Dear Alastair,
I have found your name on the ALS web site as the local contact for the
tomography beam line 8.3.2. On behalf of the GlueX collaboration (http://www.gluex.org) I would like to
request your advice regarding our rather unique needs for a
highly-collimated X-ray beam line for diamond crystal assessment.
These diamonds are large thin monocrystaline wafers (6mm x 6mm x
100microns) which our colleagues from the University of Glasgow have
obtained from the Drukkers synthetics laboratory (a division of
Debeers). These crystals are grown from a small seed to an ingot of
cm-scale dimensions. We are able to obtain samples from a number of
the ingots, from which we must select the few (perhaps 10-20%) of the
ingots whose uniformity and mosaic spread are sufficiently small for
our purposes. After final machining, these wafers are mounted in a
beam line at Jefferson Lab and used to generate highly monochomatic and
polarized gamma ray beams in the multi-GeV energy region by the process
of coherent bremsstrahlung. These beams provide unique opportunities
for high-energy nuclear physics experiments.
A synchrotron beam has unique properties of intensity and collimation
that facilitate the assessment of these large samples. A typical good
crystal has a rocking curve peak of 20 microradians width over its
entire 30mm^2 surface (natural width of diamond is 5 microradians) . A
bad one may be 50 or 100 microradians wide or have several peaks
spanning this range. Such widths are difficult to resolve with
conventional laboratory Xray diffractometers and scanning this large
area would require an excessive amount of time with a typical 10kW
copper K-alpha source. By contrast, the beam line we have used up
until now at the Daresbury SRS (U.K.) had an angular divergence of
order one arc-second and was equipped with variable slits that could
open up to a 6mm x 6mm spot. It also had a dual-axis goniometer that
enabled us to come in and run tomographs and rocking curves on a dozen
samples within just a few hours. Now that the SRS has been shut down,
we are looking for another beam line, preferrably in North America,
where we could get in for brief periods to run these diagnostic
measurements. An initial estimate might be for two periods per year
over the next few years of a total of 24-48 hours each.
The following recent NIM publication contains more details of the
diamond preparation and selection process. We would appreciate any
advice you might offer us on how we might refine our search.
Nucl.Instr.Meth.
A545 (2005) 164.
thank you,
Richard Jones, Prof.
University of Connecticut