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Re: New fast-track definition proposal:_diffrn_radiation_wavelength_nominal

But NIST keeps changing their minds about what the value is; check the  
certificates for the various SRMs.

Quoting "Brian H. Toby" <Brian.Toby@ANL.gov>:

> 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.
>>>
>>> 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
>>> _______________________________________________
>>> comcifs mailing list
>>> comcifs@iucr.org
>>> http://scripts.iucr.org/mailman/listinfo/comcifs
>>>
>>
>>
>>
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>
> ********************************************************************
> Brian H. Toby, Ph.D.                            office: 630-252-5488
> Senior Physicist/Materials Characterization Group Leader
> Advanced Photon Source
> 9700 S. Cass Ave, Bldg. 433/D003             work cell: 630-327-8426
> Argonne National Laboratory         secretary (Marija): 630-252-5453
> Argonne, IL 60439-4856         e-mail: brian dot toby at anl dot gov
> ********************************************************************
> "We will restore science to its rightful place, and wield   
> technology's wonders... We will harness the sun and the winds and  
> the  soil to fuel our cars and run our factories...  All this we can  
> do.  All this we will do."
>
>



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