Connecting NXdata to NXdetector

This is an attempt to summarize the various proposals that have been discussed on this page. Proposal 1 is, I believe ([Ray Osborn] (Ray_Osborn.html “wikilink”)), the original proposal when NeXus was first designed (although we didn’t link the data item itself as Freddie has sensibly suggested). If anyone thinks that the remaining proposals have been incorrectly summarized, please let the NeXus committee mailing list know as soon as possible.

Proposal 1

All the items within an NXdata group (data, errors, and axes) are stored as links to parent items stored in their logical location (NXdetector, NXsample, etc). The absolute address of the parent item is stored as an attribute on the data, so that other information can be conveniently located. This is the only proposal that addresses the issue of non-NXdetector axes.

This method implies a one-to-one correspondence of NXdata and NXdetector groups; the names do not need to match, though they could.

Example:

<NXsample>
   <magnetic_field link="entry/sample">
</NXsample>
<NXinstrument>
   <NXdetector>
      <data axes="time_of_flight:magnetic_field" link="/entry/instrument/detector">
      <time_of_flight link="entry/instrument/detector">
   </NXdetector>
</NXinstrument>
<NXdata>
   <data axes="time_of_flight:magnetic_field" link="/entry/instrument/detector">
   <time_of_flight link="entry/instrument/detector">
   <magnetic_field link="entry/sample">
</NXdata>

Proposal 2

There is a one-to-one correspondence between NXdata groups and the corresponding NXdetector group, which should have the same name to provide a logical association.

Example:

<NXinstrument>
   <NXdetector name="Bank1">
      <data axes="polar_angle:time_of_flight">
      <time_of_flight>
      <polar_angle>
   </NXdetector>
   <NXdetector name="Bank2">
      <data axes="polar_angle:time_of_flight">
      <time_of_flight>
      <polar_angle>
   </NXdetector>
</NXinstrument>
<NXdata name="Bank1">
   <data axes="polar_angle:time_of_flight">
   <time_of_flight>
   <polar_angle>
</NXdata>
<NXdata name="Bank2">
   <data axes="polar_angle:time_of_flight">
   <time_of_flight>
   <polar_angle>
</NXdata>

Proposal 3

There is a one-to-one correspondence between NXdata groups and the corresponding NXdetector group. The NXdata group is stored within its respective NXdetector group and linked to the NXentry.

Example:

<NXinstrument>
   <NXdetector>
      <NXdata>
         <data axes="polar_angle:time_of_flight">
         <time_of_flight>
         <polar_angle>
      </NXdata>
   </NXdetector>
</NXinstrument>
<NXdata (link only)>
   <data axes="polar_angle:time_of_flight">
   <time_of_flight>
   <polar_angle>
</NXdata>

Proposal 4

There can be multiple NXdata groups for a single NXdetector group or multiple NXdetector groups for a single NXdata item (e.g. to cope with groups of different types of detectors). Detector ID’s are used to provide the link between the two.

Example:

<NXinstrument>
   <NXdetector>
      <detector(1:N)>
      <polar_angle(1:N)>
   </NXdetector>
</NXinstrument>
<NXdata name="Bank1">
   <data(1:i) axes="detector">
   <detector(1:i)>
</NXdata>
<NXdata name="Bank2">
   <data(i+1:N axes="detector">
   <detector(i+1:N)>
</NXdata>

or

<NXinstrument>
   <NXdetector name="Bank1">
      <detector(1:i)>
      <polar_angle(1:i)>
   </NXdetector>
   <NXdetector name="Bank2">
      <detector(i+1:N)>
      <polar_angle(i+1:N)>
   </NXdetector>
</NXinstrument>
<NXdata>
   <data(1:N) axes="detector">
   <detector(1:N)>
</NXdata>

Criteria of a Good Solution

A proper solution should address the following:

• Linking - should it be used as a method of saving space or carry relational information (associating things).

• Grouping information into NXdetectors in a logical manner. For example, an instrument may want to have a NXdetector for each bank, or one for each panel, irrespective of how the NXdata is grouped.

• NXdata must be rectangular ( lxm or lxmxn so data that would be grouped together in the analysis needs to be split up to conform to the standard. How should the data be associated with each other.

• How do you properly deal with multi-dimensional detectors, i.e. single ended tubes AND linear position sensitive detectors AND area detectors all used in the same measurement.

Update 01/2015

This page is very old. I cannot remember that this has been decided upon. The current usage though is along the lines of proposal 2: Multiple detectors give rise to multiple NXdetector and NXdata groups with preferably the same name. NXdata is supposed to contain links to the relevant data items required for a default plot. There can be additional NXdata groups for special purposes. See also the description of NXsubentry in the NeXus manual.

The above is valid for raw data NeXus files. In processed data NeXus files NXdata has a different meaning and contains the actual resulting processed data.