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[Fwd: Re: New fast-track definitionproposal: _diffrn_radiation_wavelength_nominal]

  • To: Distribution list of the IUCr COMCIFS Core Dictionary Maintenance Group <coredmg@iucr.org>
  • Subject: [Fwd: Re: New fast-track definitionproposal: _diffrn_radiation_wavelength_nominal]
  • From: David Brown <idbrown@mcmaster.ca>
  • Date: Wed, 25 Nov 2009 15:17:48 -0500
Dear Colleagues,

Last September I circulated for fast track approvel a recommendation for adding a new item:                             


to the core dictionary (see below for details).  The intent was that this could be used for giving a nominal wavelength calculated e.g., from the paparmeters of a diffractometer.  It was proposed tht this might be needed for completeness even though it was not accurate enough for calculating  lattice parameters, etc.  This proposal was passed by the core CIR dictionary maintenance group by default (no one raised any objection).  When this proposal was subsequently presented for final approval to COMCIFS, Nick Spadaccini suggested a more elegant alternative, namely that by adding instead the item


we could loop the nomiinal wavelength with the  refined (and any other interesting) wavelengths.  This would be more informative and allow for future expansion.

An example of how this might be used is:

                1   1.23456   fundamental
                2   1.25      estimated

According to our rules, this proposal is being posted for comment to the coreCIF dictionary maintenance group for the next six weeks.  At the end of this time, if there are no unresolved issues, the core dictionary maintenance group will be deemed to have accepted the proposal.

David Brown


The proposal is to add the following item to the coreCIF dictionary:

2009-09-25 Proposal from the Nick Spadaccini on the COMCIFS dicsussion list to replace a withdrawn proposal.

  _name                      '_diffrn_radiation_wavelength_determination'
  _category                    diffrn_radiation_wavelength
  _type                        char
  _list                        both
  _list_reference            '_diffrn_radiation_wavelength_id'
       'Wavelength that is a fundamental property of matter e.g. MoK\alpha'

       'Estimated from secondary information e.g. monochromator angle or time of flight'

       'Based on refinement using a standard material with known cell parameters'

;              The method of determination of incident wavelength. Further information may be provided in _diffrn_radiation_special_details

The original proposal was as follows:
>> 2009-08-23 proposed by James Hester.
>> 2009-10-16 approved by the core Dictionary Maintenance Group under a fast
>>           track approval process.
> > 2009-11-25 proposal withdrawn in favour of  an alternative
>> 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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fn:I.David Brown
org:McMaster University;Brockhouse Institute for Materials Research
adr:;;King St. W;Hamilton;Ontario;L8S 4M1;Canada
title:Professor Emeritus
tel;work:+905 525 9140 x 24710
tel;fax:+905 521 2773

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