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Re: New fast-track definition proposal:_diffrn_radiation_wavelength_nominal

I disagree. Cu K alpha 1 has been calibrated directly from primary standards. Anything done at a synchrotron or neutron source, however,...

On Nov 4, 2009, at 11:15 AM, kaduk@polycrystallography.com wrote:

While I am sympathetic to the rationale, I'm not sure that this  
suggestion accomplishes very much.  The lattice parameters of the  
standard material depend on the wavelength NIST (or somebody else)  
used to determine them.  Perhaps all of the wavelengths in normal  
crystallographic work are "nominal"?

Jim Kaduk

Quoting James Hester <jamesrhester@gmail.com>:

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.


2009-08-23 proposed by James Hester.
2009-10-16 approved by the core Dictionary Maintenance Group under a fast
          track approval process.

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

    _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'
;              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'.

    _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'
;              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

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