Refjax - Lightweight jit and grad compatible reflectometry in jax

This package provides lightweight, low-level access to neutron reflectometry kernels in jax.

The provided kernels are all jit and grad compatible and easy to integrate with your favourite optimisation or uncertainty quantification library for fast parameterisation of stratified reflectometry models. Several examples for popular libraries are available in the examples folder.

Design pattern

Reflectometry models are created by creating reflectivity kernels. The call to the refjax.kernel function takes three arguments.

kernel = refjax.kernel(model, smearing, jit=True)

Currently implemented models:

• refjax.models.abeles Abeles matrix formalism for stratified media

Currently implemented smearing kernels

• refjax.smear.no_smear No smearing kernel (default)
• refjax.smear.constant_gaussian Constant gaussian smearing at every Q value
• refjax.smear.pointwise_gaussian Pointwise Gaussian approximation to the true smearing kernel (requires FWHM resolution measurements)

Once a kernel has been created, it is called with the model parameters and Q (and FWHM values for smeared kernels).

R = kernel(params, Q, FWHM) # fast beacuse jit

Check out the examples folder for more advanced usage and examples of parameter identification schemes.

Having trouble? Feature request? Raise an issue on the GitHub page.