Dear All,
so that we don't have to wait until Bill comes back...
I sent a portion of an MgO pattern taken with HRPD to Alan, and he kindly
put it on his www server: http://www.ill.fr/dif/hrpd-mgo.doc. It's a simple
text file. I converted the data to 2th, using a popular synchrotron
wavelength (0.7 A) for everybody's convenience. You can see by yourself
that the 200 peak has about 0.013 degrees FWHM. Clearly, the FWHM depends
on the wavelength you choose, so I guess I could have gotten to 0.006 using
a shorter wavelength. As I already said, what matters is the deltad/d
resolution, in this case 7x10-4. By the way, D2B at the ILL has similar
performances, albeit in a more restricted 2th region.
The pattern was truncated at high angles to reduce the file size, but the
low-angle limit is a real one with this chopper setting. The setting can be
changed, but we aren't yet flexible enough to do extended patterns for
routine structural solution. This will change quite soon.
As far as the number of points is concerned, the full data set had 17000
points (I truncated to 80 degrees to reduce the file size). Indeed, we do
things a little differently. As usual, we collect data in such a way as to
have roughly the same number of points in each profile, but we achieve this
by using constant step size in log(d-spacing). When converted to 2th, the
step size varies from 0.0018 degrees at 20 degrees to 0.01 degrees at 80
degrees.
The full detector complement of HRPD is ~1000 independent scintillators.
So, if we keep everything decoupled (which is now possible), we get 68Megs
of 4-byte integers per pattern. To focus the pattern, I used my new data
reduction software ("ARIEL"), which allows dynamic selection of the detector
area, and I specified to focus only the high-resolution rings (deltad/d
<7e-4).
With this, I rest my case.
Paolo
