Notes
Pieces of R-matrix theory written out properly — the physics first, then a worked example on a real reaction. Written for someone who wants to understand the machinery, not just use it.
R-matrix theory
When a reduced width stops being measurable
An R-matrix fit can describe the data beautifully and still contain a parameter the data cannot see at all. 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, and half the partial waves that 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 actually 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. Carbon-12(α,γ)oxygen-16 as the example.
June 2026 · 7 min read