On Wednesday, June 19, 2013 11:11:01 am Edward A. Berry wrote: > Somewhere I got the idea that a diffractometer is an instrument that measures > one > reflection at a time. Is that the case, and if so what is the term for > instruments > like rotation camera, weisenberg, area detector? (What is an area detector?).
As I originally learned the term, it meant a photon-counting device (discrete counts) as opposed to film or other analog measurement. Stout & Jensen (1968 1st ed page 149): Two general methods are available for measuring the intensities of diffracted beams. Either the beams may be detected by some sort of quantum counting device which measures the number of photons directly (diffractometer or counter methods) or else the degree of blackening of spots on diffraction photographs may be measured and taken as proportional to the beam intensity (photographic methods). All rotation cameras and Weisenberg cameras that I have encountered used film or image plates for recording data, so they fall in the "photographic methods" category. But really both of these refer to the geometry used during the experiment. In principle you could mount a Pilatus detector on a rotation camera, or kit out a cylindrical drift chamber as a Weissenberg camera (maybe). That would shift them into the diffractometer category. "Area detectors" came later. But they too come in both photon-counting varieties (multiwire detectors, pixel detectors) and analog proportional detectors (imaging plates, CCD). The line is blurred in the case of CCD/pixel detectors operating in a mode where accumulated charge is translated back into a specific number of photons. > Logically I guess a diffractometer could be anything that measures > diffraction, > and that seems to be view of the wikipedia article of that name. That does not match the historical use of the term. -- Ethan A Merritt Biomolecular Structure Center, K-428 Health Sciences Bldg University of Washington, Seattle 98195-7742
