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Re: New fast-track definition proposal:_diffrn_radiation_wavelength_nominal
- To: "Discussion list of the IUCr Committee for the Maintenance of the CIFStandard (COMCIFS)" <comcifs@iucr.org>
- Subject: Re: New fast-track definition proposal:_diffrn_radiation_wavelength_nominal
- From: "Ralf W. Grosse-Kunstleve" <rwgk@yahoo.com>
- Date: Tue, 3 Nov 2009 18:14:10 -0800 (PST)
- In-Reply-To: <279aad2a0911021852q329a5eej75d5754548218476@mail.gmail.com>
- References: <279aad2a0911021852q329a5eej75d5754548218476@mail.gmail.com>
Hi James, Not being a native speaker I have to confess that the intended meaning of "nominal" wasn't immediately obvious to me, although it is clear after reading the _definition. I'd prefer "estimated" instead of "nominal", and estimated from secondary information, in the definition. However, this clearly is a minor point. I'd be happy to support the proposal as is. Ralf ----- Original Message ---- From: James Hester <jamesrhester@gmail.com> To: Discussion list of the IUCr Committee for the Maintenance of the CIF Standard (COMCIFS) <comcifs@iucr.org> Sent: Mon, November 2, 2009 6:52:27 PM Subject: New fast-track definition proposal: _diffrn_radiation_wavelength_nominal 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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