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Re: New fast-track definition proposal:_diffrn_radiation_wavelength_nominal

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.
>
> 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.
>  ;
>
> --
> T +61 (02) 9717 9907
> F +61 (02) 9717 3145
> M +61 (04) 0249 4148
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>



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