New fast-track definition proposal: _diffrn_radiation_wavelength_nominal
James Hester
jamesrhester at gmail.com
Tue Nov 3 02:52:27 GMT 2009
Dear COMCIFS members: the following fast track proposal for a minor
change to the coreCIF dictionary has been approved by the core
Dictionary Maintenance Group after a 6-week comment period. As per
the recently approved fast track process, it is now open for comments
from COMCIFS for a further 6 week period.
James.
----------------------------------------------------------------------------------------------
2009-08-23 proposed by James Hester.
2009-10-16 approved by the core Dictionary Maintenance Group under a fast
track approval process.
Explanation
===========
The wavelength used for a measurement defines the
length scales used in the crystallographic unit cell, so it is
important that the quality of the number used for wavelength is
understood. When the bulk of experiments came from lab X-ray sources,
the wavelength value was an accurate and precise quantity that could
be relied on. In contrast, synchrotron and neutron sources have
comparatively poorly-defined wavelengths, subject to systematic errors
(for example, changes in the monochromator d-spacing due to
temperature effects, and angular offset errors). Many facilities do
not attempt to characterise the accuracy of such derived wavelength
values. Instead users may (or may not) refine against a standard
compound in order to recover an accurate wavelength value. I therefore
propose a separate tag for those wavelength values that are nominal,
that is, derived from positioning of optical elements rather than
proper standards. This removes ambiguity in determining whether or
not a wavelength has been obtained from refinement against a standard,
or is simply a nominal value based on beamline optics.
A new DDL1 item
data_diffrn_radiation_wavelength_nominal
_name '_diffrn_radiation_wavelength_nominal'
_category diffrn_radiation_wavelength
_type numb
_list both
_list_reference '_diffrn_radiation_wavelength_id'
_enumeration_range 0.0:
_units A
_units_detail 'angstroms'
_definition
; The incident radiation wavelength in Angstroms
calculated from secondary information,
for example monochromator angle or time of flight.
If the wavelength has been determined
using some type of standard,
_diffrn_radiation_wavelength should be used
;
A change in definition of an existing item. The current
definition reads 'The radiation wavelength in angstroms'.
data_diffrn_radiation_wavelength
_name '_diffrn_radiation_wavelength'
_category diffrn_radiation_wavelength
_type numb
_list both
_list_reference '_diffrn_radiation_wavelength_id'
_enumeration_range 0.0:
_units A
_units_detail 'angstroms'
_definition
; The radiation wavelength in angstroms as
determined from measurements using standards,
for example an X-ray emission line,
or when a refinement based on a standard
material has been carried out.
The details of such a refinement should be
recorded in the _diffrn_radiation_special_details
item.
;
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