International
Tables for
Crystallography
Volume C
Mathematical, physical and chemical tables
Edited by E. Prince

International Tables for Crystallography (2006). Vol. C. ch. 4.2, p. 238

Section 4.2.5.3. Filters

D. C. Creaghb

4.2.5.3. Filters

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It is usual to consider only the cases where a quasimonochromatic beam is to be extracted from a polychromatic beam. Before discussing this class of usage, mention must be made of two simple forms of filtering of radiation.

In the first, screening, a thin layer of absorbing foil is used to reduce the effect of specimen fluorescence on photon counting, film and imaging-plate detectors. A typical example is the use of aluminium foil in front of a Polaroid camera used in a Laue camera to reduce the K-shell fluorescence radiation from a transition-metal crystal when using a conventional sealed molybdenum X-ray source. A 0.1 mm thick foil will reduce the fluorescent radiation from the crystal by a factor of about five, and this radiation is emitted isotropically from the specimen. In contrast, the wanted Laue-reflected beams are emitted as a nearly parallel beam, and the signal-to-noise ratio in the resulting photograph is much increased.

The second case is the ultimate limiting case of filtering, shielding. If it is necessary to shield an object completely from a polychromatic incident beam, a sufficient thickness of absorbing material, calculated using the data in Section 4.2.4[link], to reduce the beam intensity to the level of the ambient background is inserted in the beam. [The details of how shielding systems are designed are given in reference works such as the Handbook of Radiation Measurement and Protection (Brodsky, 1982[link]).] In general, the use of an absorber of one atomic species will provide insufficient shielding. The use of composite absorbers is necessary to achieve a maximum of shielding for a minimum of weight. This is of utmost importance if one is designing, say, the shielding of an X-ray telescope to be carried in a rocket or a balloon (Grey, 1996[link]). To produce shielding that satisfies the requirements of minimum weight, good mechanical rigidity, and ability to be constructed to good levels of mechanical tolerance, shielding must be constructed using a number of layers of different absorbers, chosen such that the highest-energy radiation is just stopped in the first layer, the L-shell fluorescent radiation created in the absorption process is stopped in the second, and the lower-energy L and M fluorescent radiation is stopped by the next layer, and so on until the desired radiation level is reached.

In the usual case involving filters, the problem is one of removing as much as possible of the Bremsstrahlung radiation and unwanted characteristic radiation from the spectrum of a laboratory sealed tube or rotating-anode source whilst retaining as much of the wanted radiation as is possible. To give an example, a thin characteristic radiation filter of nickel of appropriate thickness almost completely eliminates the Bremsstrahlung and Kβ radiation from an X-ray source with a copper target, but reduces the intensity in the Cu Kα doublet by only about a factor of two. For many applications, this is all that is necessary to provide the required degree of monochromatization. If there is a problem with the residual Bremsstrahlung, this problem may be averted by making a second set of measurements with a different filter, one having an absorption edge at an energy a little shorter than that of the desired emission line. The difference between the two sets of measurements corresponds to a comparatively small energy range spanning the emission line. This balanced-filter method is more cumbersome than the single-filter method, but no special equipment or difficult adjustments are required. In general, if the required emission is from an element of atomic number Z, the first foil is made from material having atomic number Z − 1 and the second from atomic number Z + 1. A better balance can be achieved using three foils (Young, 1963[link]). The use of filters is discussed in more detail in §2.3.5.4.2[link] . Data for filters for the radiations in common use are given in Tables 2.3.5.2[link] and 2.3.5.3[link] . The information necessary for choosing filter materials and estimating their optimum thicknesses for other radiations is given in Sections 4.2.2[link], 4.2.3[link], and 4.2.4[link].

It should be remembered that filtration changes the wavelength of the emission line slightly, but this is only of significance for measurements of lattice parameters to high precision (Delf, 1961[link]).

References

First citation Brodsky, A. (1982). Editor. Handbook of radiation measurement and protection, Vols. 1 and 2. Florida: CRC Press.Google Scholar
First citation Delf, B. W. (1961). The effect of absorption in the β-filter on the mean wavelength of X-ray emission lines. Proc. Phys. Soc. London, 78, 305–306.Google Scholar
First citation Grey, D. (1996). Instrumentation developments and observations in hard X-ray astronomy. PhD thesis. The University of New South Wales, Australia.Google Scholar
First citation Young, R. A. (1963). Balanced filters for X-ray diffractometry. Z. Kristallogr. 118, 233–247.Google Scholar








































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