CanSAS Proposal NX1: For NXpolarizer

[smk78]

Oct 2024.

Background
CanSAS are considering proposals from a community-led sub-group for revisions to the NXcanSAS application definition to support Magnetic-SANS techniques.

The NXcanSAS proposals may be viewed here.

In drafting the proposals the sub-group noted that some NeXus Base Classes would benefit from additional structure. This issue is one of several detailing those additions.

CanSAS Proposal NX1

• That the NXpolarizer base class be granted the additional parameters transmission , transmission_empty , pressure , length , polarization_initial and decay_time.

Aside: As currently defined, NXpolarizer is very much predicated on a supermirror polarizer. Although it recognises a polarizer may be of type "3He", almost none of the current parameters would describe a 3He MEOP cell. In fact, it would be more sensible to deprecate the type "3He" and introduce either an umbrella type "hyperpolarization", or introduce three new types: "SEOP", "MEOP" and "DNP".

Note: NeXus has no NXanalyzer base class, it is presumed because it would be just be a copy of NXpolarizer with a different name. That works provided it can be made clear what role a given NXpolarizer instance in a file represents. At present this can only be inferred from the position of the device; ie, an analyzer must be after a polarizer. CanSAS Proposal NX4 would help to further resolve any ambiguity. Or, of course, NeXus could introduce an NXanalyzer base class!