colleague runs the “same” analysis; plots have a different energy range than expected; a cross section has no uncertainties even though you thought it should.

Cause: library='best' is a search order, not a merge: Curie takes the first library in the priority list (see Reaction How-to Guide) that contains a unique match. Two consequences are easy to miss. First, similar-looking reactions can come from different libraries — '226RA(n,2n)' resolves to ENDF/B-VIII.1, but the residual-product form '226RA(n,x)225RA' resolves to TENDL-2025 — with different grids, values and uncertainty availability. Second, the resolution can change over time, as data libraries are updated or the priority evolves between Curie versions.

Fix: print(rx.library.name) — always, before using a number — and pass the library explicitly (ci.Reaction('115IN(n,g)', 'irdff')) in any analysis you intend to reproduce or publish.

Natural target or isotopic target?

Symptom: 'natTI(p,x)48V' works by default (it resolves to the IAEA library) — but the same reaction with library='tendl_p' raises “not found”. Or: an isotopic cross section is a factor of several above the measured production on a natural foil.

Cause: the libraries differ. The IAEA monitor library evaluates natural targets (monitor foils are natural metal); TENDL’s residual-product libraries are isotopic only. And the two aren’t interchangeable: an isotopic cross section overstates production on a natural target by roughly the inverse of the isotope’s abundance, and a natural cross section understates the yield from an enriched target.

Fix: match the target to your actual material. For a natural target with only isotopic data available, build the abundance-weighted sum (the Reactions Worked Examples shows the ~6-line loop, including why some isotopes contribute nothing). For an enriched target with only natural data, there is no clean inversion — use the isotopic library.