Notes
R-matrix theory set out in full: the physics first, then a worked example on a real reaction. For readers who want to understand the machinery, not just use it.
To see the machinery at work, open one of the live R-matrix evaluations; for what the reactions do in stars, see the reaction atlas.
R-matrix theory
When a reduced width stops being measurable
An R-matrix fit can describe the data well and still contain a parameter the data cannot see. Why that happens, why the channel radius controls it, and how to tell before trusting an uncertainty.
July 2026 · 16 min read
Polarization observables in R-matrix theory
What a polarized beam measures, why it carries information no cross section can, and how the calculation adapts to the spins of the particles involved. Two worked examples: protons on carbon-12 and on nitrogen-15.
July 2026 · 15 min read
Identical particles: why half the partial waves vanish
When the projectile and the target are the same nucleus, quantum mechanics forbids asking which one went where. The consequences — a Mott cross section symmetric about 90°, and the disappearance of every odd partial wave — with carbon-12 on carbon-12 as the example.
June 2026 · 5 min read
Where radiative capture happens
Not every capture γ-ray is emitted by a compound nucleus. Two amplitudes — one from inside the channel radius, one from outside it — add coherently, and keeping them straight requires knowing exactly how a reaction is decomposed into pathways. With 12C(α,γ)16O as the example.
June 2026 · 7 min read