New fast-track definition proposal: _diffrn_radiation_wavelength_nominal
kaduk at polycrystallography.com
kaduk at polycrystallography.com
Wed Nov 4 21:07:03 GMT 2009
But NIST keeps changing their minds about what the value is; check the
certificates for the various SRMs.
Quoting "Brian H. Toby" <Brian.Toby at 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 at 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 at 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 at 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
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