Crystallography Online: the website of the International Union of Crystallography

I have used natural mica sheets in the past, but
recently tried a synthetic material called 3525 Ultralene.
I didn't notice any unusual scattering background (the
polymer film was only 4 um thick), though I have not tested
it extensively yet.

You can get it from

     SpexCertiPrep
     203 Norcross Ave
     Metuchen, NJ 08840
     phone: 1-800-LAB-SPEX

I believe Lois Pollack's group here at Cornell is
also experimenting with even thinner windows via
nanofabrication methods, but they are quite fragile.
She may describe them is some of her recent papers.

Richard Gillilan
MacCHESS, Cornell University