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Re: New fast-track definition proposal:_diffrn_radiation_wavelength_nominal

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.
;

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