Poly-propylene and polycarbonate are particularly poor in regard to their ability to act as barriers to hydrogen and helium. VYNS, polycarbonate and Formvar are poorer water vapor membranes than the other materials.

The high water permeability of Formvar is reflected in its high water absorption (~8% in comparison with values of < 0.1% for most of the other materials.) This high absorption can adversely affect the other permeability values. For example, the dry oxygen permeability value of Formvar (Table 4.3) is 0.024 x 10-10 cm-3 (STP) cm/cm2.sec.cm Hg. Increasing the relative humidity to 50 and 100% increases the oxygen permeability by an order of magnitude to 0.19 x 10-10 and 0.28 x 10-10 cm3 (STP) cm/cm2.sec.cm Hg, respectively.

The permeability values of Table 4.3 were obtained on relatively thick films (25-75 m). If thin films are to act as gas barriers, not only must the criterion of low permeability be met but the thin film must also be structurally continuous. Previous work by this author with water vapor has shown that parylene C membranes are pinhole free at least down to thicknesses of 0.030 m.

The radiation transmission of a material can be calculated from the weighted summation of the mass absorption coefficients of the elements. Using the mass absorption coefficients (m/p) measured or calculated by Henke et al., Victoreen, and Allen and the densities (r) listed in Table 4.3, the mass absorption per unit thickness may be calculated. The values are shown in Table 4.4 for the wavelength region 10 80 Å (1.24-0.55 keV). Table 4.4 can be construed as the ln(Ia/I) for a 1 m thick film (I/Iu being the fraction of radiation transmitted through a given film thickness). Comparing the various polymers at this equal thickness level, it is observed that polypropylene, due to its low thickness level, its low density and the absence of absorbing elements other than carbon, is the most transparent material within this spectral region. Below the K absorption edge of oxygen, polypropylene is followed in transparency by parylene N, parylene C, polycarbonate, Formvar, VYNS, and Mylar, the latter being the most opaque. Between the K absorption edge, parylene N and polypropylene are more transparent than the other materials.

The above is based on the assumption that all the materials listed are available or can be produced in thicknesses of 1 m. As listed in Table 4.3, the thinnest Mylar available is ~3.8 m and the thinnest commercial polypropylene is ~12.7 m. Polycarbonate can be purchased as 2.0 m films; VYNS and Formvar are sold commercially in resin or solution form. Parylenes N and C are available as thin as 0.025 m. Sinha and Bhattacharyya report that ~1.5 m Mylar can be made; Caruso and Kim have stretched polypropylene to ~1.0 m, and VYNS can be made as thin as 0.007 m. Capacitance measurements on parylene film as thin as 0.004 m have been observed.

Using the absorption coefficients of Table 4.4, the percent transmission of commercially available films of Mylar, polypropylene, parylene N, parylene C and polycarbonate can be calculated. The 1.0 m thick polypropylene of Caruso and Kim is also included here for comparison. At an energy of 1 keV, 0.1 m thick parylene will absorb less than 2% of the incident radiation, 2.0 m thick polycarbonate about 43% and 3.8 m thick Mylar about 76%. At 0.5 keV, the parylenes are ~86% transmittant, and polycarbonate ~9%; Mylar is virtually opaque. At 0.2 keV, 0.1 m parylene N will pass ~95% of the incident energy which is about 50% more than stretched polypropylene, about five times as much as 2.0 m polycarbonate and nearly 60 times that of the thinnest available Mylar.

Table 4.4 Unit mass absorptions of various window materials

In conclusion, the combination of extremely good thickness uniformity, good inherent barrier properties, absence of pinholes, and very high radiation transmission in the soft and ultrasoft x-ray region point to the parylenes as attractive materials for radiation detection applications.

The author of this section on films, M.A. Spivack, would like to thank Ted Zehnpfennig of Visidyne Corporation for his advice and Dr. Lloyd Robeson of this laboratory for the gas permeability measurements:

1. G. Brown and R. Kanaris-Sotiriou, J. Sci. Instrum. Scr. 2,2,551 (1969)
2. B.K. Sinha and R. Bhattacharyya, Nucl. Instruments Methods 64, 107 (1968)
3. M.A. Volkar and R.L. Wolke, Rev. Sci. Instrum. 40, 849 (1969)
4. B.D. Plate and L. Yaffe, Can. J. Chem. 33,15 (1955)
5. W.L. Baun, Rev. Sci. Instrum. 40, 1101 (1969)
6. M.A. Spivack and G. Ferrante, J. Electrochem. Soc. 116, 1592 (1969)
7. J.A. Victoreen, J. Appl. Phys. 20, 1141 (1949)
8. S.J.M. Allen, Handbook of Chemistry and Physics, 44th Ed., p. 2766
9. B.L. Henke, R. White, and B. Lundberg, J. Appl. Phys. 28, 98 (1957)

Figure 4.24 Foil Mount.
Document Reference: Drawing. no. 600-A-1025, Rev. B, 5/26/83, RHW, Space Sciences Laboratory, Univ. of California-Berkeley.

Unless otherwise specified, dimensions are in inches. 
Tolerances are: decimals: .xxx +/- .005; angles: +/- 1/2°
Material: Alum. 6061-T6; finish: black anodize

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