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
On Feb 9, 2005 Maher and I measured the thickness of a cathode foil that came out of the detector. The foil measured was labelled T45A. From this foil we cut the largest square that we could without getting any glue from the edges (226.0mm by 226.5mm)+/-1.0mm. We folded the foil, and weighed this foil three times on an accurate scale, getting 0.434g, 0.434g, and 0.4335g which we take to be 0.434+/-0.001g. From the measurements above we get an area density of (8.48+/-0.03)x10^-4g/cm^2. Using the foil density extracted from GEANT of 1.4065 g/cm^3, this gives an unstretched foil thickness of 6.02+/-0.03microns. The 0.6% stretched thickness would then be 5.96+/-0.03microns. This is about 0.06microns (2 sigma) thicker than from our previous measurement of a foil that 'might' have come from the same roll.
B. Jamieson, 9 Feb 2005
On Mar 4, 2005 Maher and I measured the thickness of a another cathode foil that came out of the detector. This time we took some time up front to try to estimate the thickness using a micrometer. We measured the thickness for 1,2,4,8,16,32, and 64 layers of the foil, resulting in an average thickness of 6.42+/-0.08microns, illustrated in this plot. We also measured the dimensions of the foil to be 18.2cm by 22.5cm +/-0.1cm, and weighed the foil to be .3465+/-0.0010g. These measurements result in a density measurement of 1.317+/-0.009 g/cm^3. This density is about 10 sigma lighter than the 1.4065 g/cm^3 used in GEANT. It is possible that our thickness measure using the micrometer is too large however due to extra dust between layers, and possibly due to pockets of air trapped when the foil is folded. From the measurements above we get an area density of (8.46+/-0.06)x10^-4g/cm^2. Using the foil density extracted from GEANT of 1.4065 g/cm^3, this gives an unstretched foil thickness of 6.02+/-0.04microns. The 0.6% stretched thickness would then be 5.96+/-0.04microns. This compares well with our previous measurements: 5.96+/-0.03microns, (5.887 +/- 0.018)x10^-4cm, and (5.866 +/- 0.018)x10^-4cm.
B. Jamieson, 4 Mar 2005