ReflectoFit

ReflectoFit fits specular X-ray reflectivity (XRR) curves of thin films: you give it a measured curve and a layer model with bounds, and it returns the thickness, roughness and scattering length density of each layer with their standard errors. It is a library and a command line.

The forward model is a float64 Parratt recursion compiled with numba, with Névot-Croce roughness. The instrument model adds Gaussian resolution (constant and dq/q terms), the beam footprint, Cu Kα₂, a 2θ offset, I0 and background. It agrees with refnx, a JAX implementation and the ReflectoLearn engine to 1e-10, and the resolution integral to within 1e-5 of a brute-force convolution, including at the critical edges of weakly absorbing films. A torch backend gives the same curves with gradients.

Fitting: least squares (optionally inside a trust window around the start), differential evolution, and losses on log R, raw R, Poisson deviance or χ², with AIC and BIC for the likelihood losses. MCMC, the staged refinement of ReflectoLearn and model selection follow.

ReflectoSim builds synthetic training data on this forward model, so a network trained on it and a fit of the same measurement use the same physics.

Install

Not on PyPI yet. From a clone, with uv:

git clone https://github.com/NaNIsaac/ReflectoFit
cd ReflectoFit
uv sync

First run

Simulate a curve of a 240 Å film on SiO₂ on Si with Poisson counts, then fit it from a wrong start:

uv run reflectofit calc --model examples/film.toml --counts 1e8 -o film.dat
uv run reflectofit fit film.dat --model examples/film_start.toml --loss poisson --counts-scale 1e8 -o fit

The fit prints each free parameter with its standard error and writes fit/result.json, fit/model.toml and fit/fitted.dat. The model file format, the command line and the Python API are in the README.

Conventions

Layers are listed from the top: ambient first, substrate last, and a layer's roughness belongs to the interface above it. SLD is in 1e-6 Å⁻², complex, with a positive imaginary part for absorption; lengths in Å, q in Å⁻¹, angles in degrees. The ambient must not absorb. These are pinned by tests written before the engine, and the reasons are in the design notes.