What is meant by "diffraction experiment" in `DIFFRN`?

[rowlesmr]

What is the scope and intent of the phrase "diffraction experiment" in the category description of DIFFRN?

If I collect a single diffractogram from a specimen, I can see that that is a single diffraction experiment. If I then collect another diffractogram from the same specimen using the same instrument and under the same conditions, is that the same diffraction experiment (ie able to be identified by the same _diffrn.id)?

If I collect a single diffractogram from many unrelated specimens (so I have many diffractograms) using the same instrument configuration and data collection regime, is that a single diffraction experiment (ie able to be identified by the same _diffrn.id)? What if these data were collected over multiple years (with tube intensity loss / tube changes, realignments, and everything else that normally happens)?

If I collect many diffractograms from a single specimen (eg in situ/operando, pressure, temperature, magnetic field, atmosphere, voltage... changing with time, or even just the specimen aging or reacting with a static atmosphere), is that a single diffraction experiment? (it seems that if I change something, like pressue (which has an associated data item), they are different experiments, but (technically) if I just let the specimen react by itself, its the same experiment)?

[jamesrhester]

When anything in the DIFFRN category gets a new value, then it must be a different diffraction experiment (_diffrn.id), by virtue of the fact that _diffrn.id is a key data name. Currently this means changes in ambient conditions or (single crystal) sample. To this I would add that changing the instrument or detector would also involve a new _diffrn.id but that is not as obvious from the dictionary currently.

So for powder diffraction, this would suggest that recollecting from the same sample under the same conditions is the same _diffrn.id, but changing the specimen (even to one with the same nominal composition) means a new _diffrn.id.

To capture this in pdCIF, we would add a new data name in the DIFFRN category corresponding to specimen identifier, to show that there is one sample per diffraction experiment.

[rowlesmr]

When anything in the DIFFRN category gets a new value

Does this also include categories that are subcategories of DIFFRN, like DIFFRN_ATTENUATOR?

[jamesrhester]

This would be true for any categories that are formally 'child' categories. This list has to be reassessed, I think, because DIFFRN_SOURCE does not need to be restated for every change in wavelength or temperature, and nor should we need to restate the list of attenuators. Of course, changing attenuator would change the _diffrn.id. So that's a little project we need to undertake at some point.