Jim's point is well taken and can perhaps be generalised to say one is limited either by the crystal or by the set up for recording the diffraction. To optimise things it is useful to know where the limits are.
With some tightly focused synchrotron beams you can also see your spots getting larger as you increase the detector distance because the focused beam can have a high divergence. By having a low divergence, one can gain on spot to background ratio by moving the detector further away. To fully exploit this, a larger detector (more angular resolution elements) is needed. The gain of having a lower divergence beam and larger detector will continue until dominated by the divergence introduced by the crystal (James Holton has probably covered this somewhere in his emails - I only read the bit about the beer!). One could however go further as the divergence due to the crystal will not be smooth but will contain some structure describing the crystal disorder (e.g. domain structure). By matching the beam divergence and detector angular resolution to this structure one should be able to record this and get a still better ratio of spot intensity to background. The limit is to illuminate the crystal with a coherent beam and record the scattering with an angular resolution appropriate to the whole crystal. One would then reconstruct the crystal from the oversampled scattering and average the unit cell contents in real space. It would allow one to properly handle disorder and "background" from the crystal. Most people would say that this last suggestion is way in the future and perhaps not worth doing. However, it is being investigated by those interested in looking at sub micron (nano) crystals with coherent x-ray sources. Colin -----Original Message----- From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On Behalf Of Jim Pflugrath Sent: 24 November 2009 15:25 To: CCP4BB@JISCMAIL.AC.UK Subject: Re: [ccp4bb] decrease of background with distance? The source for the X-ray background are points along the air path post-collimator including the sample with loop and cryoprotecdant (or capillary and mother liquor). So the 1/r^2 falloff is noticable going from 100 mm to 200 mm. The same counts in a 2x2 pixel area is now seen in a 4x4 pixel area. The source for Bragg reflections at a synchtrotron is upstream a couple dozen meters. The divergence is not large as well, so the spread in the spots (for a source ~30 meters upstream) goes from 1/(30.1 * 30.1)^2 to 1/(30.2 * 30.2)^2 which is really not that noticable. In contrast, a modern home lab system with Osmic multilayer optics, the source is much closer to the sample and the divergence is larger. For folks with home lab systems, you can see your spots getting larger as you increase your distance. The total background under the (sum of all) pixels of the Bragg reflections does not change that much with changing distance. It's just that everything (spot & background) are spread out over more pixels. Thus moving the detector backwards will not do the same thing that it does for you at a beamline. Folks with such home lab systems can do the experiment to see this for themselves. So when your synchtrotron buddies talk about all this, be sure to buy them beers to help with their thinking. :) Jim -----Original Message----- From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On Behalf Of James Holton Sent: Tuesday, November 24, 2009 1:46 AM To: CCP4BB@JISCMAIL.AC.UK Subject: Re: [ccp4bb] decrease of background with distance? Spots don't fall off with the inverse square law. It is a very easy experiment to do. Just take exposures at several distances and scale the data together, noting the correction for air absorption. A good reference for the underlying theory is Chapter 6 of M. M. Woolfson's book (1997). But briefly: Diffuse scattering falls off with the inverse square law as one would naturally expect from anything that spreads out in all directions. Spots, however, are "reflections" and their source point is the source of the incident beam, which is usually far away or at worst comparable to the sample-to-detector distance. For those who do not like my "long posts" I now direct you to the "delete" button which should have come with your email client. Everyone else, read on. ... It is an interesting point, however, that simultaneously increasing the detector distance and increasing the photon energy to keep the resolution at the edge of the detector constant has absolutely NO effect on the photons/spot from Bragg vs diffuse scattering. It took me a while to become convinced of this, and I still owe Colin Nave a few beers as a result. Nevertheless, the experimental evidence was given by Gonzalez, Denny & Nave in 1994. HTH -James Holton MAD Scientist Richard Gillilan wrote: > It seems to be widely known and observed that diffuse background > scattering decreases more rapidly with increasing detector-to-sample > distance than Bragg reflections. For example, Jim Pflugrath, in his > 1999 paper (Acta Cryst 1999 D55 1718-1725) says "Since the X-ray > background falls off as the square of the distance, the expectation is > that a larger crystal-to-detector distance is better for reduction of > the x-ray background. ..." > > Does anyone know of a more rigorous discussion of why background > scatter fades while Bragg reflections remain collimated with distance? > > > Richard Gillilan > MacCHESS -- This e-mail and any attachments may contain confidential, copyright and or privileged material, and are for the use of the intended addressee only. 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