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An issue that may be relevant to the accurate determination of the filter transmission on orbit is the temperature dependence of the amplitude of EXAFS. According to EXAFS theory the amplitude of the EXAFS oscillations are a function of temperature. This dependence arises from the fact that thermal vibrations of the atoms in a solid produce a phase mismatch of the backscattered electron wave function. The transmission properties of the ACIS filters were measured at room temperatures 20 C while the on orbit filter temperature is expected to be about -60 C. The temperature dependence of the EXAFS component is incorporated in a Debye-Waller type term Q(k,T) (Stern et al., 1975). For thermally induced disorders of atoms and for deviations about the average shell distance of Rj which follow a Gaussian distribution, Q(k,T), is given by,
The on orbit transmission at temperature Torb is expressed as a function of the transmission as measured on ground at temperature Tgr by the expression,
In Figure 5.12 we show the percent change in filter transmission above the Al-K absorption edge for an expected on orbit filter temperature of -60 C.