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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. 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. ;
begin:vcard fn:I.David Brown n:Brown;I.David org:McMaster University;Brockhouse Institute for Materials Research adr:;;King St. W;Hamilton;Ontario;L8S 4M1;Canada email;internet:idbrown@mcmaster.ca title:Professor Emeritus tel;work:+905 525 9140 x 24710 tel;fax:+905 521 2773 version:2.1 end:vcard
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