# RE: [Fwd: Re: New fast-trackdefinitionproposal: _diffrn_radiation_wavelength_nominal]

• To: "Distribution list of the IUCr COMCIFS Core Dictionary Maintenance Group" <coredmg@iucr.org>, "Distribution list of the IUCr COMCIFS Core Dictionary Maintenance Group" <coredmg@iucr.org>
• Subject: RE: [Fwd: Re: New fast-trackdefinitionproposal: _diffrn_radiation_wavelength_nominal]
• From: "Haltiwanger, Curtis" <chaltiwa@cephalon.com>
• Date: Thu, 26 Nov 2009 12:57:11 -0500
• References: <4B0D90EC.1090102@mcmaster.ca>

I have only used lab sources, so I have not needed other wavelengths.
That being said, I approve of including additional information.

R. Curtis Haltiwanger
Research Scientist
Cephalon, Inc.
610-738-6853

-----Original Message-----
From: coredmg-bounces@iucr.org on behalf of David Brown
Sent: Wed 11/25/2009 3:17 PM
To: Distribution list of the IUCr COMCIFS Core Dictionary Maintenance Group
Subject: [Fwd: Re: New fast-track definitionproposal:	_diffrn_radiation_wavelength_nominal]

Dear Colleagues,

Last September I circulated for fast track approvel a recommendation for

to the core dictionary (see below for details).  The intent was that
this could be used for giving a nominal wavelength calculated e.g., from
the paparmeters of a diffractometer.  It was proposed tht this might be
needed for completeness even though it was not accurate enough for
calculating  lattice parameters, etc.  This proposal was passed by the
core CIR dictionary maintenance group by default (no one raised any
objection).  When this proposal was subsequently presented for final
approval to COMCIFS, Nick Spadaccini suggested a more elegant

we could loop the nomiinal wavelength with the  refined (and any other
future expansion.

An example of how this might be used is:

loop_
1   1.23456   fundamental
2   1.25      estimated

According to our rules, this proposal is being posted for comment to the
coreCIF dictionary maintenance group for the next six weeks.  At the end
of this time, if there are no unresolved issues, the core dictionary
maintenance group will be deemed to have accepted the proposal.

David Brown

==============================================================

The proposal is to add the following item to the coreCIF dictionary:
=============================================

2009-09-25 Proposal from the Nick Spadaccini on the COMCIFS dicsussion
list to replace a withdrawn proposal.

_type                        char
_list                        both
loop_
_enumeration
_enumeration_detail
'fundamental'
'Wavelength that is a fundamental property of matter e.g. MoK\alpha'
'estimated'
'Estimated from secondary information e.g. monochromator angle or
time of flight'
'refined'
'Based on refinement using a standard material with known cell
parameters'

_definition
;              The method of determination of incident wavelength.
Further information may be provided in _diffrn_radiation_special_details
;

The original proposal was as follows:
=========================
>>
>> 2009-08-23 proposed by James Hester.
>> 2009-10-16 approved by the core Dictionary Maintenance Group under a
fast
>>           track approval process.
> > 2009-11-25 proposal withdrawn in favour of  an alternative
>>
>> 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
>>
>>     _type                        numb
>>     _list                        both
>>      _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,
>> ;
>>
>> A change in definition of an existing item.  The current
>>
>>
>>     _type                        numb
>>     _list                        both
>>     _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
>>               item.
>>  ;

-----------------------------------------
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