I put some pictures of scattering from random things (including "air") here:
http://bl831.als.lbl.gov/~jamesh/pickup/images/ It is actually more difficult than you might think to get a clean image of "air scatter" on a well-maintained camera. This is because we usually try to minimize the contribution from air. Generally, air scatter is reduced to the point where it becomes less than the most bothersome "other thing", such as fluorescence from slits and pinholes, etc. To demonstrate this, I took two 60s shots of "air scatter" with the cryostream set at 100K and 200K and then subtracted them. The result is "N2_gas_100K-200K_0_001.img". The reason why there is so much more "air scatter" with the stream at 100K vs 200K is because N2 gas at 100K is twice as dense as it is at 200K (and ~3x denser than the ambient air). In fact, you can actually confirm the sensitivity of your detector doing this, provided you know your beamline's flux. This is because the scattering from any gas in the "forward" direction (near the beamstop) is: I_gas = F^2*r_e^2/kB*P/T*thickness*flux*exposure*(pixel_size/distance)^2 where: I_gas = the number of photons hitting one pixel due to the gas F = 14 electrons (7 for atomic nitrogen, 14 for N2) r_e^2 = 7.94e-30 m^2 (Thomson cross section) kB = 1.38e-23 kg*m^2/s/K (Boltzmann constant) P = 101325 kg*m/s^2 (ambient pressure) T = 100 K or 200 K thickness = ~8e-3 m (width of cryojet nozzle) flux = 2.4e11 photons/s exposure = 60 s pixel_size = 102.5e-6 m (ADSC Q315r in binned mode) distance = 150e-3 m (150 mm sample to detector) Note that P/T/kB is just the number of molecules per cubic meter of the gas. Using the above numbers, the difference between 100K and 200K is 3074 photons/pixel falling on the detector near the beamstop. (at higher angles, you need to use a smaller "F"). You can see in the N2_gas_100K-200K*.img file there are ~3600 "counts"/pixel near the beamstop, wich is pretty close, but actually a coincidence. The "gain" of this detector is not unity, so the "counts" on the image are not photons. For a Q315r in hardware-binned mode (my default), the digitizer is set to 4 electrons per ADU (Area Detector Unit = one pixel level on the image). A single 11 keV photon will generate an average of 7.3 electrons in the CCD (according to the manufacturer), which gives us a total "gain" of 1.8 ADU/photon. Yes, this is the number you give to MOSFLM as the "GAIN", and yes it does mean that one "photon" actually generates almost two ADU. Given, this, my N2_gas_100K-200K image should have 5611 ADU/pixel. So, now its too high. This is because not all 8 mm of the cryo stream width is at the lowest temperature. There is a bit of a "gradient", which you can see in this image: http://bl831.als.lbl.gov/~jamesh/pickup/images/cryojet_net.img This is a difference image taken with the cryojet "on" vs "off" with the collimator and beamstop removed and an 0.3 mm selenium foil inserted 30 cm upstream from the sample. This is just thick enough to "stop" the main beam, so this is literally an "x-ray" of the sample area from a "point" source of Se Kalpha fluorescence. You can see the outline of the cryojet nozzle on the left and the sample magnet on the right. If you use the ADXV "line" tool you can see how the gas of the cryo stream is actually denser (colder) in the middle, and somewhat denser toward the bottom. Note that the "beam center" in the image is actually the point where the x-ray beam passes through the cryo stream, which can be useful for alignment. If I take the "relative density" across this image of the stream (assuming "1" at the center and "0" beyond the edges), I get an average of 60%, which brings 5611 ADU/pixel down to 3366 ADU/pixel, vs the observed 3600 ADU/pixel. Not too bad, I think. -James Holton MAD Scientist On Thu, Aug 1, 2013 at 9:31 AM, Pietro Roversi < pietro.rove...@bioch.ox.ac.uk> wrote: > Dear all, > > together with two fellow crystallographers, > I am writing a pamphlet to introduce schoolchildren > to X-ray crystallography. > > For the introductory chapter, > we would need a picture of X-rays scattered by air. > Or by any gas for that matter. > > I have tried Google images without much luck. > > Can anyone either kindly donate, or point me to, > an image of X-rays scattered by a gas? > > Thanks! > > Pietro > > PS of course a picture of X-ray scattering by a liquid would also be very > welcome >
