Cathode foil thickness

On March 3, 2003 Glen Marshall and I measured the thickness of a piece of aluminized mylar. The piece we measured came from a roll with the same specs as we used for the cathode foils (nominally 6.25um). The measurement is documented in TWIST Log Book #4 (start date 2002-05-27).

With Steve's help we cut a square piece of the foil about 60x60cm^2, no tension, small wrinkles. The weight of the piece was found to be (3.006+-0.002)g, that gives 8.35x10^-4g/cm^2. We estimated the error on the measurement to be about 0.3% which is mainly due to the uncertainty in the shape and size of the piece. (See the log book.)

We also measured the foil thickness directly with a micrometer. One layer is 0.00029'', four layers are 0.0011'' (translating into 0.00028'' per layer). The instrument accuracy is 0.00005'', so this measurement is to no better than 5%. Also a measurement of g/cm^2 is more relevant for GEANT, so we will ignore the micrometer result.

From the surface mass density assuming the specific density of mylar of 1.39g/cm^3 and ignoring the aluminum we get the effective thickness of 6.007x10^-4cm.

Most (?) of our cathode foils are stretched to 0.6% (a linear, not a surface factor). So the effective thickness of the foil in the detector is 6.007/(1.+0.006)^2=(5.936+-0.018)x10^-4cm, where the 0.3% error has been applied.

The thickness listed in dt_geo.00032 is 5.872x10^-4cm, which is 3.5 sigma below the new measurement.


A. Gaponenko, March 2003

On Jan 27, 2005 Maher and I measured the thickness of a piece of aluminized mylar. The piece we measured came from a roll with the same specs as we used for the cathode foils (nominally 1/4 thou). The measurement is documented here.

With Steve's help we cut a roughly square piece of foil with side lenghts of 989.5mm x 705.5mm x 1000.5mm x 700.0 mm, or roughly 702.8 mm x 995.0 mm, with no tension, small wrinkles. The weight of the piece was found to be (5.857+-0.001)g, that gives 8.38x10^-4g/cm^2. Here we will assume the error is about the same 0.3% estimated by Glen and Andrei.

From the surface mass density assuming the specific density of mylar of 1.39g/cm^3 and ignoring the aluminum we get the effective thickness of (6.026 +/- 0.020)x10^-4cm.

Most (?) of our cathode foils are stretched to 0.6% (a linear, not a surface factor). So the effective thickness of the foil in the detector is 6.026/(1.+0.006)^2=(5.954+-0.018)x10^-4cm. This is in agreement with the previous measurement.


B. Jamieson, 27 Jan 2005
Peter Gumplinger has extracted the density used in GEANT, which uses a mixture of by weight of 2.42% Al + 97.58% mylar corresponding to a foil of mylar 6.27 u thick with 0.04 u of Al on either side. The density of this mixture is 1.4065 g/cm**3. Using this number in calculations like those above, instead of the 1.39 g/cm**3 assumed in Glen and my calculations, we get stretched foil thicknesses summarized below:

Glen and Andrei's measuremnt gives (5.866 +/- 0.018)x10^-4cm, and

My and Maher's measurement gives (5.887 +/- 0.018)x10^-4cm.

These estimates are well within 1 sigma of the value in the geometry file which is 5.872x10^-4cm.


B. Jamieson, 2 Feb 2005