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Replies to "I want to hear..."

  • Subject: Replies to "I want to hear..."
  • From: "John D. Barnes" <john.barnes@nist.gov>
  • Date: Tue, 9 Sep 97 16:15:56 -0400
Dear Colleagues;

     Several people have sent responses to my message of 5 September. A 
couple of people asked for summaries of the replies, so here is a 
preliminary digest of the discussion to date.

>1. Are there methods other than Fe 55 to obtain a flood illumination for the 
purpose of
>measuring the efficiency function for an area detector on a beamline from a 
>source operating with Cu Kalpha radiation?

Two classes of answers were received:

A. Use the fluorescence from something excited by the Cu Kalpha radiation 
to produce a nearly isotropic scattering. Move the beam center off of the 
detector to minimize interference from the small angle scattering. I have 
looked at this question in the past. The absorption edge of such a system 
must necessarily lie well below the energy of the Cu Kalpha photon, which 
means that the x-ray energy of the fluorescent photon will be lower than 
the one for which we are trying to characterize the detector performance. 
Unfortunately we have reason to believe that the efficiency of our 
particular detector (a multiwire gas-proportional unit) varies with the 
incident energy. Other detectors may not suffer from this problem, but 
this is an issue that we want to put to bed. There were a couple of 
suggestions of this ilk, and we will give them a try.

B. Use a weakly modulated azimuthally constant, mainly elastic, scatterer 
to create a heuristic function that can be used for correction. This was 
suggested by Malcom Capel, and he suggested using air. It sounds like it 
is worth a try.

His suggestion reads as follows:

even in the absence of a perfectly flat flood frames you can calculate
	a 2D "flattening" or efficieny corrector matrix by calculating the 
	radial average of the flood frame ... for each pixel of the detector 
	use the radial average as a lookup table for the expectation value 
	of the flood frame as a function of the distance of said pixel from 
	the beam center...generate a matrix of corrector scalars that you 
	directly multiply raw frames with to correct for nonrandom variations
	in apparent efficiency.  we use this method with our multiwire 
	detector which has tremendous anode modulation nonuniformity...works 
	very well if the detector isnt saturated by conditions in effect to 
	measure the flood frame.

2. Does anyone have a bright (yet practical) idea for a primary absolute 
intensity standard for the same kind of setup described above?

One respondent talked about polyethylene secondary standards, but there 
have been no suggestions regarding a primary standard yet. Has anyone 
ever implemented Bob Hendricks' suggestion for using a fluid near its 
critical point?

3. Is it possible to make nice unoriented specimens that exhibit 
fine structure in their SAXS patterns?

Dieter Schneider mentioned:

	The silver salts of fatty acids produce nicely detailed small-angle
	powder patterns that can serve both as wavelength and intensity
	standards.  Descriptions are given in :
	T.C. Huang et al, J. Appl. Cryst. 26, 180-184 (1993), and
	T.N. Blanton et al, Powder Diffraction 10(2), 91-95 (1995).

I am concerned about crystal orientation, particle size, and absorption 
effects in the kind of sample that might be useful for SAXS. I read the 
Blanton article, but found that it mainly addressed the needs of 
diffractionists who needed to work below about 5 degrees 2-theta. It did 
not look to me like this system would exhibit more than one or two peaks 
in the q region that I am interested in. I will, however, give it a try. 

Dr. John D. Barnes                       email: john.barnes@nist.gov
NIST Polymer Structure and Mechanics Grp Voice: 301-975-6786
Bldg 224, Rm A209                        FAX: 301-975-4977
NIST, Gaithersburg, MD 20899             URL: http://www.nist.gov/sas
                               or http://www.ctcms.nist.gov/~jdbarnes