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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 laboratory >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 significant 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
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