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Fast track proposal approved.

  • To: Distribution list of the IUCr COMCIFS Core Dictionary Maintenance Group <coredmg@iucr.org>
  • Subject: Fast track proposal approved.
  • From: David Brown <idbrown@mcmaster.ca>
  • Date: Fri, 16 Oct 2009 14:46:36 -0400
The attached fast track proposal for a minor change to the coreCIF dictionary was circulated last September to you, the core Dictionary Maintenance Group (DMG).  It yielded one favourable and no unfavourable responses.  According to the recently approved rules for fast track approval of minor dictionary changes, this item is now deemed to have been approved by the DMG and has been passed to COMCIFS for final approval.

David Brown

 The following is a proposal for a change in the DDL1 core dictionary 

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

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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