This working group aims to draft a spec for a standard file format to be used across X-ray and neutron reflectivity. This is an important goal to move towards interoperability across facilities.

Summary of the May 2020 workshop

The outcome of the discussions during the workshop can be summed up as

We want to stick to the principles whenever possible - and at the same time we are flexible and pragmatic and pay respect to traditions.

This attitude shows up in the agreement to use SI units, but to also allow Angstrom.

Acting in this spirit a broad support for developing two file formats could be observed:

  • The strict format who’s aim is to fulfil the principles of inter-operability and reusability, most likely using a NeXus-based hierarchical data format (HDF).
    This format should fulfil high scientific standards, it will allow future analysis software to do more than just fit R(q) curves and it should match the national data policies.

  • The pragmatic format, which should be easily human-readable (i.e. ASCII format with a YAML like structure) and contain the essential information to trace back its origin and some basics about the state of the data reduction. The standardised minimum content of this format can be expanded as seen appropriate by the user.
    This format is compatible with most actual analysis software. It is easy to understand and read by new users and thus unburdens beam line scientists. And it allows fast and easy comparison of results. After all, a reflectivity curve is expected as the outcome of a reflectivity measurement - however spoilt by experimental influences (e.g. resolution) it might be.

It is in the nature of the pragmatic approach that a lot of input has been given and example files were created as a basis for further discussions.

The main work for the (near) future will be to create a series of dictionaries describing the vocabulary used in either of the two file formats:

  • list of symbols and their definitions (e.g. alpha_i := angle of incidence on the sample surface …)
  • dictionary of key words (e.g. what is the meaning of reflectivity in the context of the reflectivity data file)
  • dictionary of standard(ised) data reduction steps (e.g. geometrical footprint correction)

It was recommended to use existing definitions from NeXus or canSAS whenever they fit. If theses are not in agreement with the common sense used in reflectometry experiments (e.g. the sample coordinate system) new, specialised definitions should be written in a similar style. It was also suggested to start with the essential quantities (again coordinate system, angles, momentum transfer, …) and to expand the dictionaries later when needed. These dictionaries are to be used for the reduced data. The very technical and highly instrument specific language describing the raw data should be used in the raw data file.

A series of tasks were defined and everyone is welcome to contribute there (as well as to the general discussions):

  • Draft a header for an ASCII file. (J. Stahn)
  • Draft a strict reflectivity file in NeXus format. (A. Nelson)
  • Contribute to the dictionaries.
  • Create an overview over the national data policies.
  • Define an identification scheme.
  • Try to implement the pragmatic file format in existing data reduction software and report on problems and successes.

Message board

Open to all to read, however, an account is required to post.

Members

  • Ruipeng Li (NSLS-II)
  • Brian Maranville (NIST)
  • Tim Snow (Diamond)
  • Jochen Stahn (PSI) – Chair