X-Message-Number: 13737
From: 
Date: Wed, 17 May 2000 07:53:52 EDT
Subject: optical system in the X-ray domain

X-ray optics.

A lot of optical element is in order in a QND interferometer.
There are "classical" lenses of the Fresnel kind limited to
some millimeters in diameter. Making them at the nanometer
scale precision is a challenge in itself. Other elements are
even more uncommon, for example, elementary beams
must be bunched side by side, that may be done with high
incidence mirrors, something only possible with atom scale
precision epitaxy.

Then, outside the interferometer there must be a polarization
filter, and a detector.The detector cells will be a kind of CCD
with pixels in the ten micrometer range. This is 100 time the
scale of the detected brain elements. An optical system must
then enlarge the picture by this factor at the interferometer
exit. Even if the elementary beams are first unbunched, the
enlargement of a single 3 mm beam may need a one feet
diameter mirror or lense. Because the optical precision of
lenses are four time smaller than the mirror solution for a
given result, lenses are *the* solution.

Big X-ray lenses can't be built with the fresnel desing, the
next best way is then to use the complex refractive index
of many material in the X band to make a "classical" lense.
Most material are strong absorber in the 1 keV domain of
interest for QND interferometer, only solid hydrogen would
be suitable. Making and keepping such a lens at some
degrees above absolute zero is not a small undertaking in
itself.

Graphite could be a good alternative. It is carbon and carbon
is not crystal clear at 1 KeV, but graphite is a crystal: all
carbon atoms are keept in a stack of sheets. In two out of
three directions, a polarized beaam may propagates without
contact with the carbon atoms. That lens is at the same time
a polarizer, by desing, it has some transparency only to
vertical or horizontal polarization. This is not a problem here
because we need indeed such a polarizer at the interferometer
exit.

Yvan Bozzonetti.

Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=13737