From Wayne Faszer, in a TWIST forum posting:
https://twist.phys.ualberta.ca/forum/view.php?site=twist&bn=twist_chambers&key=1054340890

     The new target module consists of an Aluminum foil disc of
99.999+% purity.  The disc is nominally 75 um thick and 150 mm in
diameter.  It is glued to an aluminized mylar cathode plane nominally
25.4 um thick with a 120 mm diameter hole cut in the center.  The glue
is a silver loaded conductive epoxy and the radial width of the glue
joint is approximately 15 mm.

     Because there are concerns about nonuniform electric fields due
to this construction the cathode planes in layers pc6 and pc7 have
been masked with kapton masks.  There are 4 annular masks nominally
25.4 um thick with an inner diameter of 110 mm and an outer diameter
of 338 mm (which is approximately the outer diameter of the cathode
plane).

     Samples of the materials were weighed to determine their actual
thickness and the following results were obtained:

     Mylar     22.76 um + - .07 um
     Kapton    24.43 um + - .07 um
     Aluminum  71 um + - 1 um

The densities that I assumed are as follows:

Mylar        1.392 gm/cm**3
Kapton      1.42   gm/cm**3
Aluminum  2.6989  gm/cm**3