A few helpful people have asked me what my low-angle mystery scattering
looks like, so I have made images of it available here at two different
grey levels:
http://bl831.als.lbl.gov/~jamesh/pickup/bright.png
http://bl831.als.lbl.gov/~jamesh/pickup/dim.png
My question is: what is all that scattering around the rim of the
beamstop?
There was nothing at the sample position but air. The black dot in the
middle of these images is the attenuated direct beam shining through the
semi-transparent core of the beamstop. In these images, the edge of the
beamstop shadow is at a d-spacing of 430-260 A and the edge of the image
is at a d-spacing of 50 A. The detector in this case was an ADSC
Quantum 210 positioned 1 meter from the sample position. The beamstop
is a 1 mm long gold tube OD=0.3 mm ID=0.2 mm stuffed with ZnS:Cu
phosphor powder. The beam-defining aperture is a 0.1 mm round hole
laser drilled in a 2 mil thick tantalum sheet, which is located 35 mm
upbeam from the beamstop. The exit window from the beamline is made of
Be and it is another 60 mm upbeam from the pinhole. The beam in this
case was set with a convergence angle of 0.3x0.3 millirad. The support
structure for the beamstop is a 1 mm thick sheet of packing foam
(equivalent to 1 micron of polyethylene).
This can't be "air scatter" because the scattering from most any gas is
flat at these small angles. The pinhole is centered in a round holder,
and when I rotate the pinhole about the beam, the radial striations you
can see in "bright.png" rotate with the pinhole. The intensity and
shape of this low-angle scatter do not change when I walk the beam
energy across the Ta, and Zn edges, so I am concluding that this is not
fluorescence from the pinhole or from the beamstop. Moving the beamstop
around does not seem to change the shape of this scattering either.
Filling the region behind the pinhole with He also had no effect.
Anything that came from the back of the beamstop should show up in the
beamstop shadow itself, so I think whatever this is, it is coming from
something upbeam from the beamstop.
Crazy suggestions and wild speculations are welcome...
-James Holton
MAD Scientist
