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Re: New fast-track definition proposal:_diffrn_radiation_wavelength_nominal
- To: comcifs@iucr.org
- Subject: Re: New fast-track definition proposal:_diffrn_radiation_wavelength_nominal
- From: kaduk@polycrystallography.com
- Date: Wed, 04 Nov 2009 21:07:03 +0000
- In-Reply-To: <D4741A57-710F-4074-82C6-5F0972C42D31@ANL.gov>
- References: <279aad2a0911021852q329a5eej75d5754548218476@mail.gmail.com><20091104171548.28796odgwnc1b08w@webmail15.dotsterhost.com><D4741A57-710F-4074-82C6-5F0972C42D31@ANL.gov>
But NIST keeps changing their minds about what the value is; check the certificates for the various SRMs. Quoting "Brian H. Toby" <Brian.Toby@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@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@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@iucr.org >>> http://scripts.iucr.org/mailman/listinfo/comcifs >>> >> >> >> >> _______________________________________________ >> comcifs mailing list >> comcifs@iucr.org >> http://scripts.iucr.org/mailman/listinfo/comcifs > > ******************************************************************** > 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 > technology's wonders... We will harness the sun and the winds and > the soil to fuel our cars and run our factories... All this we can > do. All this we will do." > >
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